Phase autofocus on the matrix. How does autofocus work on a smartphone?

The camera's autofocus system adjusts the lens to focus on the subject and can mean the difference between a clear shot and a missed opportunity. Despite the apparent obviousness of the task of “sharpness at the point of focus,” the hidden work required for focusing, unfortunately, is far from so simple. This chapter is designed to improve the quality of your photography by providing an understanding of how autofocus works, allowing you to get the most out of it and avoid its shortcomings.

Note: Autofocus (AF) works either using the contrast sensors in the camera ( passive AF), or by sending a signal to illuminate or estimate the distance to an object ( active AF). Passive AF can be achieved using methods contrasting or phase detector, but both methods rely on contrast to achieve accurate autofocus; therefore, for the purposes of this chapter, they are considered qualitatively identical. Unless otherwise noted, this chapter covers passive autofocus. We'll also look at the active AF assist beam method towards the end.

Concept: Autofocus sensors

The camera's autofocus sensor(s) are located in different parts of the image's field of view and are the entire system behind achieving sharp focus. Every sensor measures relative focus from changes in contrast in the corresponding area of ​​the image, and maximum contrast is considered to correspond to maximum sharpness.

Changing focus:		Blur	Half focus	Sharpness
	400%	Sensor histogram

The basics of image contrast are covered in the chapter on image histograms.
Note: many are compact digital cameras the contrast sensor uses the image sensor itself (using a technique called contrast AF) and is optionally equipped with multiple discrete autofocus sensors (which are more common when using phase detection AF). The diagram above illustrates the contrast AF method; The phase detector method is different, but also relies on contrast as a criterion for autofocus.

Focusing process general outline works like this:

1. The autofocus processor (AFP) slightly changes the focusing distance.
2. AFP reads the AF sensor and evaluates how and how much focus has changed.
3. Using the information from the previous step, AFP adjusts the lens to the new focusing distance
4. The AFP repeats the previous steps sequentially until satisfactory focus is achieved.

The entire process usually takes a fraction of a second. In difficult cases, the camera may not achieve satisfactory focus and will begin to repeat the above process, which means the autofocus will fail. This is a terrible case of "focus hunting" where the camera constantly moves the focus back and forth without achieving focus. However, this does not mean that focusing on the selected subject is impossible. The next section examines cases and causes of autofocus failure.

Factors affecting autofocus

The subject you're shooting can have a huge impact on how successful an autofocus is, often more so than the differences between camera models, lenses, or focus settings. The three most important factors affecting autofocus are light level, subject contrast, and camera or subject movement.

An example illustrating quality various points focus, shown on the left; Hover over the image to see the advantages and disadvantages of each focal point.

Note that all of these factors are interrelated; in other words, autofocus is achievable even on a dimly lit subject if it has high contrast, and vice versa. This has important implications for your choice of autofocus point: choosing a focus point that is on a sharp edge or strong texture will help achieve better autofocus, all other things being equal.

The example on the left differs favorably in that the best autofocus points coincide with the position of the subject. The next example is more problematic because autofocus works better on the background than on the subject. Hover over the image below to highlight areas of good and bad work autofocus

In the photo on the right, if you focus on fast-moving light sources behind a subject, the subject itself may be out of focus if the depth of field is shallow (as is usually the case when shooting in low light conditions like the ones shown).

Otherwise, focusing on the external illumination of the subject might be the best approach, except that this illumination quickly changes location and intensity depending on the position of moving light sources.

If it is not possible to focus the camera on external lighting, a less contrasting (but more static and fairly well-lit) focus point can be selected on the model’s legs or leaves on the ground at the same distance as the model.

However, the above choice is complicated by the fact that it often needs to be made within a fraction of a second. Additional specific autofocus techniques for both still and moving subjects will be covered in appropriate sections towards the end of this chapter.

Number and type of autofocus points

Autofocus stability and flexibility are primarily a result of the number, position, and type of autofocus points that are available in a given camera model. High-end DSLR cameras have 45 autofocus points or more, while other cameras may even have just one center point. Two examples of AF sensor locations are shown below:

The examples on the left and right are Canon 1D MkII and Canon 50D/500D cameras, respectively.
For these cameras, autofocus is not possible at apertures smaller than f/8.0 and f/5.6.

Note: the sensor is called “vertical” only because it detects contrast
along vertical line. The irony is that such a sensor, as a consequence,
detects horizontal lines best.

For DSLR cameras, the number and accuracy of autofocus points can also vary depending on the maximum aperture of the lens used, as shown above. This important fact When choosing a lens: Even if you don't plan to use the lens's maximum aperture, it can still help the camera achieve higher autofocus accuracy. Further, since the center AF sensor is almost always the most accurate, for off-center subjects it is often best to use that sensor to achieve focus first (before recomposing).

Multiple AF sensors can operate simultaneously for increased reliability or individually for increased flexibility, depending on the camera settings selected. Some cameras also have "AutoGRIP", an option for group photos that ensures all points in the focus cluster fall into an acceptable degree of focus.

AF modes: Continuous (AI SERVO) or One-shot (ONE SHOT)

The camera's most widely supported focus mode is single focus, which is best suited for static images. This mode is prone to focus errors for fast-moving subjects because it is not designed for movement, and may make it difficult for the viewfinder to track moving subjects. Single focus requires achieving focus before the picture can be taken.

Many cameras also support an autofocus mode that continuously adapts the focusing distance for moving subjects. Canon cameras call this mode "AI Servo" and Nikon cameras call it "Continuous" focusing. The tracking mode operates on the basis of an assumption about the location of the object at the next moment in time based on the calculation of the speed of the object based on the data of previous focusing. The camera then focuses at the predicted distance with an advance to account for shutter speed (the delay between pressing the shutter and the start of the exposure). This significantly increases the likelihood of correctly focusing on moving objects.

Examples maximum speeds tracking are shown for various Canon cameras below:

The values ​​are valid for ideal contrast and illumination when using the lens
Canon 300mm f/2.8 IS L.

The above graph can be used to approximate the capabilities of other cameras. Actual tracking speed limits also depend on how uneven the subject's movement is, the contrast and brightness of the subject, the type of lens, and the number of AF sensors used for tracking. Also keep in mind that using focus tracking can significantly reduce your camera's battery life, so only use it when necessary.

AF assist beam

Many cameras come with an AF assist beam, either visible or infrared, which is used in the active autofocus method. This can be very useful in situations where the subject is not well lit or contrasted enough to autofocus, although using the assist beam also has its drawbacks since autofocus is much slower in this case.

Most compact cameras use a built-in infrared light source to operate AF, while DSLR cameras often use a built-in or external flash to illuminate the subject. When using the assist flash, autofocus may be difficult to achieve if the subject moves noticeably between flashes. Therefore, the use of auxiliary lighting is recommended only for stationary objects.

In Practice: Capturing Motion

Autofocus will almost always work best when shooting motion in AI servo or continuous mode. Focusing performance can be greatly improved if the lens does not have to search over a large range of focusing distances.

Perhaps the most universal way to achieve this is pre-focus the camera on the area where you expect a moving object to appear. In the cyclist example, prefocus might be on the side of the road, since the cyclist is likely to appear nearby.

Some lenses for SLR cameras have a minimum focusing distance switch; setting it to the maximum possible distance (which the subject will never be closer to) will also increase efficiency.

Note, however, that in continuous autofocus mode, pictures can be taken even if precise focusing has not yet been achieved.

In practice: portraits and other static shots

Static shots are best taken in single focus mode, which ensures that precise focus has been achieved before exposure begins. The usual requirements for the focus point regarding contrast and illumination apply here, but a slight mobility of the subject is also required.

For portraits sweet spot focusing is the eye because it is standard and because it provides good contrast. Although the center AF sensor is typically the most sensitive, the most accurate focusing for off-center subjects is achieved by using off-center focus points. If you use the center focus point to lock focus (and then change the composition), the focusing distance will always be slightly less than the actual focusing distance, and this error increases as the subject gets closer. Accurate focusing is especially important for portraits, since they typically have a shallow depth of field.

Since most commonly used autofocus sensors are vertical, it may be appropriate to worry about whether vertical or horizontal contrast is dominant at the focus point. In low light conditions, autofocus can sometimes be achieved only by rotating the camera 90° while focusing.

In the example on the left, the steps consist primarily of horizontal lines. If you focus on the farthest front step (in hopes of obtaining a hyperfocal distance), in order to avoid autofocus failure, you can orient the camera to the landscape position while focusing. After focusing, you can optionally rotate the camera to portrait position.

Note that this chapter discusses, How focus instead of on what focus. For further information on this subject, see the chapters on depth of field and hyperfocal distance.

We live in an age of speed and high technology, when everyone is in a hurry and wants to have everything at hand. Today we will talk about smartphone cameras that are capable of capturing the right shot at the right moment. And, since we all want sharp photos, there are a few things you need to know about your camera's hardware. Over the past few years, many mobile device manufacturers have been trying to improve autofocus technology, and it deserves our close attention. Let's look at what types of automatic focusing exist, as well as what advantages and disadvantages each of them has.

If we briefly discuss what the main difference between focus and autofocus is, then everything is simple. IN in this case We are talking about when the objective lens is focused on a specific object, through the refraction of rays, due to which the light is collected at one point. When everything matches, the matrix sensor is at the right point, the frame is detailed and of high quality. When a photographer focuses on the main subject by manually adjusting the lens, the photo emphasizes the foreground or background while the rest is blurred. This is the process of focusing. Today this process is much easier, since automation can do everything for us. Autofocus lets you capture sharp, detailed photos with just a point and click. And, since almost all modern smartphones are equipped with cameras with automatic focusing, it is worth considering what types it comes in.

Phase detection autofocus

This technology is based on splitting the light beam that passes through the lens into two streams, after which the light hits the photosensitive sensor. In this case, the distance between the flows that pass through the opposite edges of the lens is measured. The aiming is considered final if the separated beams reach a certain distance specified by the sensors. The device itself can essentially determine how to change the position of the lenses in order to get the picture of the required quality. Irrefutable dignity phase detection autofocus Focusing speed and accuracy is considered. This feature is very important when shooting a moving scene. It is also worth noting that this technology works faster than contrast autofocus, which you will read about below.

However, phase-detection autofocus has some disadvantages, one of which can be considered the complexity of implementation. In order for this technology to work, ultra-precise physical alignment is required, as well as meticulous digital tuning. For a good implementation of phase autofocus, good hardware is required, which not all smartphones have. In addition, the accuracy of phase detection autofocus directly depends on the lens aperture, so in low light conditions this technology will not produce the desired result.

Contrast autofocus

The operation of this technology is based on the use of special photosensitive elements that evaluate the contrast of the frame. Focusing in this case is considered accurate when the picture acquires maximum accuracy and contrast compared to the background. This solution is used in the vast majority of smartphones mainly due to the comparative simplicity in the implementation of the technology. A special sensor measures the amount of light on the lens, after which the same sensor must move the lens until maximum contrast is achieved. When maximum contrast is achieved, the subject being photographed is in focus. Let us once again note the ease of use of this technology, which does not require complex hardware.

Now let's add a fly in the ointment by noting some of the shortcomings that are inherent in contrast autofocus technology. Let’s say right away that this solution works somewhat slower than other technologies. Contrast autofocus thinks somewhere within a second, during which it focuses on the subject being photographed. If you are a slow person and are not in a hurry, then, in principle, the time spent on focusing will not strain or irritate you. Especially if the subject being photographed is also in no hurry, a snail, for example. But, if you move at super speed, like the superhero Flash, then a second will stretch for an eternity for you. If you wanted to capture a hummingbird with its super metabolism, then it might just fly away during this time. The speed in this technology suffers mainly due to the fact that contrast evaluation occurs in several stages, which takes some time. In addition, contrast autofocus is deprived of such a feature as tracking focusing; in twilight or in poor lighting, the quality of photographs is unlikely to satisfy anyone. Note that contrast autofocus technology is usually used in budget-level smartphones.

Laser autofocus

This technology works by applying the principle of a laser rangefinder, when the function of the laser emitter is to illuminate the object being photographed, while the sensor measures the distance to the object and records the time during which the reflected laser beam arrives. The killer feature of this technology can be considered the time spent on focusing. In particular, laser autofocus can cope with this task in 0.276 seconds. Of course, you already realized that phase and contrast autofocus are “smoking nervously on the sidelines.”

Laser autofocus is lightning fast and performs well in low light conditions. However, when working with this solution, one detail should be taken into account - the most good result can be achieved only when the distance to the object being photographed is within 0.6 meters. And, if the distance to the object exceeds 5 meters, then laser autofocus is powerless in this case. In this case, you only benefit from contrast autofocus.

If we debrief, we note that when choosing a smartphone in general, as well as its photo capabilities in particular, everyone is guided by their own considerations and preferences. The budget that is expected to be spent plays an important role in the choice. Moreover, if you are a fan of high-quality photographs, then the camera in your smartphone will not satisfy you in any case, in which case you just need to buy a DSLR.

Article text updated: 12/13/2018

Many modern DSLR cameras have such advanced autofocus systems that it can often be difficult to understand how to use them. Whether we're shooting with an entry-level camera or a professional camera, to ensure sharp photos, we need to try to figure out how to use the different autofocus modes. Incorrect focusing and a blurred image can destroy the positive impression of the photo, and it is impossible to correct this defect during subsequent processing in a graphics editor. Some photographers convert their photos to black and white to hide focus issues. If we learn how to focus correctly, we won't need to resort to these kinds of tricks, we will get a much better result that our viewers will love. A clear image is what people want to see when they look at our photos today. Someone might argue that sometimes a blurry picture looks “creative,” but here we need to understand: it’s one thing when we blur a photo for a specific purpose, and another thing when we ruin a photo because we don’t understand the workings of our camera’s focusing system. Once we understand how DSLR autofocus works, we can decide for ourselves when and to what extent the image will be out of focus

Photo 1. Lessons for beginners. To get a high-quality photograph, you need not only to choose the right shutter speed and autofocus mode, but also to be able to quickly rotate the zoom... Nikon D610 camera. Nikkor 70-300 telephoto lens. Settings: ISO 1000, FR-98mm, f/5.0, B=1/2500 second

In today's free photography tutorial, we'll cover the basics of autofocus modes on DSLR cameras. Since the operation of automatic focus directly depends on what type of camera and its model we use, we, of course, will not describe in detail absolutely all AF modes, but will look at a couple of examples for clarity. Since I myself now have a full-frame Nikon D610 camera, and previously had a cropped Nikon D5100 camera, more emphasis will be placed on the performance of DSLRs from this manufacturer. Well, I apologize to completely beginner amateur photographers for the fact that the photo lesson will use specific vocabulary that is more understandable for advanced photographers.

1. How the autofocus system of SLR cameras works

One of the nice differences between modern cameras and their film counterparts produced fifteen years ago is that now we don’t have to manually adjust the focus. Digital photography is much more friendly to the amateur photographer in this aspect, because, unlike film photography, we immediately see the result and can easily change the settings and retake the photo without thinking about the cost of film and photo paper. Over the past ten years, autofocus systems have become much better and even entry-level DSLRs can boast good complex automatic focusing. Well, how does such a system work in modern SLR cameras? Let's start with the very basics.

1.1 Active vs passive autofocus

There are two types of autofocus (AF) systems: active and passive. Active AF works by sending an infrared beam to our subject and capturing its reflection (the “sounder” principle). The camera makes calculations and understands how far the object is from it, and sends a signal to the lens how much to adjust the focus. A nice benefit of the active focusing system is that it can work in very low light conditions where normal (passive) autofocus would fail. The disadvantage of “Active AF” is that this mode can only be used in stationary conditions, for shooting stationary subjects, and only works at short distances: up to 5-6 meters. If we take photographs using a Nikon or Canon flash that has an AF Assist function, it will work in active autofocus mode.

The “Passive AF” system is based on a completely different principle: it does not send an IR beam and does not pick up its reflection to understand the distance between the camera and the subject in focus. Instead, special sensors inside the camera are used to contrast definitions parts of the light passing through the lens (called the “phase method”), or the camera matrix itself serves as such a sensor that determines the contrast of the image (called the “contrast method”).

What is meant by “definition of contrast”? Without going into the jungle of terminology, this is a determination of sharpness in a certain area of ​​the image. If it is not sharp, then the autofocus system adjusts the lens until sharpness/contrast is achieved.

This is why the passive autofocus system needs enough contrast in the frame to function properly. When the lens begins to "hunt" over a uniform surface (such as a white wall or some surfaces with a smooth transition of tones), this is because the camera needs objects with edges (contrast) separated from the background in order to understand how adjust focus.

By the way, if there is an AF-assist illuminator lamp on the front panel of our DSLR, this does not mean that the camera is operating in active focusing mode: all the lamp does is illuminate our subject, like a flashlight, i.e. The camera operates in “Passive A.F.”.

Many digital cameras, such as point-and-shoot cameras, camcorders and the like, most often use "contrast AF method" to achieve focus. At the same time, most modern DSLRs can be equipped with both systems for focusing correction: phase detection and contrast detection autofocus.

Since the “contrast method” requires light to hit the sensor, a DSLR camera must have its mirror in the raised position when determining focus, which means that contrast autofocus in a DSLR can only be performed in “Live View” mode.

The phase method is great for focusing on moving objects, and the contrast method is great for focusing on stationary objects. Contrast-contrast autofocus often performs better than phase-detection AF, especially in low-light conditions. The advantage of contrast focusing is that to adjust sharpness it is enough to use any part of the image (including that which is at the very edge) on the matrix, while phase focusing requires the use of one or more focusing points of the DSLR. The disadvantage of the contrast method today is that it is relatively slow.

Many professionals are confident that camera manufacturers will be able to solve this problem in the foreseeable future, since autofocus speed when shooting video is becoming increasingly important for DSLRs and some mirrorless cameras (in particular, the Micro Four Thirds standard, 4/3) are already equipped with fast contrast AF. Modern high-end mirrorless cameras have two autofocus systems: fast phase detection for working in good light and slow contrast for low light conditions. Some manufacturers, in general, managed to integrate phase sensor pixels directly into the camera matrix, which, in comparison with the traditional phase autofocus system of DSLRs, greatly increased the accuracy of the system.

If everything described above sounds confusing, don't get too upset: the technical information presented above is intended to provide a general understanding of how autofocus works in a camera. We just need to remember that focusing errors in a camera happen due to the lack of light passing through the lens and the type of focusing mode we have chosen (as explained below).

1.2 Focus points

Focus points are small empty rectangles or circles that we can find in the viewfinder of our camera. Manufacturers often differentiate between amateur and professional level cameras by integrating various systems automatic focusing. Entry-level DSLRs usually have minimal amount focus points, allowing you to achieve focus, and advanced SLR cameras are equipped with a comprehensive, highly configurable AF system with big amount focusing points. They are part of a "phase AF method" so that each point can be used by the camera's AF sensor to determine contrast.

Focus points are deliberately located in a certain part of the frame, and their number differs not only between different manufacturers, but also among different camera models. Here is an example of two different types of autofocus with a different number of focus points and their locations.

As you can see, the Nikon D5100 DSLR has 11 points, while the Nikon D810 has 51 points - a big difference in the number of sensors. Does the number of focus points matter? Absolutely - yes! And the point is not only that it is easier for us to compose a specific shot, focusing on a specific area of ​​the image, but also because the AF system can more effectively track the subject in the frame (extremely convenient when shooting sports competitions and wild animals). Although, we must keep in mind that not only the number of focus points in our camera matters, but also their type.

1.3 Types of points in the AF system of DSLRs

let `s talk about various types auto focus points in DSLRs. As stated above, the number of points is not the only important parameter of an autofocus system. The type of points is also critical to achieving accuracy. There are three types of focus points: vertical, horizontal And crusades. Vertical and horizontal work in the same direction, i.e. These are linear sensors. Cross dots measure contrast in two directions, making them more accurate to work with. Therefore, the more cross sensors in our DSLR, the more accurately the AF system works.

That’s why, when the release of a new SLR camera model is announced, in the review we can read something like: “The number of focusing points is X, of which Y is a cross type.” The manufacturer proudly emphasizes the number of dots, especially the presence of cross dots, if there are more of them in the new camera. For example, in the list of the main differences between the Nikon D7200 and Nikon D7100 from the earlier model Nikon D7000, it was indicated that they have 51 focusing points, including 15 cross-type ones, while the old lady has 39 points, 9 cross-type ones.

When we buy a new SLR camera that we plan to use for shooting sporting events or for photo hunting, we need to pay close attention for both of these parameters.

1.4 Other factors affecting the performance of a camera's autofocus system

As we can see, both the number of focus points and their type are important. Although, they are not the only ones that affect the operation of automatic focusing. The quality and quantity of light is another parameter that greatly determines the performance of autofocus. Every photographer has probably noticed that the camera focuses perfectly when shooting on a bright sunny day outside, but when we go into a dimly lit room, the lens begins to “hunt.” Why is this happening? Because, in low light conditions of the subject, it is much more difficult for the camera to measure the contrast differences in the scene. Remember that passive autofocus is entirely dependent on the light passing through the lens, and if the lighting quality is poor, then autofocus will not work satisfactorily.

Speaking about the quality of light, we must not forget about the features of the lens, and the fact that the maximum open aperture also has an impact on AF. If we shoot with old glass that has mold, dirt, too much dust, or has problems with front and back focus, then automatic focusing, of course, will not work very accurately.

That's why professional lenses at f/2.8 aperture allows you to focus much faster than amateur lenses at f/5.6. The f/2.8 aperture is the most suitable for high-speed focusing: the aperture is not too wide, not too narrow. By the way, lenses at aperture 1.4 usually focus more slowly than at f/2.8, since more rotations of the glass elements inside the structure are required to properly focus .

Focusing accuracy is important at such open apertures because the depth of field is very narrow. Ideally, the aperture should be between f/2.0 and f/2.8 for the autofocus system to function best.

Smaller apertures, such as f/5.6, will result in less light passing through the lens and making it harder for the autofocus system to work. For this reason, open apertures (with the exception of f/1.4) are preferable to closed ones.

I must also add that all are modern digital cameras focus when the aperture is open, so that no matter what aperture number we choose (for example, f/22), the aperture only changes at the moment of shooting .

Finally, the overall quality and durability of the autofocus system is of the utmost importance. For example, the top-end professional DSLR Canon 1D Mark III, designed for shooting sports competitions and photo hunting, after its release into series, spoiled its reputation due to problems with autofocus. And it took forever for Kenon to release firmware to fix these shortcomings that annoyed professional photographers. Many of them switched to Nikon cameras precisely because of focusing problems. The camera was equipped with all autofocus modes, but it did not work correctly under certain conditions.

If we want to get better system For automatic focusing in modern DSLR cameras, especially for shooting sports and wildlife, you should choose from Nikon or Canon (although other manufacturers are quickly catching up with the market leaders).

2. Auto focus modes of digital SLRs

These days, most DSLR cameras have the ability to shoot in a variety of focus-off modes depending on the specific situation.

It’s one thing when we photograph a portrait of a calmly sitting person, and another when we photograph a running athlete or a flying hawk. When we shoot a stationary subject, we focus once and take a photo. But if the subject is in continuous motion, we need the camera to automatically adjust the focus at the moment we take the picture. The good news is that our camera has a built-in function for efficient work in such situation. Let's look at each of the focusing modes in more detail.

2.1 Single focus tracking mode

Single-shot tracking focusing in Nikon cameras is designated “AF-S”; in Canon cameras this type is called “One-shot AF”. And it provides a simple way to directly focus the lens. We select a focusing point, and the camera measures the contrast simply one point at a time.

If we press the shutter button or the assigned AF button (if such assignment is possible in our model) halfway, the camera is focused, but if the subject moves, the focus does not reset, even if we continue to hold the shutter button halfway. That is, the focus remains “locked”.

Typically, in single-servo AF mode, in order for the shutter release to fire, the camera must first be in focus. Therefore, if it was not possible to focus, or the subject has moved, pressing the shutter will do nothing (due to a focus error). In some camera models it is possible to change the camera’s reaction to lack of focus (for example, on the Nikon D810 we can set the “AF-S Priority Selection” setting in the “Release” custom settings menu, which will allow us to take a photo even if the camera does not focus) .

There are some things to keep in mind about AF-S mode: If we have installed an external flash that has a red AF-assist beam, it will require setting the camera to AF-S mode for it to work. The same is true for the autofocus assist lamp built into the front panel of the camera: it only works in AF-S mode.

2.2 AI Servo Focus Mode

Another focusing method available in modern DSLR cameras is called "continuous-servo AF or AF-C" by Nikon and "AI Servo AF" by Canon. It is used to track moving subjects, and is absolutely necessary when photographing sports, wild animals and other non-stationary objects. The operating principle of this mode is based on analyzing the movements of objects and predicting where it will be in the next moment, and setting the focus to this point.

The advantage of this mode is that focus automatically adjusts if the photographer or subject moves. All you need to do is continue to hold the shutter button (or the key assigned to AF, if there is an assignment option) half-pressed. The autofocus system will automatically track the subject. Compared to AF-S single-servo focus, AF-C continuous focus mode generally has a large number of settings (especially in the most expensive DSLRs) and can perform complex tasks such as tracking objects along one or more focus points.

2.3 Hybrid single-shot and focus-servo mode

Some cameras also have another mode called Auto Servo AF “AF-A” on Nikon cameras or “AI Focus AF” on Canon cameras. It is a kind of hybrid that automatically switches between single-frame and continuous focusing. If the camera detects that the subject is stationary, it switches to AF-S mode, and if the subject is moving, it switches to AF-C.

Cheap DSLRs have AF-A enabled by default and work fairly well in many situations. Many professional cameras do not have an Auto Tracking AF mode because it was designed for beginners.

2.4 Continuous focus tracking

The constant focus tracking mode, designated by Nikon as “AF-F”, was introduced by the company for the new Nikon D3100 and D7000 models. It is intended primarily for shooting in Live View format. In this mode, the camera tracks the subject and automatically adjusts the focus while shooting video. Although the name sounds nice, in real life this mode does not work very well when shooting fast moving objects. Nikon Corporation engineers still have a lot of work to do to perfect the “AF-F” mode. If you do not shoot video with a DSLR, then you should not enable this mode.

Many professional photographers in photography lessons for beginners note that most of the time they have the AF-C continuous focus tracking mode turned on and only when the camera cannot focus in low light conditions do they switch to AF-S.

2.5 Changing focus modes

If you do not know how to change the auto focus mode on your camera, it is better to read the instructions for it, as different models it happens in different ways. For example, for entry-level cameras Nikon D5300 or Nikon D5200 you need to press the “Info” button and select the focus mode with the joystick. And expensive DSLRs have a special button on the front panel, with which you can quickly switch between different modes. Here, for example, is how to change the AF mode on a Nikon D610 camera: press the AF mode button and simultaneously turn the control wheel.

The letter “C” appears on the auxiliary screen, which means the camera is operating in AF-C continuous focus tracking mode, switch to “S” - single-frame focus is activated. Pressed “M” - switched to manual control of camera focusing.

3. Auto Focus Area Modes

To confuse novice photographers even more, many DSLR cameras have menu items called something like “AF Area Mode”, which allow the amateur photographer to select several options for how focusing will work in AF-S, AF-C, AF- modes. A and AF-F.

For entry-level DSLRs, such as Nikon D3100 or Nikon D5200, settings can be changed through the menu, and for advanced cameras, such as Nikon D300s, Nikon D700, Nikon D3s or Nikon D3x, they are changed with a special selector on the rear panel (for DSLR cameras Nikon D810 and Nikon D4S cannot reassign control of this parameter to other buttons). Let's see what choosing an auto focus area gives us.

3.1 Single point focus area

When we select “Single Point AF” mode in a Nikon camera or “Manual AF Point” in a Canon camera, we use only one focusing point through the viewfinder to achieve focus. That is, when we switch from one point to another with the joystick, the camera measures the contrast only in this particular area of ​​​​the image, using vertical or cross sensors (depending on which one we chose). Many professional photographers recommend using single-point focus mode when shooting landscapes, architecture, and other stationary subjects.

3.2 Dynamic Focus Area Mode

In the “Dynamic AF” mode for Nikon or “AF Point Expansion” for Canon cameras, we select one focus point, and the camera first adjusts the focus according to it. Next, once the focus is set, if the subject moves, the camera uses surrounding dots to track it and keeps focus on the subject. We expect it to follow the subject's movement and keep it in focus while keeping the camera close to the initially selected focus point. If the camera selects surrounding/other points - this will not be visible in the viewfinder, but will be noticeable in the finished photo.

Dynamic AF mode works great when shooting fast-moving subjects like birds, as we have a hard time keeping the bird in focus while it's flying. Advanced DSLRs, for example, Nikon D7100, Nikon D7200 or Nikon D800, allow you to choose the number of points surrounding the main one: 9, 21 or 51 pieces.

Thus, when we want to monitor a small area in the frame, we select 9 points, and if we need to track movement across the entire field of the frame, we assign 51 points.

IN Lately Many Nikon DSLR models also have a “3D tracking” mode - when we assign a point, and the camera then decides how many auxiliary ones it needs to track the change in the position of the object in the frame. The advantage of 3D tracking mode is that the camera uses its built-in pattern recognition system to automatically read colors and follow the subject itself, and you simply compose the photo as the subject moves.

For example, we photograph a white heron walking among black birds. System 3 DTracking will automatically focus on the white bird and follow it even if the bird moves or the camera moves, allowing us to compose the shot .

If we compare the “Dynamic AF” and “3D Tracking” modes, then in the first case a certain number of points will be used, and in the second, all available ones will be used to track the subject. At the same time, “Dynamic AF” uses certain “zones”, activating only the surrounding focus points (as many as we selected in the settings). For example, we selected 9 points, tracking will work as long as the object is in the area of ​​9 focus points surrounding the main one. If the subject leaves this area, the camera will not be able to focus. But in 3D tracking mode, the camera will continue to monitor the object (the newly selected points will appear in the viewfinder), even if it moves significantly away from the initially selected point.

Professionals use the dynamic autofocus mode when photographing birds and wild animals, using a small number of points: 9 or 21 pieces. There are different opinions about 3D tracking, since it is not as fast as with, for example, 9 points of dynamic AF.

3.3 Automatic focus area selection mode

On Nikon cameras it is designated as “Automatic AF Point Selection”, on Canon it is “Automatic AF Point Selection” and is a “point and shoot” focusing method. The camera automatically chooses what to focus on. This is a complex system that can recognize the color of a person’s skin in a frame and automatically focus on it. If there are several people in the frame, the focus will be on the one closest to the camera. If there are no people in the frame, then, usually, the camera focuses on a near or distant object. If we have selected AF-S and Auto-area AF modes, the active focus point will be displayed in the viewfinder for a second, allowing us to confirm the area on which the camera has focused.

The same is possible with Canon cameras, but their mode is called “Automatic AF point selection in One-Shot AF mode”. It's hard to say why this mode is needed, since professionals like to control all the shooting parameters, rather than letting the camera do it for them.

3.4 Group focus mode

The latest models of Nikon SLR cameras, such as the Nikon D810 and Nikon D4S, have a new “Group AF” focus area selection mode. Unlike "Single-point AF", not one, but five focusing points are used to track subjects. This mode is better suited for setting the starting point of focusing and tracking subjects compared to Single Point AF or Dynamic AF, especially when it comes to photographing small birds that constantly flit from branch to branch and can be difficult to catch. focus and follow them. In such cases, “Group AF” can greatly help the photographer and produce better results than “Dynamic AF”, since it is more accurate and provides consistency from shot to shot.

How does Group Focus Area mode work? We see 4 focusing points in the viewfinder, the fifth, in the center, is hidden. We can move the group by pressing the joystick on the back of the camera (ideally we want to stay in the center, because the focus point in the middle of the frame is a cross, which is more accurate). Once we've locked onto a subject, all five points are activated simultaneously to initially focus on the closest subject.

This is in contrast to “Dynamic AF” with 9 points, which has priority according to the selected central point. If it was not possible to focus on the central one (low contrast), the camera will try the other 8 pieces. Initially, the camera always focuses on the central point, and only then moves to the other 8 pieces.

In turn, “Group AF” uses all 5 points simultaneously and tries to focus on the nearest subject, without giving any advantage to any of the 5 points.

Group AF mode is especially useful for shooting birds, wildlife and non-team sports. In the example above of the cyclists, if our goal is to focus on the athlete in front, Group AF would be a better option because this mode will allow the camera to follow the athlete closest to it.

Another good example: the bird sits slightly above the photographer so that the background behind it is almost invisible. In Dynamic AF mode, no matter where you aim, the camera will first try to catch focus. If we point the lens directly at the bird, the camera will focus on it. If we accidentally aim at the background, the camera will focus on it.

Therefore, photographing small birds can be slightly difficult, especially in bushes, or if the branches they are sitting on are constantly swaying. Choosing the starting point of focus is very important, and the sooner we choose it, the higher the chances of catching the bird in focus and tracking it, especially if it suddenly decides to fly away. As stated above, in the “Group AF” mode, there are no advantages to any single focus point; all 5 are activated simultaneously. In this case, since the bird is sitting closer than the background, once the group of 5 points is close to it, the camera will always focus on the bird and not the background. Once we have selected focus, the camera in Group AF mode will follow the subject, but again only if one of the 5 points is close to the subject. If the subject is moving quickly and we do not have time to turn the camera in the same direction, focus will be lost, just as would happen in the 9-point Dynamic AF mode.

Some photographers say that Group AF mode allows you to catch focus quite quickly, but no one has really measured whether it is faster than 9-point dynamic focusing. Perhaps the latter will be faster in some situations.

Another important fact to note is that when we enable Group AF mode while single focusing A.F.— S, the camera turns on the face detection function and tries to focus on the eyes of the person closest to it, who stands out from the group. For example, if we photograph someone standing between tree branches and foliage, the camera will always try to focus on the subject's face instead of focusing on the leaves. .

Unfortunately, face detection is only possible in AF-S mode, so if we're photographing a group of fast-moving athletes and we need the camera to lock focus and follow the subjects' faces (rather than focusing on nearby objects), we're better off using the Dynamic mode. AF" for Nikon or "AF Point Expansion" for Canon cameras.

Here is a schematic comparison of each of the autofocus modes for Nikon cameras.

When viewing images clockwise: Single-point AF mode, Automatic AF area selection (9, 21 and 51), 3D tracking and Group AF.

3.5 Other focus area selection modes

The latest DSLR models have new area selection modes, for example: “Face-priority AF”, “Wide-area AF”, “Normal-area AF” and “Subject-tracking AF”. These modes are used when shooting video with a DSLR camera. Most likely, these functions will be built into the entire line of Nikon DSLRs capable of shooting video. We will not discuss these modes in detail, since their functioning is slightly different in different cameras and may be changed in the future.

Canon also has its own autofocus area selection modes, for example, “Spot AF”, in which we can fine-tune the focus within the focus point. This mode is highly specialized; it can be found, for example, in Canon EOS 7D cameras.

3.6 In what cases to choose one or another type of automatic focus

Why do we need to know how and when to use the different AF area selection modes? Because each of them can be combined with a focus mode! To understand this better, let's make a table with examples (for Nikon DSLR cameras).

AF area selection mode	Nikon Focus Modes
Single-point AF	The camera is focused only once and only at the selected focus point.	The camera is focused on one selected point, and when the object moves, the focus is re-adjusted.	The DSLR detects whether the subject is moving or stationary and automatically decides which mode to use: AF-S or AF-C. In any case, only one point applies.
Dynamic AF	Disabled, just works like single point autofocus.	We select a starting focus point and, once the camera is aimed at the subject, surrounding dots are turned on to track its movement. In the camera menu you can select the number of auxiliary points.	As in the previous case, but for a group of points.
	As in the previous case	Instead of using a specific number of focus points, all possible ones are used and color recognition is used to track the subject. The photographer specifies a starting point, and the camera tracks the subject across the frame automatically, allowing him to recompose the shot without losing focus on the subject.	Similar to the previous one
	The camera activates 5 focusing points and aims at the nearest object. If it determines that there is a person in the frame, it will focus on him.	The camera automatically focuses on the nearest subject and follows him in the frame as long as he is close to 5 points. Face recognition doesn't work.	Not available.
Automatic AF area selection	The camera itself selects a point depending on what is in the frame.	The camera itself sets a point on a moving object and follows it.	Similar to previous cases.

Note to the table explaining the focus area selection modes above: in various models Some options may be missing.

3.7 Changing focus area selection modes

To understand how to change the focus area selection mode specifically for your camera, it is better to read the instructions. For entry-level DSLRs such as Nikon D3100 or Nikon D3300, you need to enter the “Shooting Mode Menu” section, and advanced cameras have a switch on the rear panel. Here, for example, is what the auxiliary display looks like on the Nikon D600 and D610 SLR cameras.

Press the AF button at the base of the mount, and without releasing it, rotate the front and rear control wheels.

4. Autofocus scenarios and examples

Well, we've learned a lot of technical information about what each of the auto focus and AF-area modes are. Let's go through some more scenarios and examples to ensure we have a good understanding and grasp of the data presented earlier. The camera settings described below are taken for Nikon cameras.

4.1 Scenario No. 1 – Filming sports competitions on the street

Which autofocus mode and type of AF area metering will we choose when photographing, for example, football? Let's start by choosing the right focus mode. Obviously, the AF-S single-servo focusing mode will not work, since we need the camera to focus constantly as long as the shutter button is half-pressed (or the button that we assigned to AF). Therefore we must use either AF-C or AF-A mode. Professionals want complete control of the shooting process, so they switch to AF-C continuous-servo autofocus mode in this situation.

What about AF area selection? Should we enable Single Point AF, Dynamic AF, Group AF or 3D Tracking? Professional photographers, when photographing sporting events such as football, basketball or outdoor hockey, will include 3D tracking, allowing the camera to follow the athletes while a person composes the shot. If it suddenly turns out that 3D tracking does not work correctly and makes mistakes often, then you can switch to “Dynamic AF” with a sufficiently large number of focus points, especially if we are standing close to the scene of action. The Group AF mode will only work well if we are standing very close to our subjects. Here is a set of focus mode settings for the described cases:

1. Auto Focus Method:AF-C
2. AF area metering mode: 3D Tracking, Dynamic or Group AF
3. Custom Settings => Dynamic AF: 21 or 51 points
4. Custom Settings => AF-C Priority Selection: Focus priority

4.2 Scenario No. 2 – Shooting people on the street

When we're photographing people posing outdoors on a sunny day, either focus mode should work well. If we chose AF-S, the camera will focus once as soon as we half-press the Shutter, so we just need to be sure that our subject doesn't move after focusing. By default, the camera will not allow you to take a photo in AF-S single-servo focusing mode if the focus is not achieved.

If we shoot in AF-C continuous focus tracking mode, we just have to make sure that the focus is set correctly before pressing the button. Also, AF-A is good for shooting portraits.

As for choosing the AF metering area, it is more convenient to shoot with “Single-point AF”, since the subject is motionless.

1. Modeautofocus: AF-S, AF-C or AF-A
2. AF metering area: single point
3. Custom Settings => AF-S Priority Selection: Focus priority
4. Custom Settings => AF-C Priority Selection: release priority

It probably goes without saying that we should always focus on the closest eye of our model, especially if she is close to us.

4.3 Scenario #3 – Taking portraits indoors

Shooting people inside a building in poor lighting can be a bit difficult. If the room is dark, we can switch to AF-S single-servo focus mode and let the assist illuminator assist us if needed. If we have an external flash, AF-S mode will allow us to turn on the red beam to adjust the focus.

You cannot use this function in AF-C mode. AF-A autofocus should also handle this situation, but professional photographers will prefer to turn on AF-S.

As for AF area metering, it is more convenient to use the central focus point for greater accuracy in low-light conditions.

1. Autofocus mode: AF-S
2. Metering: Single point AF
3. Custom Settings => AF-S Priority Selection: Focus priority

4.4 Scenario No. 4 – photographing birds in flight

Bird photography is an extremely difficult genre of photography because it is difficult for us to predict their behavior and they often fly very quickly. As noted above, when photographing, it is better to select the “Continuous-servo AF” (AF-C) mode, and the focus area - either “Group AF” or “Dynamic AF” with 9 or 21 points (I would like to photograph with 21 points , but usually 9 pieces are faster). Professional photographers say they've tried using 51 focus points and 3D tracking, but these modes are slower and less accurate than using fewer points.

One of the photographers told me that 99% of the time he focuses on the birds on the central point, changing it only when the birds are sitting high on some branch. Once again: the central focusing point in most cases gives the best result. If we're shooting small birds and don't have time to set the starting focus point, we can try the Group AF mode (if available on your camera).

1. Autofocus mode:AF-C
2. AF area metering: Dynamic or Group AF
3. Custom Settings => Dynamic AF: 9 or 21 points
4. Custom Settings => AF-C Priority Selection: release priority

4.5 Scenario #5 – Shooting landscapes and architecture

All focusing modes are suitable for these types of shooting, but it is still more convenient to use AF-S, since we do not have objects to follow.

In low-light conditions, we won't be able to use the AF-assist illuminator function because the distances are very long. In this case, you can set the camera on a tripod and switch to Live View to focus on the bright object in our scene using the contrast method. If this does not help, there is only one thing left to do: turn off automatic focus and focus manually.

When shooting landscapes or architectural objects, we need to be more attentive to what our camera is focused on and remember that the need to clearly understand what depth of field (DOF) and hyperfocal distance is is of particular importance.

Regarding metering the autofocus area, one thing can be said: we definitely need the “Single-point AF” mode to focus precisely on a specific point in our frame.

1. Autofocus mode: AF-S
2. Autofocus area selection method: Single point AF
3. Custom Settings => AF-S Priority Selection: Focus priority

4.6 Scenario #6 – Photographing large animals

On safari, when photographing large animals, professionals prefer to use the AF-C continuous focus tracking mode and the Dynamic AF or 3D tracking AF area metering method, both of which work perfectly. Animals are usually not as nimble as birds (although sometimes they can move even faster), so if we are not shooting fast-paced events, it is better to use the Dynamic AF mode with more focus points or use 3D tracking.

1. Auto Focus Mode:AF-C
2. AF area selection: Dynamic Focus or 3D Tracking
3. Custom Settings => Dynamic AF: maximum number of points or 3D
4. Custom Settings => AF-C Priority Selection: release priority

Hopefully, the scenarios listed above will make it easier to understand when and how to choose a particular focus mode and focus area metering. Now it's time to go back to the table above and check if we understood everything well.

4.7 Scenario No. 7 – Photographing small groups

Beginners often ask what mode to focus in when we're shooting a group of several people. Before we talk about autofocus mode, there are some important things to discuss. If we use a lens with a standard focal length or a telephoto lens with an open aperture, we need to remember the distance to the subject. When we stand close to our group and shoot at f/1.4-f/2.8, it can happen that only a couple of people will be in focus, and the rest will be blurred, unless they are standing in the same plane. There are two solutions here: either clamp the aperture to f/5.6 or f/8, or move further away to increase the depth of field. Or you can use both of these tips.

If we want to blur the background and shoot at a large aperture, we can only put everyone in a row, strictly parallel to the camera. Let's imagine how people would need to stand if they pressed the back of their heads against a flat wall - this is how our models should be positioned.

As for the focusing modes, in the daytime they will all work well, but it is more convenient to use single-point focusing.

1. Modesautofocus: AF-S, AF-C or AF-A
2. Measuring method: Single point AF
3. Custom Settings => AF-S Priority Selection: Focus priority
4. Custom Settings => AF-C Priority Selection: release priority

Note: As you can see, in all modes the priority selection for “AF-S” and “AF-C” is set to “focus priority” and “release”, respectively. And that's why. By setting AF-S single-servo focusing mode and “focus priority,” we are telling the camera not to allow a photo to be taken if it cannot achieve focus. Professional photographers don't use AF-S very often, but when they do, they want their shots to be sharp.

For AF-C's continuous-servo autofocus mode, "release priority" works great in most situations: the camera fine-tunes focus but doesn't allow too long shutter lags, allowing the photographer to shoot when they want. For AF-C mode, there is no point in thinking about what priority to set: release or focus. In "release priority" the camera doesn't care whether the focus is good or bad (why do you need autofocus then?), but in "focus priority" it won't let you take a good photo until the focus is locked. If we need the focusing to be so precise, we switch to A.F.— SThen. We just set this parameter as shown in the examples above and forget about them forever .

5. Tips for improving autofocus performance in low light

As noted earlier, in good, sunny shooting conditions, cameras do an excellent job of autofocusing. But when photographers start shooting in low light, they face a lot of problems, especially if they shoot indoors. Here are some tips to make the autofocus system work better when there is not enough light:

1. Using the central focus point. It doesn’t matter whether our camera has 9 or 51 focusing points, we still focus on the central one, and not on the outer ones, if we shoot in poor light, since it works more accurately. There is usually a cross-type sensor in the center that works better than any other point on our camera.

But what then should we do with framing and composition if we have to focus on a central point? The solution seems to be to reassign the autofocus function from the shutter button on the camera to another one located on the back of the camera. Then you can focus on the subject and recompose the frame. Most DSLR cameras, including entry-level ones for beginners, allow you to do this. Professional DSLRs have a button (usually called “AF-On”) that can be enabled through the menu by selecting “AF-ON Only” in the autofocus activation settings. But we have to be careful after we recompose a shot, especially when photographing with a shallow depth of field and an open aperture. When we focus and then move the camera, the focus is likely to shift and we need to be careful to keep our subject sharp.

2. Turn on the AF-assist illuminator function on the camera or on an external flash. Whenever you have to shoot in low light, this feature helps photographers. To activate it, you need to make sure that the AF-assist illuminator is turned on in the menu and the focus mode is set to Single Servicing - AF-S.

3. Selecting contrasting objects and edges. Instead of trying to focus on a flat, monochromatic surface, look for “high contrast” objects that stand out from the background.

4. Add a little light or turn on lamps. It sounds simple, but if we have trouble focusing, what could be easier than adding a little more light or turning on more bulbs in the room? One professional photographer told how he had to photograph dancing at a party. There was so little light that I had to shine a flashlight on the models to focus. Then he approached the organizer and asked to turn on general lighting in the hall - all the problems resolved themselves, and he was able to take excellent photographs.

5. Watch your shutter speed. We may think that we have problems with focusing, but we must not forget that the shutter speed must be sufficient for handheld shooting. The rule for determining exposure time using the formula B=1/(2*FR) is described in detail in a separate photo tutorial on DSLR settings.

6. We use a tripod. By using a tripod, we can achieve more accurate focusing in low light without worrying about camera movement.

7. Let's use the contrast focusing feature in Live View mode. When we have installed the camera on a tripod, we can try to focus in Live View mode, in which, as we remember, we can use a more accurate method of focusing based on the contrast of objects in the frame. Many professional photographers note that whenever they have to photograph on a tripod, they try to use contrast focusing because it gives better results. And, in general, it is more convenient to focus in Live View mode, since the image on the camera screen is larger than in the viewfinder.

8. A useful thing is a bright flashlight. If our camera model does not have a built-in autofocus illuminator, we use a bright flashlight and ask someone to shine it on our subject to try to focus. As soon as sharpness is captured, we switch to manual focus mode and turn off the flashlight, taking photographs “with the self-timer.” I've seen advice from professionals to use a laser pointer to focus when shooting night landscapes (don't forget that if you hit a person or animal in the eye, you can burn the retina).

9. Using manual focus. This advice does not correspond to the title of the article, but we should be able to manually adjust the focus and not be afraid to do it. Sometimes manual focusing will be even faster than in automatic mode. Many landscapes, macro and architectural photographs are shot using manual focusing.

Photo 13. Another landscape shot with manual focusing. HDR of three frames. Nikon D610 camera. Lens - Samyang 14/2.8. Sirui T-2204X tripod.

P.S. Dear friends, colleagues and site guests! If you think that the article can be useful to other photographers, I would be grateful if you share a link to it on social networks, on specialized forums, or publish it on your blog. I just ask you to put an active link to the source! My wife spent the whole day drawing all these frames on the photographs... Her work cannot be in vain. Thank you! Good luck with sharp photos to you.

The first decades of photographic cameras were large and consisted of a simple but bulky accordion-shaped structure connecting the lens and the cassette part to the photographic plate. Before shooting, a ground glass (focusing screen) was inserted in place of the photographic plate, and the photographer manually moved the lens (usually a single lens) to focus the image, covering himself with a dark blanket to increase brightness and contrast. This process was slow, but there was no need to rush: the sensitivity of photographic plates at that time was low, the shutter speed was minutes, so they shot mostly static scenes - landscapes, still lifes and portraits of people who had to sit still for this.

Handmade

By the beginning of the 20th century, the sensitivity of photographic materials increased, the format decreased, cameras became much more compact and convenient, but it became difficult to focus the lens on an image on a small focusing screen even with the help of a magnifying glass. This problem could be solved in several ways. First, focus the lens at the hyperfocal distance, so that most of objects in the frame were depicted sharply. Secondly, mark the distance scale on the lens and focus, setting the required values ​​“by eye”. And thirdly, it was possible to apply a fundamentally new solution by equipping the cameras with a device for measuring distance - a rangefinder. This simple optical device consisted of a beam-splitting prism and a rotating mirror, spaced at a certain distance (base). The photographer, looking through the rangefinder window, turned the mirror until the images were combined. Using triangulation, based on the angle of rotation and base, it was possible to find the distance to the subject and set this distance on the lens (manually). Cameras began to be equipped with such devices from the beginning of the 20th century, and in 1916, in the 3A Autographic Kodak Special model, designers for the first time mechanically combined distance measurement with simultaneous lens focusing. This device gained real popularity thanks to the Leica company, which began equipping its cameras with rangefinders starting with the Leica I model (1925) - in fact, such cameras began to be called rangefinders.

Remove split

In 1976, at Photokina, Leica introduced a camera with the Correfot system (which it had been developing since 1960), the world's first autofocus system. According to one legend, despite public interest, the company refused to release it “because customers already know how to focus the lens correctly.” In fact, the system was simply too power-hungry (a set of six batteries lasted less than an hour of filming) and was generally “crude.” Therefore, the first serial autofocus camera was the Konica C 35 AF in 1977, equipped with Honeywell's Visitronic system. This system was based on a classic rangefinder and triangulation, only the two images were brought together not by the photographer himself, but by electromechanical automation, comparing signals from two CCD matrices.

Canon took a slightly different path, deciding to do without complex electromechanics. The Canon AF35M (1977) introduced active autofocus, which was an optoelectronic version of the classic rangefinder: an LED emitted an infrared pulse, and the distance was determined by the angle of its reflection from the subject, measured using a CCD sensor. The next model, the Canon AF35ML (1981), already used passive autofocus based on "solid-state triangulation": no moving parts, and the images were "converged" electronically- by the difference in signals on two CCD matrices.

In the first rangefinder cameras, the photographer combined images, read the distance and set the resulting value on the lens focusing scale. The 3A Autographic Kodak Special camera combines these procedures into one.

Phase shift

The first autofocus SLR camera was the Minolta Maxxum 7000 (1985). This model used the Through The Lens (TTL) phase detection autofocus (AF) system, which is still widely used today. The principle of its operation is based on the fact that rays passing through the two halves of the lens are reflected by a mirror and focused at two different points on the AF sensor - two CCD arrays. The distance between these points for ideal focusing is precisely known, and if the measured distance between the peaks does not coincide with this value, the control system begins to move the lens in the desired direction until the peaks are in the desired places. In real life, of course, everything is much more complicated - the image is not a point, may not be located on the optical axis, etc. These problems are solved by introducing various masks and additional condenser lenses, but the principle is the same.

Automatic rangefinders and real AF The Konica C35 AF was equipped with an electromechanical rangefinder with two CCD sensors. Signals from the sensors were compared, their coincidence meant accurate focusing.

Phase detection autofocus is very fast (the system immediately knows in which direction to move the lens, and thanks to this it can even track the movement of an object in the frame), does not require much processing power and has no moving parts. The main drawback of this system is its poor performance in low light, and the fact that it only works when the mirror is down: at the moment of shooting, the mirror is raised, and all the light through the lens hits the film or sensor, and not the AF sensor. This means that this system is not suitable for cases where the frame is visualized on an LCD screen (LiveView), that is, for most compact digital cameras and smartphones.

And the first real AF appeared in the Minolta Maxxum 7000 camera. It was a full-fledged phase-detection autofocus system through the lens (TTL) - the ancestor of all modern phase-detection AF systems.

In the image and likeness

For digital cameras, which have replaced film cameras since the early 2000s, a new autofocus principle had to be invented. Well, not exactly new. How does a person aim a lens manually? Rotates the focus ring until the observed picture becomes sharp, that is, with maximum contrast. Contrast autofocus works in exactly the same way: it moves the lens, achieving maximum contrast in the image on the photosensitive matrix.

Such a system works with the main matrix and does not require complex optical circuits or additional sensors. But, unlike phase detection autofocus, it cannot determine in advance which direction the lens should be moved, and begins to do so in a random direction - just like a person would do. Therefore, the focusing speed sometimes leaves much to be desired - especially in low-light conditions or when shooting low-contrast objects, when the system simply cannot “see” sharp details (exactly like a person). However, for a long time, there were no alternatives to contrast autofocus for compact digital cameras and especially smartphones.

The Canon EOS 70D camera was the first model equipped with a Dual Pixel CMOS AF system. Unlike a hybrid AF system, which uses dedicated dedicated photodiodes on a common CMOS sensor, dual-pixel AF uses all of the sensor's photodiodes for both focusing and shooting.

Hybrid approach

In 2010, Fujifilm released the FinePix F300EXR camera with a new hybrid autofocus system. On the camera matrix, in addition to the usual light-sensitive photodiodes (pixels), two types of specialized ones were evenly scattered - “right” and “left”, that is, they perceive light only from the right or left part of the lens (the other part is covered with an opaque mask). The AF system compared the image on the submatrices formed by the “left” and “right” pixels. The exact alignment of these two images indicates accurate focusing, and the offset indicates how much and in which direction the lens should be moved. Sounds like phase AF, doesn't it? Almost, but not quite: the resolution of the submatrices is significantly less than that of the entire matrix, and with very small deviations from precise focusing, the system is unable to see the difference, so at the final stage, contrast focusing is used.

Nothing extra

Hybrid autofocus advantageously combines the advantages of phase and contrast AF systems, but it also has disadvantages. To improve AF performance, you need to increase the number of pixels that “work” only by 50%, and this leads to a decrease in the overall light sensitivity of the matrix. But matrix developers have come up with an ingenious way to get around this limitation.

In 2013, the Dual Pixel CMOS AF system was first tested in the Canon EOS 70D camera. And in 2016, the first smartphone with a camera equipped with a Dual Pixel system appeared on the market - a flagship Samsung Galaxy S7.

There is a way to make “everything sharp” without autofocus at all. In the era of film cameras, cheap models were usually equipped with a simple focus-free lens at the hyperfocal distance. Such a lens allows you to more or less sharply image all objects located at a distance from half the hyperfocal (usually 0.5−1 m) to infinity. Cheap digital cameras and the first smartphones with cameras were equipped with similar lenses. However, this principle only applies to cheap wide-angle lenses with large minimum value diaphragm. Another case is the use of a plenoptic camera, or “light field camera.” It records not only the distribution of illumination in the focal plane, but also the direction of incoming rays (light field). Such an image can later be “refocused” in any desired way (in any plane). The idea for such cameras was put forward in 1908, and a few years ago the Lytro company decided to produce digital versions, although they have not yet received much distribution.

Each pixel of the Dual Pixel matrix consists of two separate photodiodes - “right” and “left”. Thus, during autofocus, the entire matrix is ​​divided into two submatrices, “right” and “left”, with the same resolution as the main matrix. Comparing the signals from the two halves provides accuracy higher than that of hybrids, and speed is much higher than that of contrast AF systems (say, in the Samsung Galaxy S7, focusing time is less than 0.2 s). Since Dual Pixel is a phase detection AF system, it allows you to track the movement of the subject in the frame. And at the time of shooting, both submatrices work as a single unit, there is no drop in photosensitivity, which is important for smartphones with their small matrices. Therefore, such a system today represents the pinnacle of the evolution of AF systems. Of course, until the engineers come up with something new again.

Sonars, radars and lidars

A separate branch on the evolutionary tree of autofocus is occupied by external (relative to the camera’s optical system) rangefinders with direct distance measurement. One of the first cameras with an autofocus system was the Polaroid SX-70 Sonar OneStep (1978), equipped, as its name implies, with a rangefinder based on ultrasonic sonar. Archaic? Not at all, sonar rangefinders for cameras still exist. They are produced, for example, by the company RedRockMicro - however, not for automatic, but for remote manual focusing of professional cameras. A newer principle of determining distance, laser ranging, is now actively used not only in construction and military equipment, but also in some smartphones (LG G3) - in addition to the usual contrast autofocus system. Sony patents mention radar autofocus, but production samples similar type not available on the market.

The editors would like to thank Markus Kohlpayntner for his assistance in preparing the article.

Two sides of the coin

Camera Nikon D5200. The pinpoint accuracy of the 39-point AF system with nine cross-shaped sensors ensures exceptionally sharp images by keeping your chosen subject in sharp focus.

Modern cameras now, as a rule, use the so-called passive principles of autofocus - phase and contrast. By the way, there are also active ones - ultrasonic and infrared, but they are used very little (in cameras with snapshots and for some compacts).

Phase detection autofocus

Phase detection autofocus is traditionally used in SLR cameras (sometimes in compact ones), while contrast autofocus is originally used in non-DSLR models. In both cases, for successful focusing main role plays the contrast of the focal point.

The main function in ensuring the operation of phase detection autofocus is performed by special sensors. Their number varies depending on the camera model, for example, the Nikon D3200 has eleven, while the Nikon D800 has fifty-one. Using special mirrors, sensors catch light fluxes and distribute them to light-sensitive sensors. Sensors record the distance between the streams, and if it corresponds to a certain standard parameter, the aiming is accurate; if the distance is greater or less, you need to focus again. Thus, when pointing the lens, the camera initially determines whether the focus is correct, and if not, in which direction it needs to be changed to achieve the best result. The focusing speed of the phase-phase system is high and mainly depends on the efficiency of the lens motor.

Nikon Advanced Multi-CAM 3500FX is the most advanced autofocus module on this moment. 51 sensors, 15 of which are increased accuracy. It allows individual selection and configuration of coverage area settings in 9, 21 and 51 points.

The number of touch sensors is limited; they cover only a certain part of the frame. The two most common types of sensors are vertical and horizontal. Combining both options, we obtain the greatest focusing accuracy. The latter approach is quite expensive and technically complex, so the number of such sensors is usually not as large as vertical ones. There is also a nuance with the selection of lenses. And here, models with higher aperture (for example, f/2.8) are preferable. A high aperture allows you to increase focusing accuracy, while the use of lenses with a lower aperture, on the contrary, reduces its focusing speed and sometimes leads to the fact that the lens begins to make characteristic sounds, which in professional jargon is often called “yaw”.

Let's look at the autofocus system using the Nikon D800 as an example. It has 51 sensors, of which 15 are cross-shaped. They are located vertically in the center in three rows. All 15 work with lenses with a maximum aperture of f/5.6 or less. At f/5.6 - f/8.0 apertures there are nine of them, and at f/8.0 there are only one cross-shaped (central) and ten regular ones. The sensor layout is similar to the Nikon D4 autofocus system.

Note that the number of active vertical sensors also varies depending on the lens aperture; the “darker” the lens, the fewer there are. You can remain without autofocus by using teleconverters, for example, Nikon TC-20E III with a magnification factor of 2.0x. If you take a lens with an f/5.6 aperture, the actual value of the parameter when using the mentioned teleconverter will decrease by two full stops and will be f/11. In this case, you will have to focus manually.

Contrast autofocus

What about contrast autofocus? It has gained a lot of popularity in non-DSLR digital cameras in recent years. The essence of the work is quite simple: the system reads the image from the matrix, analyzing the degree of contrast of the focal point, then decides to adjust the lens focus to find the optimal value. The process is repeated until the system finds the best contrast value. All this takes time, complicating the situation is the fact that autofocus at the beginning of the process may mistakenly go forward or backward, this will take a little more time. We think many people have seen how the camera focuses in one extreme position, then in another, and only after that it begins to “identify” the subject. There is another point - while the camera reads information from the sensor, some time passes. As a result, you can observe the situation - the shutter button is pressed, the shot is taken with some delay, and the moment is lost. But there is a silver lining: a simple working circuit (sensor and lens) makes it cheaper and much more compact, in addition, this approach provides more accurate autofocus.

The Nikon 1 system combines phase detection and contrast detection autofocus to deliver fast, accurate focusing anywhere, anytime.

Let's take a break from DSLR cameras and remember that Nikon engineers managed to combine both approaches in the Nikon 1 line. The J and V series cameras use a hybrid autofocus system: good conditions When shooting, a phase approach is used; in limited lighting, a contrast approach is used. This made it possible to implement a full-fledged autofocus tracking system and significantly speed up the process itself.

There are times when none of the approaches will help, there is no way to focus due to minimal lighting and/or low contrast levels. For such occasions, cameras are equipped with an active autofocus system, namely backlight. It significantly reduces the speed of the focusing process, but allows you to take a successful photo.