All About Chromatic Aberration

If you are an amateur, and you heard somewhere that some lens gives strong CA (color aberration, which is how they are most often called for short), and then you looked at the photo for a long time and did not see anything - do not get upset. Nowadays the struggle over CA is in full swing. Nowadays high-quality optics is very good at fighting with CA, so it is really hard to see them.

There are axial, spherical, oblique, and other kinds, but they all boil down to the distortion of a picture.

The main point of chromatic aberration definition comes from dispersion, simply put, when a white beam splits into its spectrum - this is due to the fact that rays with various wavelengths of light (different colors) are refracted at various angles as they pass through the lens. Just remember a physics lesson about the refraction of rays in a prism, and a lens is two prisms connected by bases. You get a layered picture. Often CA adds different color spots and stripes, it is especially pronounced in the transition of contrasting objects, often cited as an example with trees. Of course, there are several lenses (and it is lenses that give CA), which still strongly suffer from this malaise. But as the saying goes: optimists invent the airplane, pessimists - a parachute, so you have to fight CA to get a good picture.

Types of Chromatic Interference

How to define chromatic aberration? Very often CA is pronounced in the in-front or behind-the-focus point. Popular aberrations, usually in the fuzzy area, are called 'fringing around'. Transitions in the area in front of the sharpness area are colored purple, and out of the sharpness area, they are colored dark green.

A spherical aberration example is the blurring of boundaries between contrasting elements in the sharpness area of a photo. For example, above, the model's white shirt flows smoothly into the dark background, erasing the border. Monoculars suffer greatly from spherical aberration. Sometimes people confuse purple and red halos in a photo with CA. Halos are simply parasitic glare from a bright source in the frame. This is why this type of optical interference can be red, magenta, yellow, and any other hue. We are talking about the dispersion of light, which Newton discovered back in 1672. One of the simplest and most understandable phenomena of dispersion is the rainbow or glare of light playing in cut gemstones.

Impact of Chromatic Interference

Distortion is a type of lateral chromatic aberration, where the unnaturally convex or concave-looking contours of objects are not the photographer's artistic intent. In the first case (with inwardly concave lines) this distortion is called "barrel distortion", in the second case (with convex lines) it is called "cushion distortion".    

Distortions are caused by the change in linear magnification provided by the optics across the field of the frame. As we said, light rays passing through the lens merge at a point large distance from the lens, as opposed to those rays passing around the edges. Pincushion distortion is usually caused by using the maximum zoom of the camera, while barrel distortion is caused by using the minimal zoom. Photographers using wide-angle lenses encounter such distortions most often.

Cometic Interference

A chromatic aberration in the lenses, which affects only the light rays passing through the lens at an angle, is called a cometic (or simply a coma). In the photos, a coma can be seen in the "tails" of the comet stretching behind the single points of the image. The "tail" of such a comet can be directed toward the center of the photo (negative coma) or its edge (positive coma). The latter distortion is more noticeable the closer the image point is to the edge of the frame. Rays of light enter clearly through the center of the lens and are not objects to comatic aberration.    

Most geometric aberrations can be corrected by interacting with the aperture - by adjusting it. By narrowing the aperture the photographer at the same time reduces the amount of light that reaches the edges of the lens. However, this should be used with extreme caution because excessive aperture narrowing leads to an increase in the amount of diffraction.

Diffraction

Diffraction is chromatic aberration meaning an optical system effect that limits the detail of a photograph regardless of the set resolution. Diffraction is caused by the scattering of light as it passes through the aperture. Many novice photographers, to increase the depth of field, cover the aperture so much that the sharpness achieved by this manipulation is completely outweighed by the diffraction smoothing effect.

This effect is called the diffraction limit in the photo world. Knowing its limits makes it possible to avoid image detail problems. Anyone can calculate the diffraction limit.

How to Avoid Chromatic Interference? 4 Best Tips

When choosing a camera, remember that even chromatic lenses with no aberration simply do not exist. At least not until today. Even the most "premium" optics has some image distortions. Because no matter how hard lens makers fight, automatic correction of one type of aberration leads to the strengthening of the second and third ones, etc., making the process of lens improvement endless. However, to be considered a good photographer you don't have to wait for the impossible - the invention of the perfect lens. All you have to do is study the characteristics of a particular lens (your own "tripod" in particular) and correct its flaws with your own hands - using all your skill. In the next section, we will tell you the easiest way to avoid aberrations.

1. Avoid high-contrast shots. Chromatic aberrations result from the high-contrast scenes. In particular, if the light source is behind the subject, especially in photos taken against a white background, landscape photos were taken at sunrise. In such situations, you should try to recompose the scene you are shooting. Choose a different backdrop that is more in contrast with your subject, or wait for more suitable lighting conditions. If the scene you are shooting needs to be captured in this way, or there is simply no other way to shoot, then switch to RAW format, and you will have to make adjustments in post-processing the final photo.
2. Control the focus length of the zoom lens. It's nice to have a zoom lens with a wide range of focal lengths, but the reality is that CA shows up on the short and long ends of the focal length. So being able to choose a focal length that is close to the average focal length usually helps remove the issue of visible lens chromatic aberration. An example of chromatic aberrations, photographing with a focus lens at wide focal lengths results not only in but also in various other defects in the image. Consider shooting a panorama with several frames at a focal length that doesn't exhibit visible CA, and then merge the images in the graphic editor.
3. Increase the numerical value of the aperture. As a rule, "squeezing" the aperture helps reduce noticeable defects in most lenses, including CA. Remember that you need to set up hight the shutter speed or ISO value to compensate for light loss.
4. Position the subject closer to the center of the image. Longitudinal chromatic aberration becomes more pronounced as you move from the center of the frame to the edges. This is mainly due to the uneven curvature of the lens. Positioning the main subject closer to the middle of the frame can help reduce or even completely eliminate CA, and post-process the frame to compose it.

It is also worth mentioning that in many modern cameras CA is corrected automatically. The correction is performed by the camera's processor while converting the file to JPEG and does not decrease the sharpness of the photo.

Let's look at a few examples

Here you can see some examples. At a glance, it is clear what CA is.

Fix CA in Luminar Neo

What does chromatic aberration do? A color fringing, halo, or line around an object. It turns out that to remove this optical distortion, you should find it and discolor it. This is the principle of chromatic aberration removal. Here's how you can do it in Luminar Neo:

• Open the image;
• Create a duplicate layer. To do that, in the Layers/Layer palette, right-click on the layer and choose Duplicate Layer;
• Select the duplicate you just made. Go to Filter/Filter - Blur - Gaussian Blur. The blur radius depends on the size of the file and is chosen experimentally. But you can go for values from 2 to 7 pixels;
• Apply the blur. After that, set the blur layer to Color/Color blending mode. Don't confuse it with Color/Hue.

This method will not work in all cases of chromatic aberration, it is much easier to avoid them at the stage of photography than to correct them in the photo editor. After all, suppressing the source of the aberration leads to strengthening the other one.

Conclusion

It is also recommended to just remember that CA will define with high contrast elements in the frame (sun and sky, trees and sky, dark and light areas), when you take a photo of a really high contrast element, just remember that the photo will need to be refined if there are CA.

Most of the CA appear at an open aperture with wide angle and fast lenses, therefore CA is visible almost on all cheap and mid-priced lenses at a fully open aperture (in certain conditions).

Telephoto lenses have the most CA at the long end (maximum permissible focus at the different positions).

A very easy (but not quite correct) way to get rid of CA is to make the photo black and white.

If you want to get rid of CA completely in direct photography, mirror-lens lenses, which do not have them at all, can help.