Coma causes rays from an off-axis point of light to create a comet-like or seagul-like blur directed away from the center of the frame (optical axis). The image produced is sharp in the center of the field, but increasingly blurred towards the corners.
The aberration gets its name from the resemblance of edge blur to the shape of a comet tail. It is caused by differences in magnification accross the field preventing all the rays from one point to refocus into a single point. Each ray takes a different path and falls on the sensor in the comet-like shape. In some cases, rays at the periphery arrive at the image plane closer to the axis than light rays coming from the center of the lens. This is called negative coma. In others, peripheral rays get focused farther down the axis and create a larger image. This is positive. Depending on this positive or negative aberrration, the tail shape will point toward the center or away.
Coma is particularly severe in misaligned lenses. Asymetrical coma (stronger in one corner of the frame than in others) is a sure sign of decentering or some other fault with aligement.
Coma is one of the most problematic aberrations because of the asymmetry it produces in blurs. In scientific applications it renders measurements of positions impossible and in artistic photography, it creates ugly shapes on highlights.
Since “normal” photography rarely requires excellent coma correction, the level correction is not directly related to the price of the lens (see this account, for example). So, if coma correction is important to you, it’s best to test for yourself.
Coma produces the troublesome bird-like images seen above. The same picture is presented below at 100%. The effect is also visible on the moon. This picture also illustrates the most extreme case of purple fringing I was able to produce with this lens (notice also the green fringe on the moon).
Bokeh can also take on a strange character.
As a fun experiment, find one of these old thick-domed magnifying glasses and let the sun shine through it onto a flat surface (do NOT look at the sun through it!!!). As you tilt the magnifier, you will see coma form on the surface.
Coma is best seen in pinpoint highlights on dark backgrounds. Starry skies are a prime testing ground and astrophotographers are the most likely to encounter this aberration and find it a problem. But the sun shining through leaves or specular highlights can be good test subjects.
Make a sharp exposure of these highlights and examine their shape. Resist the temptation of using a microcope to pixel-peep. I’ve yet to test a lens that doesn’t show traces of coma … All you need to ensure is that the level you encounter is not too severe for your intended use.
Stopping down the lens prevents the rays from entering from the periphery of the lens, so it reduces coma.
There is no post-processing miracle to cure coma once it has set in.
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