COMA in Camera Lens
If you focus on a field of point light
sources, such as the starry sky, you may discover the effect called
coma - although the central points' image appear sharply, those out
toward the frame edge show as short lines or teardrops, radiating away
from the center; the farther away from the center, the greater the
The effect may also be visible with any
other subject that is richly detailed with a small pattern, whether
regular or irregular. An enlarger lens with coma may produce this
effect in the enlarged film grain, for instance. Coma may sometimes
appear, also, when an otherwise satisfactory lens of normal focal
length is used for close-up photography or photomicrography.
The effects of coma can generally be
reduced by closing the lens aperture. In close-up work, reversing the
lens, back to front, on the extension tubes or bellows will usually
much improve the image, and the coma effect may disappear completely.