Autofocus- this is a mechanism (device) that makes it possible to focus as accurately as possible with one press of the shutter button optical system lens on the subject. Almost all modern cameras have an autofocus function. The point where the rays reflected from the photographed object converge is called the focus. Autofocus is designed to adjust the sharpness of the lens optics on a specific object, group of objects or any individual point. The convenience of the autofocus system allows you to take photos quickly and without loss of quality, which is very important when a photographer needs to capture the moment.
Active autofocus systems
In 1986 the company Polaroid pioneered the use of an active autofocus system in their cameras. Principle of operation ultrasonic system consisted of the following: a powerful generator sent a certain number of pulses in the direction of the photographed object, the timing system was instantly activated, and when the sensor caught the echo, the mechanism, based on the data received, calculated the distance and commanded the drive to move the lenses to a certain position. This method commonly called active, it has a high focusing speed and is completely independent of the characteristics of the lens. But with all the advantages, this method has a significant drawback. Cameras with ultrasonic systems are not able to focus through a transparent obstacle.
For example, if you need to photograph an object through glass, the camera will not be able to do this.
The development of the active autofocus system continues with the infrared distance estimation system.. This system is based on three methods: triangulation, estimation of the magnitude of reflected radiation and time estimation.
Sound in air has a speed of approximately 300 m/s, and the speed of light is 300,000 m/s. Infrared radiation is directly related to the light spectrum, so the effectiveness of infrared radiation is much higher than the ultrasonic system.
The main obstacle infrared system distance estimates are objects heated in the sun, flames, household heating appliances - everything that has infrared radiation. The distance to the subject with a high light absorption coefficient also affects. There is a definition in physics black body - P surfaces with zero light reflectance. Surfaces There is no absolute black body in nature, but there are objects with weak reflective surface properties. It turns out that when the infrared distance estimation system encounters a material with very weak reflective property, it fails.
In this case, you have to focus manually. But this system does have advantages: the infrared system is able to focus both in poor lighting and in the dark. Previously, this system was actively used by video camera manufacturers, but later they came toTTL- method.
Passive autofocus systems
Principle of operation phase detection autofocus consists in the use of special sensors, which receive fragments of the transmitted light flux from different points of the image using lenses and mirrors. Inside the sensor, the light is divided into two parts, then each part hits its own light sensor. Focusing and precise focusing are achieved only if the two light streams are at a certain distance from each other, specified by the design of the sensor. The sensor calculates the distance between the light streams and automatically calculates how much the lens needs to be moved in order to achieve accurate focusing. Phase detection autofocus is good when you need to photograph a moving subject; it is fast and accurate. A large number of sensors makes it possible to evaluate the movement of an object, that is, it allows you to enable the tracking shooting mode. This is why phase detection autofocus is widely used today in SLR, film and digital cameras.
Below is a visual representation of how autofocus works; by moving the slider you control the focus; the animation is taken from here.
Figure No. 1
By title " contrast method“You can understand that the camera recognizes whether the image is in focus by the arrangement of the lenses, which produces the maximum contrast of the image. The principle of contrast autofocus is as follows: the shutter is raised and the camera receives an image. From this image, the camera cannot determine where to move the lenses to get a sharper image, and therefore a more accurate focus. Therefore, the camera begins to move the lenses in a certain direction, for example, forward. Then it reads the data again and checks the value of the contrast (sharpness) of the image with what was previously. Decreased contrast means the lenses have moved in the wrong direction. Now the camera moves the lenses in the opposite direction, only even further than they were at the very beginning. The shift distance is programmed into the camera firmware. Contrast autofocus is used in almost all mirrorless digital cameras. But some of them are Lately began to be equipped with a faster phase focusing system.
Figure No. 2
Autofocus motor
Not a single autofocus mechanism that moves lenses can do without a motor. The quality of focusing depends on the accuracy and speed of the motor, but it also affects the durability of the camera’s batteries. Today two types of devices are very popular - “ screwdriver" And " ultrasonic", they appeared not long ago. "Canon" was one of the first to use the new drive in their cameras " ultrasonic motor" for the lens. And after them, other companies introduced similar improved devices. You can tell that a motor is present by the index on the lens frame: USM - for Canon, HSM - for Sigma, SWM - for Nikon and SSM - for Minolta and Sony. Budget lens models are equipped primarily with a “screwdriver” motor, while more expensive lenses are equipped with an “ultrasonic” motor.
In their early days, autofocus systems were truly hocus-pocus. Now we can’t imagine life without autofocus, but just recently everyone used it and didn’t even imagine that automation would be able to clearly capture the subject of shooting.
People first started talking about autofocus in the 70s of the last century. Then the German company distinguished itself Leica, which developed the first autofocus lens, and introduced in 1976 the first camera equipped with an autofocus system. She became Leica Correfot, shown as a prototype at the exhibition Photokina-1976.
But the German company was in no hurry to produce autofocus systems and sold the technology to the company Minolta, which, thanks to the effective implementation of autofocus in its DSLRs, by the mid-1980s. rapidly became a leader in photographic equipment sales. In parallel, other corporations were developing automatic focusing systems ( Canon, Seiko, Polaroid, Pentax etc.) and the technology reached the masses.
Go into too much technical details We will not operate automatic focus systems yet. But let’s try to tell you how they function.
Today there are two main types of autofocus: phase And contrasting, as well as their symbiosis, which is called hybrid.
Phase detection autofocus
This type of focusing is fully used in DSLRs. It is based on the principle of the phase difference of the light flux that enters the lens. The difference is determined by special sensors that are placed in close proximity to the camera matrix.
The principle of operation of the phase detection autofocus system is clearly demonstrated in the picture below. The light flux enters through the opposite edges of the lens to the main mirror, where it is divided into parts: part goes to the viewfinder, and another part goes directly to the additional mirror, which reflects the rays onto the focusing sensors. If the light rays, after passing through the mirror and focusing lens, are focused at one point, then the subject is in focus. If the lens is focused closer or further away from the subject, the distance between the rays will be smaller or larger accordingly. In this case, the processor comes into play, which calculates the direction and amount by which the focusing lens needs to be moved.
Even naked eye There is a direct dependence of autofocus performance on the lens aperture. Indeed, the more light that enters the front element of the lens, the more of it will be reflected and the better the autofocus sensors will work. In this case, it doesn’t matter how much you close the aperture - it will close to the set value only at the moment the shutter is released, and during focusing the aperture will be open to the maximum. Those. Having a lens with f/1.2-1.4 aperture in your arsenal, you can count on higher focusing speed and accuracy. On the other hand, this is offset by the fact that faster lenses have a more complex and massive lens system, which means it is more difficult for the motor to move this entire mechanism. In addition, a large aperture implies a much shallower depth of field into which phase sensors need to penetrate. Bright to that example - one of the slowest (if not the slowest) lens from Canon - EF 85mm f/1.2L II USM.
The following illustration clearly shows the phenomena of back focus and front focus:
- focus closer – back focus;
- focus further – front focus.
The phase sensors themselves can be linear(horizontal And vertical) And crusades(incl. double crosses). We will look at them in more detail in upcoming materials.
Contrast autofocus
This focusing method is widely used in compacts and mirrorless cameras. They are not shy about installing contrast sensors in DSLRs - they provide focusing in LiveView mode when phase sensors cannot work.
The operation of the contrast autofocus system is based on the principle of comparing the contrast of the image that enters the camera matrix. The camera's processor analyzes the histogram and shifts the lens to see how much the contrast changes. If the contrast level goes down, the focus point will begin to shift reverse side. If the contrast increases, the focus point will continue to shift in this direction until it is possible to reach maximum value contrast. Those. the process continues until the focus point reaches maximum contrast and returns to the point after which its level began to decrease. In this case, the subject will be focused. Big advantage contrast focusing over phase focusing is that with it there is no back and front focus.
To view, move the mouse cursor to the upper right corner and move the slider forward/backward (visualization - http://graphics.stanford.edu/courses/cs178/applets/autofocusCD.html)
Hybrid autofocus
Today, this type of automatic focusing system is becoming increasingly popular. And for good reason - it combines the advantages of both systems and eliminates their disadvantages.
It works approximately as follows: phase sensors, which are located directly on the camera matrix, provide primary focusing. Subsequently, contrast sensors are connected, which correct the difference in image contrast and finally focus the camera on the subject.
Perhaps one of the main advantages of hybrid autofocus systems is the absence of back and front focus. This is explained by the fact that focusing occurs directly on the camera matrix. Another important advantage is the compact size of the hybrid autofocus system and the absence of the need to adjust this mechanism. But there is also a fly in the ointment - the speed of operation in the tracking mode hybrid autofocus still does not reach the phase level.
If you want to learn more about the operation of automatic focusing systems (with formulas and calculations), write in the comments. If there are enough people interested, we will definitely write a separate material on this topic.
Operating principles of the autofocus system.
Focusing is a sore point for most amateur photographers (and professionals too). Believe it or check it: any photographic forum will convince you, and camera tests always contain a section devoted exclusively to autofocus performance.
Discussions of autofocus on photographic forums most often end in mutual accusations of ignorance or virtual grabbing of the lapels of a jacket shouting “Who are you?!” I thought I’d start educating myself and figure out, at the everyday level, how autofocus works in modern digital cameras. It turned out that there are very few materials on the Internet, and even fewer that are understandable to a person without special education. The search results and compilation of information (thanks to LenzRentals!) are outlined below.
Modern digital cameras use two autofocus systems: contrast autofocus and phase detection autofocus. Let's start with a simpler (and less common in DSLR) autofocus system: contrast autofocus.
Contrast autofocus
Contrast autofocus works like this: the processor evaluates the histogram received from the camera sensor, moves the lens slightly - shifting the focus point, then re-evaluates it to see whether the contrast has increased or decreased. If the contrast has increased, the camera continues to shift the focus point in the selected direction until the image has maximum contrast. If the contrast has decreased, the lens is instructed to shift the focus point to the other side. The process is repeated until maximum contrast is reached (which essentially means moving the focus point slightly past the maximum contrast position and returning to the point where contrast began to decrease). An image “focused” using contrast autofocus is an image with maximum contrast.
If your camera shows a histogram in Live View, you can manually focus by contrast.
With contrast autofocus, the image is evaluated from a small area of the matrix - used as a sensor and coinciding with the focusing point selected by the photographer. This allows you to select the subject you want to focus on, and frees the camera processor from having to evaluate the contrast of the entire image - only the contrast at the selected AF points is evaluated.
Disadvantages of contrast autofocus
The main disadvantage of contrast autofocus is its slowness. The multi-step process of “shifting the focus point/lens - assessing - shifting - assessing” takes time, and the camera may start by moving the focus point in the wrong direction - then you have to go back. Due to the extremely low speed and the impossibility of tracking focusing, contrast autofocus is not suitable for dynamic scenes. The slowness makes it difficult to even photograph stationary subjects. Contrast autofocus, much more than phase autofocus, depends on good lighting, and - which is obvious - it requires good contrast of the object being focused on.
Benefits of contrast autofocus
Contrast autofocus also has advantages, thanks to which it is not only still used in cameras, but is also increasing its presence. Firstly, the contrast autofocus system is simpler. It does not require additional sensors and chips that are needed for phase detection autofocus. Simplicity reduces cost and (and for many, price is more important than speed) is the main reason for using contrast autofocus in compact digital cameras. (Another reason is that the depth of field of compact cameras is inherently greater and the requirements for autofocus accuracy are significantly lower.)
The simplicity of the Contrast AF system reduces its size. For example, recently emerging mirrorless digital cameras with interchangeable lenses tend to be miniature, and the contrast autofocus system does not require “moving” the image away from the camera matrix: this means that prisms, mirrors and lenses necessary for a phase-phase autofocus system are not needed. Miniature is one of the most important advantages mirrorless cameras with interchangeable lenses - they all use contrast autofocus.
The second advantage is that the contrast autofocus system uses the camera's sensor. There is no need to “retract” the light beam through special prisms and mirrors to additional sensors, which may not be adjusted in relation to the camera matrix. With contrast autofocus, the real image on the camera matrix is evaluated, and not a separate image, which must be (and “should” does not mean that it is) precisely aligned with the matrix.
It is for this reason that contrast autofocus provides more accurate autofocus than phase detection autofocus. Let me emphasize: “when using a matrix for contrast focusing.” Olympus and Sony DSLR cameras use an additional, smaller sensor for contrast autofocus in Live View mode, which means - as with any system that requires alignment - there is still the possibility of incorrect alignment.
Overall, contrast detection autofocus is simpler, cheaper, smaller, and theoretically more accurate than phase detection autofocus. But it's much slower. Manufacturers are making every effort to speed up contrast autofocus, there are successes, but it will remain slower in the near future.
Phase detection autofocus
Basic principles
The phase matching autofocus system (also known as phase matching) was introduced by Honeywell in the 1970s; it was first used commercially in the Minolta Maxxum 7000 camera. Honeywell sued Minolta for patent infringement and won; so the manufacturers had to pay Honeywell for the right to use the phase detection autofocus system.
Phase detection autofocus is based on the principle that rays emanating/reflecting from a point in focus will equally illuminate opposite sides of the lens (“will be in phase”). If the lens is focused in front of or behind this point, these light rays pass through the edges of the lens differently ("out of phase").
Majority existing systems phase detection autofocus uses mirrors, lenses or prisms (beam splitters) to split beams passing through opposite edges of the lens into two beams; and a secondary lens system to refocus those beams onto the autofocus sensor (usually a CCD). This sensor determines where light rays passing through opposite edges of the lens fall. If the point is in focus, the rays hit the sensor at a certain distance from each other. If the lens is focused closer or further than the desired point, the distance between these rays will be smaller or larger. A lot of words, let's try to look at the graphical display of the process - (Fig. 1).
Rice. 1 How phase detection autofocus works
I’ll make a reservation right away: the description and figure give a very simplified explanation of the principle of operation of phase detection autofocus - just to get an idea of “how it works”. The physics and mechanics of the process, the description of which would take more than one page full of formulas, numbers and other incomprehensibility, remained “behind the scenes”.
The figure clearly shows that the camera processor in the phase detection autofocus system immediately determines whether the lens is focused too close or too far from the subject, so one of the disadvantages of contrast autofocus (the camera does not know which way to shift the focus point) is initially absent - instead of moving forward and back and determine in which direction the greater contrast lies, in phase detection autofocus the processor immediately sees in which direction to shift the focus point.
And then comes the process. Every autofocus lens has a microprocessor that tells the camera its presence and state, for example, "I am a 50/1.4 lens and my focusing element is at a position 20% closer than infinity" - or something similar. When you press the shutter button halfway, the following happens:
The camera reads data from the autofocus sensor, consults a data array containing information about the properties of autofocus lenses from that manufacturer, makes some calculations and tells the lens something like “Move the autofocus point this far to infinity.”
The lens contains sensors and chips that measure either the amount of current supplied to the focusing motor or how much the focusing element has moved. The lens shifts the focusing element and sends a signal to the camera that it is “almost there.”
The camera rechecks the data from the autofocus sensors and sends a signal to the lens for more precise adjustments; the precise focusing process can be repeated several times until the lens focuses “right on target.” If something goes wrong, the infamous "yaw" of the lens occurs.
After focusing, the camera orders the lens to lock focus and informs the photographer (with sound and an indicator in the viewfinder). The whole process takes a fraction of a second. Very fast.
Phase detection autofocus circuit
The autofocus sensor cannot be in front of the sensor, so manufacturers use partially transparent areas in the mirror that transmit light to the secondary mirror, from which it is reflected to the autofocus sensor (Fig. 2).
Rice. 2 Phase detection autofocus circuit
Typically, the autofocus sensor is located under the main mirror (Fig. 3) along with exposure metering sensors. The red arrow shows the autofocus sensor of the Canon EOS 5D camera. Image taken from Canon USA website
Rice. 3 AF sensor location
Types of phase detection autofocus sensors
Each sensor can only evaluate a small part of the image. Horizontal sensors work more accurately with vertical parts. In most images, vertical details predominate, so there are more horizontal sensors. There are also vertical sensors, usually arranged crosswise with horizontal ones (Fig. 4). Some cameras are even equipped with diagonal phase detection autofocus sensors.
Some autofocus sensors (almost always centrally located), using different lenses and the size of the sensor itself, achieve greater autofocus accuracy, especially when using fast lenses. Most often, they only come into play when using lenses with an aperture of f/2.8 or faster. Figure 4, for example, shows that using an f/2.8 lens will use a cross-shaped sensor, while darker lenses will only use a single, less accurate AF sensor.
Rice. 4 Cross-shaped autofocus sensor
Early phase detection autofocus systems (and some modern medium format cameras) had only one sensor in the center of the image. As computing power and engineering prowess grew, more and more sensors were added. Nowadays, most cameras have from seven/nine to 52. Depending on the requirements of the scene being photographed, you can choose one, all, or a group of sensors. You can tell the camera which sensor/sensors to use.
Numerous phase detection autofocus sensors, together with the camera processor, are capable of remarkable things. By determining which sensors have a moving object in focus and how that changes—measuring the subject's movement and taking readings at short intervals—the camera can predict where a moving object will be after a certain period of time. This is what tracking autofocus works on.
Effect of lens aperture
Regardless of the sensor type, autofocus will be more accurate when using fast lenses. During focusing, the camera opens the lens as much as possible, closing the aperture to the value you select only when the shutters open. Phase detection autofocus is more accurate the wider the angle of the light rays. In the diagram below, the angle of rays received from the f/2.8 lens ( blue lines), will be greater than that of an f/4 lens (red lines), which in turn is greater than that of an f/5.6 lens (yellow lines). When using a lens with a maximum aperture of f/8, only the most accurate sensors are capable of working, but focusing will be slow and less accurate. This is why f/5.6 lenses stop autofocusing when we try to use a teleconverter that reduces their maximum aperture to f/8 or f/11.
Advantages of phase detection autofocus
We have already mentioned the main advantages of phase detection autofocus:
It is much faster than contrast - fast enough for shooting moving objects.
The camera is able to use a group of sensors to estimate the movement of the subject, which gives us tracking/predictive autofocus.
There are also less obvious advantages. Groups of phase detection autofocus sensors can be used for "electronic DOF" – preliminary assessment depth of field. Some cameras (though there are not many of them) are equipped with a trap autofocus function - they take a photo at the moment when something hits the camera. active point focusing. If the sensors detect movement in a static scene, they may report that the camera is moving abnormally. But the main thing is speed and tracking autofocus
Disadvantages of phase detection autofocus
Firstly, phase detection autofocus system requires physical alignment. The path of light to the camera sensor must be matched with the path of light to the AF sensor so that an object that is in focus on the AF sensor is also in focus on the sensor. Each lens must contain a chip that provides feedback with the camera and informing it of the exact position of the focusing element, the distance the element moves when a certain current is applied to the autofocus motor. All this must be precisely coordinated and calibrated in such a way that the lens shifts the focus point exactly where the camera pointed it, and the camera knows the exact position of this point. The slightest inconsistency leads to inaccurate focusing.
Secondly, the system requires software settings . Each camera and lens is programmed by the manufacturer and entered into memory a large number of data. This data ensures that the camera and lens work together in harmony, and autofocus accuracy can sometimes be improved through firmware updates. These updates are often released following the release of new lenses.
Manufacturers hide the operating algorithms of their phase detection autofocus systems. Third-party lens manufacturers are forced to experimentally read and decode the signals exchanged between the camera and lens and, based on this data, develop their own microprocessors and their own algorithms. As a result, autofocus accuracy may be lower when using third-party lenses. Changing algorithms by camera manufacturers leads to autofocus on third-party lenses refusing to work (they need to be reprogrammed, as recently happened with the Sigma AF 120-300/2.8 and Nikon D3X).
As already mentioned, lens aperture affects the accuracy of phase detection autofocus. Fast lenses able to focus more difficult conditions. Typically, aperture dependence does not cause problems because dark lenses have a large depth of field. However, there are maximum aperture values (usually f/5.6 or f/8) when phase detection autofocus simply refuses to work. (Remember, we're talking about about the maximum aperture of the lens - the camera automatically fully opens the lens aperture during focusing, so the set value does not affect autofocus if the maximum aperture of the lens corresponds to the capabilities of the camera).
Because light hits the autofocus sensors only when the mirror is down, they stop working the moment the photo is taken, and don't start working until the mirror returns to its original position. This is why phase detection autofocus does not work in Live View mode, and tracking autofocus may fail during continuous shooting.
There are other problems that we do not notice. Linear polarizing filters interfere with phase detection autofocus. There are only a few linear polarizers left now, but it happens that having bought one “cheaply” the owner is later surprised by the inaccuracy of autofocus. Phase autofocus can simply be blown away in some scenes (such as a chessboard or lattice), but contrast autofocus can easily cope with them.
Live View:
I highlighted Live View because it is what forces manufacturers to work on improving contrast autofocus and creating hybrid systems. As mentioned, contrast autofocus has its advantages, and overcoming its limitations will benefit all photographers.
Olympus and Sony have already created systems that split the light beam, sending some to the viewfinder and some to an additional image sensor. This system allows you to use phase detection autofocus even in Live View mode. But the risk of inaccurate focusing also increases, because not a matrix is used, but an auxiliary sensor.
Canon described a system that uses phase detection autofocus initially and then fine-tune focus using contrast detection autofocus.
Nikon appears to have filed a patent application for the principle of using certain pixels on a camera's sensor as phase detection autofocus sensors. This, in my opinion, will simply be a revolution.
FujiFilm has already released a line of compact digital cameras with hybrid autofocus system.
Wait and see. But it is obvious that for the first time in last years changes to autofocus systems may be revolutionary rather than evolutionary. Which, you see, holds a lot of interesting and exciting things for amateur photographers.
The article is very useful! Thank you!
And again thank you so much for good words and reviews! I am very glad if you found the material useful and interesting.
Can I ask a question?
Is the sensor sensitive to the spectral composition of light, and how does this affect focusing accuracy?
Thank you.
it’s written in the title “JUST ABOUT AF”, where the hell is that simple? Of course it’s written in an accessible way, but sooooo complex language, not an ounce of simplification
The evolution of mobile autofocus:
from contrast to Dual Pixel
When shooting with a smartphone, it is very important that the photos come out clear. To do this, the subject must be in focus before you click on the “Take Photo” button. Recently, a number of manufacturers have been working on improving autofocus technologies, and today we will look at how they differ from each other.
When choosing a camera phone, many pay attention to the number of megapixels - they say, whoever has more is cooler. However, it is often more important and useful to look at other factors that have an equally serious impact on the quality of photographs. Among them is the camera's autofocus type. Apple, Samsung, LG and other manufacturers are now actively rushing into this area, and many have actually managed to make significant progress.
What is autofocus and why do we need it?
The autofocus system adjusts the lens to focus directly on your subject, making the difference between a clear shot and a missed opportunity.
In a simplified way, the principle of operation of the camera is that rays of light are reflected from the objects being photographed and then hit the sensor, which converts the stream of photons into a stream of electrons. After this, the current is converted into a set of bits, the data is processed and recorded in the camera’s memory. CMOS sensors are now especially popular among smartphone manufacturers, which convert charge into voltage directly in the pixel, subsequently providing direct access to the contents of an arbitrary pixel.
In theory, it works like this: lenses focus light onto a sensor, which then creates a digital photo. In reality, everything is not so simple. The angle of the incoming light rays depends on the distance at which the photographed object is located. The diagram on the left shows lenses that focus light rays on a blue object: the green and red objects are out of focus and will be blurred in the final photo. If we want to focus on green or red objects, we need to change the distance between the lenses and the sensor.
At the dawn of camera phone manufacturing, most devices had a fixed focus. Modern smartphones provide the ability to adjust the distance between the lenses and the sensor. Therefore, you get high-quality detailed images. Currently, three methods are mainly used to implement autofocus in smartphones: contrast, phase and laser.
Contrast autofocus
Contrast autofocus is a passive type of autofocus. This solution is still used in most smartphones, largely because it is one of the simplest. Using the sensor, the amount of light on the object is measured, after which it moves the lens depending on the contrast. If the contrast is maximum, then the subject is in focus.
In general, contrast autofocus copes with its task quite well and has a significant advantage - it is quite simple and does not require any complex hardware.
But it also has several disadvantages. Contrast AF in particular is slower than others, typically taking about a second to focus on a subject. During this time, you may change your mind about taking a photo, or, for example, if you wanted to capture a fast-moving object, the moment will be missed. This is due to the fact that the lion's share of time is occupied by the process of “shifting the focus point/lens of the lens – contrast assessment – shift – contrast assessment.” In addition, contrast autofocus lacks tracking focusing capabilities, and in low light conditions it is unlikely to impress you. Therefore, this type of autofocus today is used mainly in budget smartphones, such as Lenovo A536, ASUS Zenfone Go and others.
Phase detection autofocus: a fast and advanced alternative
One of the pioneers here was Samsung company, which borrowed technology from digital SLR cameras and equipped its Galaxy S5 smartphone with phase detection autofocus. The point is that in in this case special sensors are used - they catch the passing light flux from different points of the image, using lenses and mirrors. Inside the sensor, light is divided into two parts, each of which hits an ultra-sensitive sensor. The distance between the light streams is measured by the sensor, after which it determines how much the lens needs to be moved for accurate focusing. For example, the Samsung Galaxy S5 takes only 0.3 seconds to focus on an object.
The first and main advantage of phase detection autofocus is that it is much faster than contrast autofocus, it’s easy must have for shooting moving objects. In addition, the camera can evaluate the movement of an object using sensors, hence the possibility of tracking autofocus.
But there are also disadvantages. Phase detection autofocus, like contrast autofocus, doesn't do its job very well in low-light conditions. It also requires more powerful hardware, so it is usually available in high-end smartphones. Among them, for example, Huawei Honor 7, Sony Xperia M5 and Samsung Galaxy Note 5.
Some manufacturers have gone further and decided to use laser autofocus in smartphones (more on this later), while others are actively engaged in improving phase detection autofocus technology. For example, Apple uses so-called “focal pixels” in its iPhone 6s and iPhone 6s Plus: the point is that the technology uses part of the pixels as a phase sensor, and shooting with Apple smartphones is really fast.
But the Dual Pixel technology that Samsung uses in its Galaxy S7 and Galaxy S7 Edge smartphones really differs from standard phase focusing. Although it is a type of phase detection autofocus, it still has some differences and subtleties. In smartphones, phase detection autofocus is somewhat limited in its capabilities - in order to assign a focal sensor to each pixel, you need to greatly reduce it, which results in noise and blurred photos. Typically, about 10% of light-sensitive points are equipped with sensors; some manufacturers, however, do not go beyond 5%.
In Dual Pixel, each pixel is equipped with a separate sensor due to the increase in pixel sizes. The processor processes the readings of each pixel, but does it so quickly that autofocus still takes tenths of a second. Samsung says that Dual Pixel technology is similar to focusing using the human eye, but this is more of a metaphor. Nevertheless, we must recognize the innovativeness of this approach to phase detection autofocus. Now this is a real exclusive for the Galaxy S7 and Galaxy S7 Edge.
Laser autofocus: the most active
Like phase detection, laser autofocus is an active type of autofocus. This direction for a long time was carried out by LG, which first implemented laser autofocus in its G3 smartphone. The technology is based on the principle of a laser rangefinder: a laser emitter illuminates an object, and a sensor measures the time of arrival of the reflected laser beam, determining the distance to the object.
One of the main advantages of this autofocus is time. According to LG, the entire laser autofocus process takes 0.276 seconds. Significantly faster than contrast autofocus and slightly faster than phase detection.
The obvious advantage of laser autofocus is that it is incredibly fast and does its job well in low light conditions. But it only works at a certain distance - the most best effect achieved if the distance from the smartphone to the object is less than 0.6 meters. And after five meters - hello, contrast autofocus.
How does autofocus work in a smartphone? Which type of autofocus works best? Pros and cons of laser, phase and contrast AF. Why is dual-pixel so good?
How does autofocus work on a smartphone? There is no easy answer to this question. You need to understand each type of autofocus and study the features of a specific focusing technology. Only after this can we draw any conclusions. Therefore, now we will talk about the types of automatic focusing technologies, and the advantages and disadvantages of each of them.
What is camera focus and autofocus
Everything is simple here: the objective lens refracts the rays and collects all the light at one point - the focus. And if the matrix sensor is located at this point, then the frame turns out to be more detailed and of higher quality. Naturally this physical phenomenon All photographers use it. They bring some part of the frame into focus, manually adjust the lens, and focus the viewer's attention on the foreground or background, the main subject or a secondary detail. The rest of the picture will be blurry.
Well, novice photographers can use the automatic focusing system, when the automation captures one or more objects in the frame “in focus”, controlling both the lens and the matrix. And these objects (or object) turn out to be as sharp and detailed as possible. And no skill or sense of frame is needed here.
This is probably why digital photography has become more popular than the film and paper version of art. After all, autofocus in a phone camera or a cheap camera allows you to take a detailed photo without any extra effort. The whole process comes down to simple rule: Point and click.
Types of autofocus and basic principles of their operation
The camera lens focuses rays reflected from an object located in space in front of the lens. When focusing, the camera is guided by the distance to the object and the intensity of the glow emanating from it. Today there are two types of auto focus modes:
- Active option - it is based on measuring distance using a locator-rangefinder.
- Passive option - it works with a light beam, measuring its intensity.
The first (active) mode uses laser infrared or ultrasonic radiation with a known wave propagation speed in the air. The emitter module emits a directed flow, which is reflected from the object and captured by the receiver module after a certain period of time. Next, the autofocus computer multiplies this time by the known wave speed and divides the result by two, obtaining exact value distances. By pointing the emitter at the desired area, the user obtains optimal focusing, directing the viewer's attention to this particular area of the photo.
The second (passive) mode is structured somewhat differently. It uses special sensors (photodiodes) that measure the intensity of the glow and a special processor that determines the focus based on the value of this parameter. In practice, it looks like this: sensors record the intensity of the glow, then the processor shifts the focus, after which the intensity is measured again; if the flux density has increased, then the focusing is considered acceptable. If not, the focus shifts again. And so on until the maximum intensity is detected. The matrices of serious cameras contain up to 40-60 photodiodes.
Most work based on these principles known systems focusing: phase, laser, contrast and dual-pixel. And further in the text we will discuss each option, evaluating along the way their basic advantages and disadvantages.
Advantages and disadvantages of laser autofocus
In this case, a laser emitter and receiver are built into the phone camera model. The first generates a narrow beam, the second receives the reflected signal. As a result, the focusing speed is reduced to thousandths of a second. Typically we are talking about 250-300 milliseconds, since the laser travels at the speed of light.
The main advantage of laser focus is the high response speed of the module, and the main disadvantage is frequent failures. A narrowly directed laser emitter sometimes “shoots” past the target, and the reflected signal is easily lost, especially in open spaces. Therefore, laser autofocus in a smartphone camera in most cases works in tandem with phase or contrast targeting.
Features of phase focusing
The technology is based on splitting the beam passing through the lens into two streams. This is done in order to measure the distance between the flows passing through the opposite edges of the lens. If this distance fits within certain values specified in the data array, the picture is considered focused. To record the distance, special sensors that react to light are used. Their signals are processed by a processor, which compares the read parameters with the base data array and gives a signal to shift the focus in the desired direction.
The main advantage of the technology is its readiness to focus on a moving object. In addition, this option is faster than contrast autofocus. This system can also be used to calculate such a parameter as depth of field.
The main disadvantage of phase technology is its complex implementation. The system of prisms, mirrors, and lenses requires ultra-precise physical adjustment and no less scrupulous software settings. In addition, the accuracy of such a focus depends on the lens aperture, and mobile phones There are big problems with this parameter.
Pros and cons of contrast focus
The technology does not change either the matrix or the optical system of the smartphone camera. Either the entire photosensor or part of it is used as a sensor. The processor reads the current histogram from the sensor and evaluates the contrast of the frame. And then the lens is given a command to shift focus, after which a new histogram is read with a re-estimation of contrast. And the whole cycle repeats until it reaches maximum level contrast in the selected area of the frame that is being focused.
The main advantage of the technology is the combination of ease of implementation, low cost design and compact size. All manufacturers of budget smartphones use such autofocus.
The key disadvantage of this option is the very slow speed. Sometimes the processor goes into an eternal “hunt for focus” mode, which ends with the loss of a rare frame.
Dual Pixel technology
This focusing technology is used in expensive DSLR cameras. In mobile devices, it is so far used only in Samsung’s flagship models, deliberately lowering the resolution of the photographic matrix while simultaneously increasing its physical dimensions.
These tricks are used because of the desire to attach an individual sensor to each pixel of the photographic sensor that reacts to the intensity of the glow. Then the signals from the sensors are processed using both phase and contrast focusing algorithms, achieving not only a perfectly sharp, but also the most contrasting image.
If, in the case of classical phase focusing, the share of sensors is no more than 10% of total number pixels in the camera, then in the case of Dual Pixel they are divided in a 50/50 ratio. Simply put, each pixel is a light-sensitive element and a sensor at the same time. This technology provides more accurate and faster focusing.
One of the disadvantages of Dual Pixel is the very complex implementation of such solutions. Only flagship devices are equipped with such tricks, for example, devices from the S-series of Samsung (from the seventh model and above). There is something similar in the latest iPhones (from the sixth model and above), but Apple calls this focusing technology Focus pixels, and it is closer to regular phase detection autofocus than to Dual Pixel.
