Definition: Aberration usually seen when point of light far from centre of the image is projected as a comet-shaped blur of light, usually with the head of the 'comet' pointing away from the centre of the image. * Oblique rays from an off-axis object produce an asymmetric patch of light resulting from increase in magnification in the direction away from the optical axis towards the periphery. * Difficult to correct, but symmetrical designs may be effective.
The last coma
Stopping down the lens aperture reduces coma as the smaller aperture vignettes off the oblique rays that are the cause. Unfortunately this is no help in the very instances coma is a problem, namely in photographing lights at night. The black background in these situations makes comatic defects all too apparent -- but the last thing you want to do is to stop down.

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