Chromatic aberration - what is it and how to avoid it? Chromatic aberrations as an artistic device

You may have heard more than once from photographers that you have a critical element in your photographs. "chromatic aberration"? And the words sound as if it is a disease. Let's look at what this term means and how to remove aberrations from photos, if necessary, using advanced technology.

And although modern lenses and cameras are designed with technology aimed at reducing such glitches, the fact remains that they still pop up in our photographs. Read below to learn how you can correct chromatic aberrations in Photoshop.

What does chromatic aberration look like in photography?

Here is an image that may serve good example chromatic aberration. Part of the image is slightly out of focus, resulting in some loss of detail. Notice the blue halo on the left side of the image and similar colored spots scattered throughout the area in different areas.

The lens can change the passage of light, the lens affects the frequency when light passes through the lens, different wavelengths (colors) move at different speeds and are fixed in different places on the sensor. Sometimes, the inappropriate use of a lens not intended for a particular shooting (eg macro) can cause chromatic aberrations. As a result, red, green and blue do not converge in the right point and create a display of channels that do not match.

There are, of course, more technical description what chromatic aberrations are, but for our purposes such a simple explanation is quite enough to understand how chromatic aberrations affect the image and how to deal with them.

This animation clearly shows how the image shifts. Each channel can be quite sharp, but when combined, the image quality degrades. It may be possible to correct the images using post-processing programs. For example, Photoshop.

Elimination of CA in an advanced way

Open the image with HA. This method is based on correcting aberrations by tuning and aligning the channels to match each other. This can be a problem since the light tends to speed up at different points in the lens, creating more distortion in some areas, but we'll try to work around it.

Start your work by making a copy of the image. We'll be making most of our adjustments on this layer, but we'll also need the original background image when we're done.

Let's start by adjusting the green channel to match the red channel. The frequency of light speeds up as light moves from red to green to blue, so we'll adjust the green and blue channels to match the slower frequency of red. So. Select the green channel. You should make sure that the blue channel is turned off so that you can see how the green and red channels overlap.

With the green channel selected, press Ctrl+A to select everything and then press Ctrl+T to free transform.

The channel must be carefully transformed to match the outer edges. When you're done, press Enter and then Ctrl+D to get rid of the selection. Don't worry if some aberrations remain on part of the image. We'll fix this later.

The same goes for adjusting the blue. Turn off the green channel, then select and turn on the blue channel as shown in the figure.

Make a similar adjustment to the blue channel: select everything with Ctrl+A, and then transform the channel with Ctrl+T. When you're done and your picture clearly etches the outer edges, press Enter to make the transformation.

When you return to the combined RGB channels, you will see that significant aberrations in the photo have been eliminated. Such a “repair” is not perfect, part of the image will be somewhat blurred as a result of your actions, so you need to continue working and eliminate the remaining shortcomings.

Repair blur from channel shift

You can different ways eliminate the blur resulting from the fight against aberrations, but we offer the most optimal method, in our opinion, that will help eliminate blurriness.

Create a copy of the main layer again.

Select the green channel, select all the image points in it using Ctrl+A before you sharpen the entire image. Repeat the same with the blue channel, following only the semantic center of the image.

Create an image mask. Use a brush to blend the mask and image together. When you're done you have a layer made up of two sublayers, create a new mask for the group. With its help, you can return any of the areas you are interested in to return to the original image.

Remove jagged edges caused by channel conversion.

Repeat this procedure as many times as you feel is necessary to correct the aberrations to a level that appears visually optimal to you. It may even take several channel changes to get the image you're happy with. Is there a better, simpler, and more convenient way to deal with chromatic aberration? Tell us about it!

Translation by S. Zavodov

If you are an amateur and have heard somewhere that some lens gives strong HA(chromatic aberrations, which is what they are most often called for shorthand), and then you looked at the photo for a long time and still didn’t see anything - don’t be upset. Nowadays, the struggle over CA is in full swing. Nowadays, high-quality optics are very good at combating CA and therefore it is really difficult to notice them.

There are axial, spherical, transverse and other types, but they all boil down to distortion of the picture.

Main essence of chromatic aberration comes from dispersion, in other words, when a white beam is decomposed into its spectrum - this is due to the fact that rays with different lengths waves ( different color) are refracted at different angles as they pass through the objective lenses. Suffice it to recall the lesson in physics with the refraction of rays in a prism, and a lens is two prisms connected by bases. It turns out to be a layered picture. HA often adds different color spots and stripes, this is especially clearly expressed in the transition of contrasting objects; the example of trees is often given.

Below is a photo filled with aberrations.

Of course, there are a number of lenses (namely, lenses that give CA), which are still suffer greatly this illness. But as they say: optimists invent the airplane, pessimists invent the parachute, so you need to fight CA in order to get high-quality images.

Chromatic aberrations. Freezing

Very often, CAs are pronounced in the blur zone. Popularly, aberrations, usually in the blur zone, are called fringing (‘Purple fringing’). Transitions in the area in front of the sharpness zone are colored violet (magenta), and behind the sharpness zone - green. This is clearly seen in the example above. Many lenses have aberrations precisely in the blurred zone.

Spherical aberration is the blurring of boundaries between contrasting elements in the sharpness zone of a photo. Example above, White shirt the model smoothly flows into the dark background, erasing the border. Monocles suffer greatly from spherical aberrations. More details in the section.

How to be? How to treat a lens for this?
Modern lenses try to use low disperse elements. Nikon denotes them as ED(Extra-Low Dispersion - super small dispersion). These elements provide less refraction for rays of different wavelengths (for rays of different colors).

They have also now started to use aspherical lenses, which allow you to get the final picture with less CA.
Modern advanced cameras have automatic control of aberrations - I don’t know how modern cameras do this (most likely using special algorithms), but they are excellent at removing aberrations.

I also recommend just remembering that aberrations appear when there are highly contrasting elements in the frame(sun and sky, trees and sky, dark and light areas), when photographing a really high contrast element, just remember that the photo will need to be improved if HAs appear.
CA appears most at an open aperture when used on wide-angle and fast lenses, because HA visible on almost all cheap and mid-range lenses when the aperture is wide open (under certain conditions).

Telephoto lenses have the most CA at the long end (the maximum allowable focal length).
A very easy (but not entirely correct) way to get rid of CA is to make the photo black and white.

If you need to completely get rid of CA during direct photography, then mirror-lens lenses will help with this, in which they are completely absent; an example of such a lens can be.

Thank you for your attention, Arkady Shapoval

Chromatic aberration in photography is the last thing you should worry about. Chromatic aberration is a property of the lens, so you won't affect it in any way. And in order not to torment yourself with doubts every time, it is best to properly understand what it is.

Aberrations, or distortions, are geometric and chromatic (color). With geometric ones everything is very simple, you can and should fight them if it is not an artistic idea. In modern lenses, geometric aberration, also called distortion, is largely corrected by an “aspherical” element. That is, a lens that does not have a spherical profile, but a more complex one:

You can successfully correct residual distortion in the editor. But with chromatic aberration everything is a little more complicated. The nature of chromatic aberration in glass dispersion. Dispersion is different refractive indices for different wavelengths:

It is clearly seen from the figure that the rays different colors focused in different planes on the optical axis. This is the first type of chromatic aberration, also called “axial aberration”. Axial aberration is stronger when the aperture is open, leading to blurred images. This has already been discussed in the article about how to take a sharp photograph. By decreasing the aperture you get rid of axial aberration, this is one of the reasons why it is not recommended to photograph at the maximum aperture. Axial aberration cannot be corrected in the editor, so manufacturers provide the lens the whole system“low-dispersion elements”, which in total compensate for aberrations from other lenses. You've probably seen a picture like this and wondered why there are so many elements:

Yellow elements are low-disperse, fighting chromatic aberration. Blue ones are aspherical, helping against distortion.

Low-dispersion elements are located both individually and as part of an achromatic lens:

The point is that the achromatic lens is glued together from two types of glass: crown and flint. Crown with low dispersion, but also low refractive index. Flint is the opposite - the refractive index and dispersion are greater.

In addition to axial aberration, there is “transverse aberration.” This is focusing different colors of rays at different points on the sensor. Transverse aberration does not depend on the aperture; it is useless to fight it with settings, but it can be corrected in the editor. Whether you need it is another matter. Modern lenses are so good that I had a hard time finding an example of a photograph where chromatic aberration was noticeable.

If you look closely at the trees in the title photo, you can see a colored border around the contrasting contours of the branches. This is chromatic aberration:

In the latest generation cameras, chromatic aberrations are automatically corrected by the processor at the JPEG conversion stage; I had to ask Kostya for a photo, since his old D40 knows nothing about aberrations. That’s why I never shoot in RAW, my Nikon D700 camera does everything for me and saves me a lot of precious time, it just needs to be configured correctly.

In any case, you're better off thinking about what you're photographing, rather than how susceptible your lens is to aberrations. The lens contains no noticeable aberrations a large number of expensive low-dispersion and aspherical elements, therefore it costs significantly more than its counterparts with aberrations. In addition, a variable focal length (zoom) design is much more complex than a fixed one, since aberrations appear differently at different focal lengths. This is why primes are better quality and cheaper than zooms.

The presence of low-disperse elements in Nikon lenses marked with the letters ED. If you see ED in the lens markings, then you can forget about chromatic aberrations.

Thank you for your attention.

, telescope, etc.). In this case, white light is decomposed into its constituent colored rays, as a result of which images of an object in different colors do not coincide in image space.

In addition, chromatic aberrations include chromatic differences in geometric aberrations (see below).

Chromatic aberration leads to a decrease in image clarity, and sometimes also to the appearance of colored contours, stripes, spots on it that are not present in the object.

Chromatism of position

Optical glass and other optical materials have dispersion, meaning the refractive index is different for rays of different colors.

The chromatism of the position causes significant blurring of the image, therefore, when shooting in black and white with a monocle and periscope, in which it is not eliminated, after setting it to sharpness, you need to introduce an additional correction for the position of the lens relative to the photosensitive element, determined by the formula:

where is the conjugate focal length; - focal length of the monocle or periscope.

The need for correction is caused by the fact that during visual guidance, the image, due to the increased sensitivity of the eye to yellow rays, is established at their focus, and not at the focus of blue-violet rays, to which black-and-white non-sensitized photographic material is most sensitive. The latter, being out of focus, form significant circles of dispersion, reducing the sharpness of the image.

Positional chromatism can be corrected by combining a converging and diverging lens made of glasses with different dispersion. When passing through the first lens, the beam is deflected towards the optical axis and disperses; entering the second lens, it deviates slightly reverse side and redisperses, but in the opposite direction. As a result, the chromatic aberration of the first lens is compensated by the second, negative, lens, and the rays various colors will gather at one point. Such lenses that correct chromatic position are called achromatic lenses (achromats).

Achromatic lenses are used in many modern lenses. Achromatize separate element or a combination is not always necessary; it is enough that all elements as a whole compensate each other’s dispersion.

At the design stage, chromatic aberrations can also be reduced if optical elements such as lenses made of special optical glasses (Kurzflint, Langkron), mirrors or zone plates are used in the design of the optical device.

Chromatism increase

Also called chromatic magnification difference.

Chromatic aberration, in which images of the same object in rays of different colors have several different size. Does not decrease with aperture, nor does it decrease with enlargement.

For digital color images, magnification chromatism can be corrected to some extent in software. To accurately converge the three components of the image (red, green and blue), it is necessary to change the scale for two of them, leaving fixed the point where the optical axis passed (usually the center of the frame). Many RAW file converters have this function, but optical correction is preferable, since complex lenses also contain other aberrations that cannot be corrected by simple conversions and are individual for each lens model, as a result of which it becomes difficult to highlight the chromatic magnification programmatically. Good magnification chromatism correction is not possible when the lens performs poorly in backlight. Correcting magnification chromaticity on a computer improves image quality, but it is still preferable to shoot photographs with lenses that have minimal aberrations. Thus, fixed focal length lenses usually have significantly less aberration than zoom lenses.

Chromatic differences of geometric aberrations

Chromatic differences in general, each geometric aberration depending on the color. So, for example, spherical aberration can be different for blue and red rays, in this case it is called “spherochromatism”), and the chromatic difference in the aberrations of inclined beams. All this can also be considered chromatic aberration, since it gives side effects, generally similar to the chromaticism of position and magnification.

Chromatic aberrations in photography

In all modern Nikon cameras, chromatic aberrations are automatically corrected for all lenses. Canon has just begun to introduce this practice and has automatic corrections only on the 5D Mark3, and then you need to load profiles for each lens into the camera.

see also

Notes

Literature

• Volosov D. S. Photographic optics. M., “Iskusstvo”, 1971.
• Tamitsky E. D., Gorbatov V. A. Educational book on photography. M., “Light Industry”, 1976
• A short photographic guide. Under the general editorship of D.T. n. Puskova V.V., ed. 2nd, M., Art, 1953.

Wikimedia Foundation. 2010.

• Christ's Resurrection
• Chromaffin cells

See what “Chromatic aberration” is in other dictionaries:

CHROMATIC ABERRATION- (by this, see chromatism and aberration). The obscurity of the image occurs due to the fact that white light, passing through the glass, is decomposed into different colors, as a result of which different colored images of the object appear, one next to the other. Dictionary… … Dictionary of foreign words of the Russian language

CHROMATIC ABERRATION- (from the Greek chroma color), one of the main. optical aberrations systems, due to the dependence of the refractive index of transparent media on the wavelength of light (see DISPERSION OF LIGHT). X. a. appears in optical systems including elements from refractive... ... Physical encyclopedia

CHROMATIC ABERRATION- image distortion due to the fact that light rays of different wavelengths are collected after passing through the lens at different distances from it; As a result, the image is blurred and its edges are colored. There is no chromatic aberration in... ... Big Encyclopedic Dictionary

CHROMATIC ABERRATION- (from the Greek chroma color) one of the main. optical aberrations systems, due to the dependence of the refractive index of transparent media on the wavelength of light (see Dispersion of light). X. A. appears in optical systems including elements from refractive... ... Physical encyclopedia

chromatic aberration- Focusing defect, in which electrons emanating from one point at different speeds are focused at different points on the beam axis. [GOST 17791 82] Topics: electrovacuum devices... Technical Translator's Guide

CHROMATIC ABERRATION- one of the aberrations (see (2)) of optical systems, which is caused by the dispersion of white light (the dependence of the refractive index of transparent media on the light wavelength). It manifests itself in the fact that light rays of different colors are collected after... ... Big Polytechnic Encyclopedia

Chromatic aberration- one of the main aberrations of optical systems (See Aberrations of optical systems), caused by the dependence of the refractive index (See Refractive index) (RI) of transparent media on the wavelength of light (see Dispersion of light). H. a. Maybe… … Great Soviet Encyclopedia

chromatic aberration- optical systems, image distortion due to the fact that light rays of different wavelengths are collected after passing through the lens different distances from her; As a result, the image is blurred and its edges are colored. Chromatic... ... encyclopedic Dictionary

chromatic aberration- spalvinė aberacija statusas T sritis fizika atitikmenys: engl. chromatic aberration vok. chromatische Aberration, f rus. chromatic aberration, f pranc. aberration chromatique, f … Fizikos terminų žodynas

CHROMATIC ABERRATION- See aberration, chromatic... Explanatory dictionary of psychology

In the comments to one of my articles with lens tests, they noticed that there is such an important parameter as longitudinal aberrations, or in English LoCA(longitudinal chromatic aberration). This type of aberration has many names, but if you call it a purple glow around the branches of a tree in the center of the frame, then I think everyone will recognize it.

longitudinal aberrations, loCA, chromatic aberrations (Wikipedia)

Now I mentioned this phenomenon and everyone remembered it. But how often does it prevent you from getting good pictures?

However, in theory, if you are going to shoot with backlit people or the sun shining through dark branches, you will encounter it and your or your clients’ individual intolerance to violet light will continue to play a role. The glow it could be different strengths, depending on the degree of correction of longitudinal chromatic aberration in the lens.

Where does longitudinal aberration come from?

The thing is that light rays are refracted in lenses at different angles, depending on the wavelength. Blues, for example, are refracted at a greater angle than reds.

refraction of light rays in a regular lens

Accordingly, when the moment of focusing on the matrix plane comes, the light rays are focused in different places and that same violet glow appears. This is especially visible when there is a sharp transition in brightness and the light decomposes from white to all the colors of the rainbow.

How to distinguish longitudinal aberration from other chromatic aberrations

Longitudinal chromatic aberration is easy to distinguish from transverse aberration, since it occurs more often in the center of the frame and can spread throughout the entire frame. At the same time, transverse (also called magnification chromatism) appears only at the edge of the frame, although it also looks like longitudinal - purple and green halos around dark objects on a white background.

Another way to distinguish Longitudinal from Transverse aberration:

Longitudinal aberration creates a fringe of one color around an in-focus object and a different color around an out-of-focus object. Transverse produces multi-colored edges around one object in focus (usually purple and green).

Longitudinal is “treated” by diaphragming. The transverse one does not depend on it.

An example of longitudinal aberration.

Photo taken Canon EF 85/1.2 L at full aperture, the image on the left is in focus, the image on the right is slightly out of focus to produce a green fringe instead of a purple one. This is one way to “fight” this aberration if the purple border annoys you.

An example of transverse aberration from the same source.

Photo taken with lenses:
A: Cosina 3.5-4.5/19-35 @ 20 mm
B: Cosina 3.8/20
C: Carl Zeiss Distagon 2.8/21

All lenses at aperture value F11 and on camera Canon 5D

How longitudinal aberration is corrected optically

Longitudinal aberration is corrected using so-called achromats, i.e. gluing lenses, where one lens is crown and the other is flint. Kron is a lens with ultra-low light dispersion and low refractive index, and flint is the opposite.
Crowns used to be made from fluorite glass, which was quite expensive. I mention this because the lenses I am considering belong precisely to the period of use of fluorite (Calcium fluoride). Flint was made from lead oxide. Later they replaced them with cheaper types of glass with low dispersion, and lead oxide was also replaced with something else due to the unenvironmental friendliness of such production (this was already in the 2000s).

In total, they try to obtain the effect of minimizing longitudinal aberration, i.e. ultimately bringing together light rays of different wavelengths to one point. This is ideal, but in reality the ideal is impossible and small aberrations are always present.

Achromat

achromatic lens bonding (dablet)

Achromat is capable of correcting longitudinal aberration for two wavelengths.

Apochromat

Later, Apochromats were invented; this is already a triplet, allowing the correction of three wavelengths.

We will not dwell on any patent litigation for achromats and apochromats and will only present a diagram of an apochromat.

triplet Apochromat

This is how we are in simple language We figured out what longitudinal chromatic aberration is and how to combat it. Now, if you want to find a lens with the least longitudinal aberrations, you first look at the name of the lens. Achromats are now almost never found even on the secondary market (I haven’t seen many of them), but there are plenty of apochromats. They have a prefix in their name APO. Most often these are quite expensive lenses. At the same time, even in lenses without such an attachment, you can look for apochromatic triplets and the manufacturer’s mention of the use of fluorite glass - such lenses will have less longitudinal aberration in advance.

If we generally talk about optical systems and their aberrations, then chromatic and spherical aberrations are reduced in achromats. In apochromats, these same A. are compensated much more accurately. In aplanates, chromatic and spherical A. have been corrected, as well as. If, in addition to these apertures, the curvature of the field is also eliminated, then the lens is called an anastigmat. Orthoscopic systems are those with corrected distortion.

But be careful. Along with the imperfection of optical systems, their individual pattern is also eliminated.

Longitudinal aberration test for Carl Zeiss lenses

Carl Zeiss Makro-Planar 60/2.8

I took the picture at apertures F4 and F5.6 specifically so that you can clearly see that aperture in this case affects the reduction of longitudinal aberration. Although the picture becomes darker, clearer and aberrations are less noticeable, they do not completely disappear.

In this case, the lens Carl Zeiss Makro-Planar 60/2.8 exhibits moderate longitudinal aberrations. But it cannot boast of a large aperture ratio, and longitudinal aberrations are a problem with high-aperture primes.

Canon 50/2.5 Macro

I don’t dare to make a comparison here, since the sun is very blinding and with this lens I was able to get into focus much more accurately. At the end of the article there will be a test in a studio environment, where I will focus more precisely.

Carl Zeiss Planar 50/1.7

Carl Zeiss Aposonnar 200/2

Aposonnar, optical design. An achromatic doublet with a fluorite lens is indicated

and also all APO:

Tele-Apotessar 300/2.8
Tele-Apotessar 500/5.6
Tele-Apotessar 800/8

Carl Zeiss Distagon 21/2.8

The use of an achromatic doublet is declared. But perhaps not fluorite (nothing is said about it).

Studio tests for longitudinal aberration

For all 50mm lenses (including 60mm), the distance to the target is 40cm. The tilt of the optical axis of the lens relative to the target is approximately 40 degrees.

50/[email protected](left) vs 50/ [email protected]

Carl Zeiss Planar 50/ [email protected](left) vs Carl Zeiss Planar 50/ [email protected]

Carl Zeiss Planar 50/ [email protected](left) vs Carl Zeiss Makro-Planar 60/ [email protected]

Carl Zeiss Planar 135/2.8 vs Jupiter-37A 135/3.5

Jupiter-37A 135/ [email protected](left) vs Carl Zeiss Planar 135/2.8@F4

conclusions

I didn’t post further pictures because in my opinion everything is so clear. Among non-apochromats, longitudinal aberrations are quite strong, but they do not at all interfere with shooting good scenes, if you do not shoot powerful backlight, like the sun. In general, you can shoot the sun, but with well-covered apertures. There was no mysticism here. Uncorrected lenses of the same focal length have approximately the same CA. Better adjusted in my opinion Carl Zeiss 85/1.4, but when I try to use it at F1.4, its longitudinal aberrations are no less than the others, since it is placed in disadvantageous conditions. Carl Zeiss 50/1.4 too, as a rule, it is used at a fully open aperture and, accordingly, in its photographs you will more often see purple edges.

Therefore, you should try to avoid harsh backlighting for most lenses.

For all their remarkable optical characteristics, they cannot provide absolute convergence of beams in such conditions. The light source should be a little higher or lower, but not “head-on”. Or significantly covered.

For example, there is light through the fog and there are no problems.

There are also a number of fast prime lenses from less famous manufacturers, such as Samyang And Sigma. Some of them, and in particular Sigma 85/1.4 demonstrate an almost complete absence of longitudinal aberrations.

How can this be explained? The most respected manufacturers are losing out to the lesser known ones.

But in the process of digging for information on this topic, I came across interesting material, which described how to improve the image clarity of binoculars (manufacturers Zeiss, Nikon etc.). It turns out that since chromatic aberrations reduce the sharpness of the image, some manufacturers in binoculars sacrifice the red spectrum and filter it at the stage of passing through the binoculars, using glass that transmits little to the red spectrum. As a result, such a system can only focus the remaining light rays. And the fewer rays, the easier it is to focus them in one plane and, accordingly, the higher the sharpness.

Then what prevents you from installing glass that filters some parts of the spectrum into the lens and thereby reduce chromatic aberrations and increase sharpness?
These are still only guesses and require confirmation. But it all once began with guesses.
In general, this would be a dishonest tactic...

Now you need a target with a continuous color gradient and photograph it with different lenses (in particular Samyang 85/1.4) in order to establish the truth.

Happy photos everyone! :)

Transverse chromatic aberrations (lateral chromatic aberrations, axial chromatic aberrations) arise due to the fact that light rays of different wavelengths are refracted differently when passing through glass. As a result, they come out at different angles and the closer to the edge of the lens, the greater the difference in the angle of refraction. For the same reason, in the center this type of aberration is practically absent (that’s why one of the names of aberration is axial) and is strongly manifested towards the edges of the image.

They may have different colors. Most often purple-green or red-blue.

What they look like