Like any "non-ideal" optical system, the human eye is characterized by optical defects - aberrations that reduce the quality of vision, distorting the image on the retina. Aberration is any angular deviation of a narrow parallel beam of light from the point of ideal intersection with the retina as it passes through the entire optical system of the eye.
In technical optics, quality optical system is determined by the aberrations of the plane or spherical front of the light wave as it passes through this system. Thus, an eye without aberrations has a flat wavefront and gives the most complete image on the retina of a point source (the so-called "Airy disk", the size of which depends only on the diameter of the pupil). But normally, even with a visual acuity of 100%, optical defects of the light-refracting surfaces of the eye distort the course of the rays and form an irregular wavefront, resulting in a larger and asymmetric image on the retina.
The quantitative characteristic of the optical quality of the image is the root-mean-square value of the deviation errors of the real wavefront from the ideal one. The German mathematician Zernike introduced a mathematical formalism using a series of polynomials to describe wavefront aberrations. Polynomials of the first and second, i.e., lower orders, describe optical aberrations familiar to ophthalmologists - myopia, hyperopia and astigmatism. Less well known are polynomials of higher orders: the third corresponds to a coma - this is a spherical aberration of oblique beams of light incident at an angle to the optical axis of the eye. It is based on the asymmetry of the optical elements of the eye, as a result of which the center of the cornea does not coincide with the center of the lens. Fourth-order aberrations include spherical aberration, which is mainly due to the uneven refracting power of the lens at its various points. Higher orders are known as irregular aberrations.
How the wavefront is measured
An optical system is considered good if the Zernike coefficients are close to zero and, therefore, the rms value of the wavefront errors is less than 1/14 of the light wavelength (Marechal criterion). Based on the data of this coefficient, visual acuity can be predicted by modeling the image of any optotypes on the retina. To determine the aberrametry of the human visual system, a special device is used - an aberrometer. In clinics, Excimer uses a Wave Scan aberrometer from VISX Inc (USA).
Currently, there are several methods for determining eye aberrations based on different principles.
The first of these is analysis of the retinal image of the target (retinal imaging aberrometry). Two parallel laser beams with a wavelength of 650 nm and a diameter of 0.3 mm are projected onto the retina, one of which falls strictly along the visual axis and is a reference, and the other is located at a given distance from it. Next, the degree of deviation of the second beam from the fixation point of the reference beam is recorded, and thus each point within the pupil is analyzed sequentially.
The second principle is analysis of the reflected beam leaving the eye (outgoing refraction aberrometry). Widely used in astronomy to compensate for aberrations in telescopes when passing through the atmosphere and space. Using a diode laser with a wavelength of 850 nm, a collimated beam of radiation is directed into the eye, which, having passed through all the media of the eye, is reflected from the retina, taking into account aberrations, and at the output enters a matrix consisting of 1089 microlenses. Each microlens collects undistorted rays at its focal point, and the aberrated rays are focused at some distance from it. The received information is processed by a computer and presented in the form of an aberration map. Wave Scan is based on this principle.
The third principle is based on the compensatory adjustment of the light beam incident on the foveola. Currently, this method is used as a subjective aberrometer, requiring the active participation of the patient. During the study, a beam of light is directed into the eye through a rotating disk with 1 mm holes, located on the same optical axis as the pupil. When the disk rotates, narrow parallel beams of light pass through each point of the pupil and, in the absence of aberrations, are projected onto the foveola, where another beam is directed with a control mark in the form of a cross. If the patient has myopia, hyperopia, astigmatism or other aberrations of higher orders, then he will notice the discrepancy between these points and the cross and with the help of a special device he will have to match them. The angle by which it shifts the point reflects the degree of aberration.
A variety of ophthalmic devices designed with the latest technology and based on different principles action, makes real not only a qualitative, but also a quantitative assessment of aberrations of lower and higher orders, as well as the factors influencing them.
The main causes of aberrations in the optical system of the eye
- Forms and transparency cornea and lens; state of the retina; transparency of intraocular fluid and vitreous body.
- Enlargement of pupil diameter. If at a pupil diameter of 5.0 mm 3rd order aberrations prevail, then with its increase to 8.0 mm the proportion of 4th order aberrations increases. It is calculated that the critical pupil size, at which higher-order aberrations have the least effect, is 3.22 mm.
- Accommodation. It is noted that aberrations increase with age, and in the period from 30 to 60 years, higher-order aberrations double. Perhaps this is due to the fact that over time, the elasticity and transparency of the lens decreases, and it ceases to compensate for corneal aberrations. The same happens with accommodation spasm.
- Spasm of accommodation occurs quite often in humans different ages. In ophthalmology, an accommodation spasm is understood as an excessively persistent tension of accommodation, due to such contraction of the ciliary muscle, which does not disappear under the influence of conditions when accommodation is not required. Simply put, an accommodation spasm is a prolonged static overstrain of the eye muscle, for example, due to prolonged work at a computer and the occurrence of a computer syndrome as a result of this. Spasms of accommodation can develop with all refractions (including astigmatism). Spasm of accommodation causes false myopia or enhances true myopia.
- The state of the tear film. It was found that when the tear film is destroyed, higher-order aberrations increase by 1.44 times. One type of tear film disorder is dry eye syndrome.
Dry eye syndrome occurs due to the drying of the surface of the cornea from rare blinking and continuous looking at the object of work. Studies have shown that when working on a computer, as well as when reading, a person blinks three times less than usual. As a result, the tear film dries up and does not have time to recover. The causes of dry eye syndrome can be: heavy loads on the eyes when reading and working at a computer, dry indoor air, malnutrition with an insufficient amount of vitamins, high air pollution, taking certain medications. - Wearing contact lenses. It was found that soft contact lenses can cause wave monochromatic aberrations high order, while rigid contact lenses significantly reduce 2nd order aberrations. However, the aspheric surface of hard contact lenses can cause spherical aberrations. Aspheric contact lenses can cause greater visual acuity instability than spherical contact lenses. Multifocal contact lenses can induce coma and 5th order aberrations.
Currently, a technique has been developed for individualized vision correction ( Super Lasik, Custom Vue) based on aberrometry, which allows, as much as possible compensating for all possible distortions in the visual system, to achieve excellent results in almost any difficult cases.
Optical distortions appeared along with the lenses, this is, as it were, their small property. But if it is really small, there will be no problems with it. To minimize the problem of optical distortion, read our article!
Expensive lens doesn't mean perfect
Any lens has an optical defect, which is why it does not create an exact copy of the object that we photograph. Of course, every year manufacturers try to create more and more ideal optics, despite the fact that there is no way to make a lens that does not suffer from distortion to some extent.
Indeed, a high price does not always mean quality in terms of optical defects. And what is important? This is the type and design of the optics. The price plays a role, but the focal length is much more important.
For example, the wider the angle of the lens, the harder it is for a straight line not to be curved. Reducing the focal length also contributes to distortion because it is not possible to correct deviations at each focal length.
No one claims that a prime lens is flawless, but the larger the zoom range, the more noticeable these distortions become.
Distortion Test
Mirrors in a car are made curved, so they will expand the viewing angle, moving away everything that is reflected in them. Something similar happens in the lens - as a test, you can photograph a sheet of paper “in a box” and then examine it in Photoshop (for this you need to turn on the Ctrl-R rulers and “drag” the guides from them with the mouse of blue color- it will be easier to see the curvature of the resulting cells)
Types of distortion
There are quite a few types of distortions, but we will focus on the most important ones.
Curvilinear. There are several subspecies of them, of which the most common is barrel-shaped. How does it arise? If you use an ultra-wide lens, then lines that were straight become convex. Now there is a trend to shoot with a fisheye, so this is this distortion, just used in an enhanced form and as a feature.
Cushion-shaped. Mostly seen in long telephoto lenses. It is opposite to the previous one, that is, the lines are concave inward. In principle, this is hardly noticeable, but if you scale the object during shooting or processing, it will be visible.
Chromatic aberrations. This is a huge problem in modern photography. Its essence is that the color of the edging appears in the pictures, it is especially noticeable even without enlarging the photo. This happens with lenses of any focal length, but especially with the cheapest models or with “soap dishes”.
Vignetting in other words, darkening areas at the edges of the frame. It can usually be seen on wide-angle lenses at their widest aperture. This effect is quite rare.
Help editor
Adobe Photoshop has good tools for rescuing a distorted photo.
We do not work directly with the original background (I think you are aware). So the first thing we're going to say is Duplicate Layer / Duplicate Layer.
After: Filter / Filters> Distort / Distortion> Lens Correction / Optical distortion. By clicking, you will see a window with heaps of settings, of which we only need the top block on the right, immediately below the keys.
There we manually move the slider until we get the desired result. You need to move very carefully, since, as a rule, very few lenses distort more than -7. This means that you need to edit until the value +4 or +5 appears, which, in most cases, is enough for many compact digital cameras. You can also drive in these numbers by hand, controlling the result on the grid located in the preview field of the filter itself. You can do it even easier by clicking the button in the upper left corner and then “drawing” an imaginary line from the edge to the center (again, very carefully).
We will try to make several attempts to correct and when the result is reached, click “OK”. It would seem that everything...
There is, however, a small problem: the filter appeared only in CS2. If you are using more early versions Photoshop, increase the canvas size by 30% ( Image/Image> Canvas Size/Canvas size> in percent 130 vertically and horizontally) and open Filters / Filters > Distort / Distortion > Spherize:
In it, on the contrary, the slider must be moved to a negative position (in our case, -5). Click OK.
After editing, you may “sag” the edges of the image, so you will need to use the “crop” tool.
Let's compare the result:
After
There are alternative, but quite expensive options for editing optical distortions. So, Thomas Nieman once released the ptLens plugin, which remained free until it reached its design capacity. Today it costs about $15. The advantage is the built-in profiles of lenses and cameras, their automatic detection from the exif data of the image file and the correction of other distortions, such as vignetting (darkening to the edges of the frame) and chromatic aberration (blue or red halos around high-contrast objects). It is worth downloading and at least trying to fix no more than 10 frames. There is also a set of more expensive filters from DxO optics that are rumored to work better.
Aberrations in photography are called distortions of images formed by the optics system. Depending on the nature of the origin, aberrations are chromatic and geometric. The reason for the occurrence of chromatic (that is, color) aberrations is the imperfection of the optics of cameras. In fact, this kind of distortion can be called a property of the lens, because in one way or another it is inherent in any of them. The lower the quality of the optics used, the more color distortions are observed in the pictures. Often in photographs taken with cheap "soap dishes", there is a bright multi-colored border framing contrasting objects. This is chromatic aberration.
To minimize this type of distortion, special achromatic lenses, consisting of two different types of glass. One of them - crowns, has a low refractive index, the second - flint on the contrary, high. The right combination of these two materials can reduce visible chromatic aberration to almost zero. The very same optical phenomenon, in which rays of light with different wavelengths are refracted at different angles, is called glass dispersion.
Geometric aberrations are no less a headache for novice photographers than color ones.
Distortion, in which points of an object located outside the optical axis, are displayed in the image as shadows or lines, is called astigmatism. Objects in a photo with astigmatism look twisted, curved, and slightly blurry. Thus, astigmatism, along with chromatic aberrations, affects image sharpness (albeit to a lesser extent).
If the contours of objects in a photograph are unnaturally concave or convex, and this is not an artistic intent, this type of geometric aberration is called distortion. In the first case (when the lines are concave inward) we are talking about barrel distortion, in the second - about pincushion.
Distortions result from a change in the linear magnification provided by the optics across the image field. In other words, light rays passing through the center of the lens merge at a point located farther from the lens than rays that pass through its edges. The appearance of barrel distortion, as a rule, is facilitated by the use of minimum value zoom, pillow-shaped - respectively, the maximum. The most obvious distortion occurs when using wide angle lenses.
Aspheric optics are used to reduce distortion. Due to the inclusion of a lens with an elliptical or parabolic surface in the design of the lens, the geometric similarity between the photographic object and its image is restored. Of course, the cost of producing such lenses is much higher than the cost of manufacturing spherical optics.
Minor manifestations of distortion are easily corrected by means of a graphic editor.
The type of geometric aberration that prevents a lens from forming a flat image is called image field curvature. With this distortion, either the center of the image or its edges can be in focus.
The curvature of the image field is corrected by making changes to the lens assembly. In this case, a prerequisite is the observance of the Petsval rule, which determines the quality of the lens elements. If the reciprocal of the product of the focal length and the refractive index of one element, in total with total number elements gives zero, so this element is good. The result of these calculations is called the Petsval sum.
Interestingly, photographers did not master the technique of correcting field curvature until mid-nineteenth century. But this did not stop them from doing artistic photography. Blurred corners and fuzzy edges were covered with intricate vignettes, and portraits (in order to minimize distortion) were framed in oval frames.
A complex aberration that affects only light rays passing through the lens at an angle is called coma(or just coma). In the pictures, the coma manifests itself in the blurring of individual points of the image in the form of a comet. The "tail" of the comet can be directed to the edge of the image (positive coma) or to its center (negative coma). This distortion is more noticeable the closer the point is to the edge of the image. The same rays of light that pass clearly through the center of the lens are not subject to coma aberration.
Most geometric aberrations can be reduced by adjusting the aperture. By reducing its diameter, the photographer simultaneously reduces the number of rays falling on the edges of the lens. But you need to use this opportunity carefully. Because excessive diffraction leads to an increase in the amount of diffraction.
- This is an optical effect that limits the detail of the image, regardless of the image resolution set. The reason for its occurrence is the scattering of the light flux when passing through the diaphragm. Many beginners, in an effort to increase depth of field, cover the aperture to such an extent that the sharpness achieved is offset by the smoothing effect of diffraction. This effect is called the diffraction limit. Knowing its value avoids problems with image detail. To calculate the diffraction limit, a special calculator is used, available for free download on most specialized sites.
When choosing a camera, you should remember that lenses without aberrations do not exist. Anyway, for now. Even the most expensive optics show some image distortion. The correction of one type of violation leads to the strengthening of another - and this process has no end. But in order to become a good photographer, it is not necessary to wait for the invention of the ideal lens. It is enough to study the features of a particular lens - and level its shortcomings with your own skill.
Fundamentals of photography. Optical distortions of objects.I think many readers have noticed more than once that the image in the photograph is different from what we see with our own eyes. This is partly due to the peculiarities of the transmission of perspective at different focal lengths. You can read more about this in the article about. In addition, defects may appear in the image in the form of color halos in contrasting areas, darkening of the frame at the edges, and changes in the geometry of objects. These shortcomings can be safely attributed to optical distortions of lenses, so we'll talk about them in today's article.
distortion
Distortion is the geometric distortion of straight lines when they appear curved. Do not confuse distortion and perspective distortion, in the latter case, straight parallel lines become converging, but not curved. There are two types of distortion according to the type of impact on the picture: pincushion - when the lines are concave and barrel-shaped - when they are convex.
Pincushion distortion, normal image and barrel distortion
Of course, in practice, the image rarely takes such ugly forms as in the diagram. More a real example The photo at the beginning of the article with a slight barrel-shaped distortion can serve as an effect.
First of all, distortion is visible on zoom lenses, and the greater the zoom factor, the more noticeable it is. Usually in the wide-angle position you can observe the “barrel”, and in the body - “cushion”. Between the extreme positions of the lens, the flaws in the optics become less noticeable. In addition, the level of distortion may also change from the distance to the object, in some cases a close object may be affected, and a distant one will turn out fine in the photo.
Chromatic aberration
The second type of optical distortion that we will consider is chromatic aberration, you can often see the abbreviated “XA”. Chromatic aberrations are caused by decomposition white light into color components, which is why the object in the picture has a little different sizes in different colors and as a result, colored contours appear along its edge. Often invisible in the center of the frame, they become visible on objects closer to the edges of the image. XA do not depend on the value of the focal length, nor on the aperture, but more often and more strongly manifest themselves again in zoom lenses. This is due to the need to introduce into the optical scheme additional elements to eliminate the effect, which is much more difficult for zoom lenses than for primes.
In the picture on the left, CAs are especially noticeable on the hair (purple outline) and on the window grille (turquoise).
It cannot be said that chromatic aberrations greatly spoil the picture, but on contrasting objects, especially in backlighting, they become very noticeable and quite striking.
Vignetting
The last point is vignetting, in other words, darkening the areas at the edges of the frame. It can usually be seen on wide-angle lenses at their widest aperture. This effect is quite rare.
Do not confuse vignetting caused by optics flaws and appearing due to additional accessories. In the picture above, the edges turned out black due to several fairly thick filters wound onto the lens. A similar effect can be obtained when screwing on a long hood.
Initially, all optical distortions are directly dependent on the class and type of optics you use. Expensive lens series have complex schemes lens arrangement and many additional elements, which minimizes such undesirable effects. Cheaper lenses, especially zooms, are much more prone to these problems due to the simplification of their design.
I hasten to disappoint readers, there are simply no lenses completely devoid of the above problems. To one degree or another, even expensive models of optics with a fixed focal length still distort the picture, although this is noticeable mainly at the edges of the frame. The good news is that for the most part these effects do not spoil the picture very much and can be eliminated programmatically quite easily (we will talk about this in the next article). In addition, on cameras with a non-full-format matrix, and these are all amateur DSLRs, the edges of the image are cut off in any case, and when using good optics, visible distortions are minimal.
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Photographic lens aberrations are the last thing a beginner photographer should think about. They have absolutely no effect on artistic value your photos, and their impact on the technical quality of the images is negligible. Nevertheless, if you do not know what to do with your time, reading this article will help you understand the variety of optical aberrations and how to deal with them, which, of course, is priceless for a real photo erudite.
Aberrations of an optical system (in our case, a photographic lens) is an imperfection of the image, which is caused by the deviation of light rays from the path they should follow in an ideal (absolute) optical system.
Light from any point source, passing through an ideal lens, should form an infinitesimal point on the plane of the matrix or film. In fact, this, of course, does not happen, and the point turns into the so-called. stray spot, but optical engineers who develop lenses try to get as close to the ideal as possible.
There are monochromatic aberrations, which are equally inherent in rays of light with any wavelength, and chromatic, depending on the wavelength, i.e. from color.
Coma aberration or coma occurs when light rays pass through a lens at an angle to the optical axis. As a result, the image of point light sources at the edges of the frame takes the form of asymmetric drops of a drop-like (or, in severe cases, comet-like) shape.
Comic aberration.
Coma can be noticeable at the edges of the frame when shooting with a wide open aperture. Because aperture reduces the amount of light passing through the edge of a lens, it generally eliminates coma aberrations as well.
Structurally, coma is fought in much the same way as with spherical aberrations.
Astigmatism
Astigmatism manifests itself in the fact that for an inclined (not parallel to the optical axis of the lens) beam of light, the rays lying in the meridional plane, i.e. the plane to which the optical axis belongs are focused in a different way from the rays lying in sagittal plane, which is perpendicular to the meridional plane. This ultimately leads to an asymmetric stretching of the blur spot. Astigmatism is noticeable at the edges of the image, but not in its center.
Astigmatism is difficult to understand, so I will try to illustrate it on simple example. If we imagine that the image of the letter BUT located at the top of the frame, then with the astigmatism of the lens it would look like this:
meridional focus. |
sagittal focus. |
When trying to reach a compromise, we end up with a universally unsharp image. |
Original image without astigmatism. |
To correct the astigmatic difference between the meridional and sagittal foci, at least three elements(usually two convex and one concave).
Obvious astigmatism in a modern lens usually indicates the non-parallelism of one or more elements, which is an unambiguous defect.
By the curvature of the image field is meant a phenomenon characteristic of very many lenses, in which a sharp image flat The object is focused by the lens not on a plane, but on a certain curved surface. For example, many wide-angle lenses have a pronounced curvature of the image field, as a result of which the edges of the frame are focused, as it were, closer to the observer than the center. For telephoto lenses, the curvature of the image field is usually weakly expressed, and for macro lenses it is corrected almost completely - the plane of ideal focus becomes really flat.
The curvature of the field is considered to be an aberration, because when photographing a flat object (a test table or a brick wall) with focus on the center of the frame, its edges will inevitably be out of focus, which can be mistaken for lens blur. But in real photographic life, we rarely encounter flat objects - the world around us is three-dimensional - and therefore I tend to consider the field curvature inherent in wide-angle lenses more as their advantage than disadvantage. The curvature of the image field is what allows both the foreground and background to be equally sharp at the same time. Judge for yourself: the center of most wide-angle compositions is in the distance, while closer to the corners of the frame, as well as at the bottom, are the foreground objects. The curvature of the field makes both sharp, saving us from having to close the aperture too much.
The curvature of the field made it possible, when focusing on distant trees, to get sharp blocks of marble at the bottom left as well.
Some blurring in the sky and on the far bushes on the right did not bother me much in this scene.
However, it should be remembered that for lenses with a pronounced curvature of the image field, the auto focus method is unsuitable, in which you first focus on an object closest to you using the central focus sensor, and then recompose the frame (see "How to use autofocus"). Since the subject will then move from the center of the frame to the periphery, you risk getting front focus due to the curvature of the field. For perfect focus, you will have to make the appropriate adjustment.
distortion
Distortion is an aberration in which the lens refuses to portray straight lines as straight. Geometrically, this means a violation of the similarity between the object and its image due to a change in the linear increase in the field of view of the lens.
There are two most common types of distortion: pincushion and barrel.
At barrel distortion linear magnification decreases as you move away from the optical axis of the lens, causing straight lines at the edges of the frame to curve outward and the image to appear convex.
At pincushion distortion linear magnification, on the contrary, increases with distance from the optical axis. Straight lines curve inward and the image appears concave.
In addition, complex distortion occurs, when the linear increase first decreases as you move away from the optical axis, but closer to the corners of the frame it starts to increase again. In this case, straight lines take the form of a mustache.
Distortion is most pronounced in zoom lenses, especially with high magnification, but is also noticeable in lenses with a fixed focal length. Wide-angle lenses tend to tend to have barrel distortion (fisheye or fisheye lenses are an extreme example of this distortion), while telephoto lenses are more likely to have pincushion distortion. Normal lenses tend to be the least affected by distortion, but only good macro lenses correct it completely.
Zoom lenses often exhibit barrel distortion at the wide end and pincushion distortion at the tele end of the lens at a near-distortion-free mid-focal range.
The degree of distortion can also vary with focusing distance: with many lenses, distortion is obvious when focused on a nearby subject, but becomes almost invisible when focusing at infinity.
In the 21st century distortion is not big problem. Almost all RAW converters and many graphic editor allow you to correct distortion when processing photographs, and many modern cameras and do it on their own at the time of shooting. Software correction of distortion with the proper profile gives excellent results and nearly does not affect image sharpness.
I also want to note that in practice, distortion correction is not required so often, because distortion is noticeable naked eye only when there are deliberately straight lines along the edges of the frame (horizon, walls of buildings, columns). In scenes that do not have strictly rectilinear elements on the periphery, distortion, as a rule, does not hurt the eyes at all.
Chromatic aberration
Chromatic or color aberrations are caused by the dispersion of light. It is no secret that the refractive index of an optical medium depends on the wavelength of light. At short waves the degree of refraction is higher than that of long ones, i.e. Blue rays are refracted by the lens of the objective more than red. As a result, images of an object formed by rays of different colors may not coincide with each other, which leads to the appearance of color artifacts, which are called chromatic aberrations.
In black and white photography, chromatic aberrations are not as noticeable as in color, but, nevertheless, they significantly degrade the sharpness of even a black and white image.
There are two main types of chromatic aberration: position chromatism (longitudinal chromatic aberration) and magnification chromatism (chromatic magnification difference). In turn, each of the chromatic aberrations can be primary or secondary. Also, chromatic aberrations include chromatic differences in geometric aberrations, i.e. different severity of monochromatic aberrations for waves of different lengths.
Position chromatism
Positional chromatism, or longitudinal chromatic aberration, occurs when light rays of different wavelengths are focused in different planes. In other words, blue rays focus closer to the rear principal plane of the lens, and red rays focus farther than Green colour, i.e. blue is in front focus, and red is in back focus.
Position chromatism.
Fortunately for us, the chromatism of the situation was learned to be corrected back in the 18th century. by combining converging and divergent lenses made of glasses with different refractive indices. As a result, the longitudinal chromatic aberration of the flint (collective) lens is compensated by the aberration of the crown (diffusing) lens, and light rays with different wavelengths can be focused at one point.
Correction of position chromatism.
Lenses in which position chromatism is corrected are called achromatic. Almost all modern lenses are achromats, so you can safely forget about the chromatism of the position today.
Chromatism magnification
Magnification chromatism occurs due to the fact that the linear magnification of the lens differs for different colors. As a result, images formed by beams with different wavelengths have slightly different sizes. Since the images different color are centered along the optical axis of the lens, magnification chromatism is absent in the center of the frame, but increases towards its edges.
Zoom chromatism appears at the periphery of an image as a colored fringe around objects with sharp contrasting edges, such as dark tree branches against a bright sky. In areas where such objects are absent, the color fringing may not be noticeable, but the overall clarity still falls.
When designing a lens, magnification chromatism is much more difficult to correct than position chromatism, so this aberration can be observed to one degree or another in quite a lot of lenses. This is especially true for high magnification zoom lenses, especially at wide angle.
However, magnification chromatism is not a cause for concern today, as it can be easily corrected by software. All good RAW converters are able to eliminate chromatic aberration in automatic mode. In addition, more and more digital cameras are equipped with aberration correction when shooting in JPEG format. This means that many lenses that were considered mediocre in the past can now provide quite decent image quality with the help of digital crutches.
Primary and secondary chromatic aberrations
Chromatic aberrations are divided into primary and secondary.
Primary chromatic aberrations are chromatisms in their original uncorrected form, due to varying degrees refraction of rays of different colors. Artifacts of primary aberrations are colored in the extreme colors of the spectrum - blue-violet and red.
When correcting chromatic aberrations, the chromatic difference at the edges of the spectrum is eliminated, i.e. blue and red beams begin to focus at one point, which, unfortunately, may not coincide with the focus point green rays. In this case, a secondary spectrum arises, since the chromatic difference for the middle of the primary spectrum (green rays) and for its edges brought together (blue and red rays) remains not eliminated. These are the secondary aberrations, the artifacts of which are colored in green and magenta.
When talking about chromatic aberrations of modern achromatic lenses, in the overwhelming majority of cases they mean precisely the secondary magnification chromatism and only it. Apochromats, i.e. lenses that completely eliminate both primary and secondary chromatic aberrations are extremely difficult to manufacture and are unlikely to ever become mass-produced.
Spherochromatism is the only noteworthy example of chromatic difference in geometric aberrations and appears as a subtle coloration of out-of-focus areas in the extreme colors of the secondary spectrum.
Spherochromatism occurs because the spherical aberration discussed above is rarely corrected equally for rays of different colors. As a result, patches of blur in the foreground may have a slight purple border, and in the background - green. Spherochromatism is most characteristic of high-aperture telephoto lenses when shooting with a wide open aperture.
What is worth worrying about?
It's not worth worrying. Everything you need to worry about, your lens designers have most likely already taken care of.
There are no ideal lenses, since correcting some aberrations leads to the enhancement of others, and the designer of the lens, as a rule, tries to find a reasonable compromise between its characteristics. Modern zooms already contain twenty elements, and you should not complicate them beyond measure.
All criminal aberrations are corrected by the developers very successfully, and those that remain are easy to get along with. If your lens has any weak sides(and such lenses are the majority), learn to bypass them in your work. Spherical aberration, coma, astigmatism and their chromatic differences are reduced when the lens is stopped down (see "Choosing the optimal aperture"). Distortion and magnification chromatism are eliminated during photo processing. The curvature of the image field requires extra attention when focusing, but is also not fatal.
In other words, instead of blaming the equipment for imperfections, the amateur photographer should rather start improving himself by thoroughly studying his tools and using them in accordance with their merits and demerits.
Thank you for your attention!
Vasily A.
post scriptum
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