Editing Lens (section)

=== Coma ===
{{Main|Coma (optics)}}

''Coma'', or ''comatic aberration'', derives its name from the [[comet]]-like appearance of the aberrated image. Coma occurs when an object off the optical axis of the lens is imaged, where rays pass through the lens at an angle to the axis {{mvar|θ}}. Rays that pass through the centre of a lens of focal length {{mvar|f}} are focused at a point with distance {{math|''f'' [[Tangent function|tan]] ''θ''}} from the axis. Rays passing through the outer margins of the lens are focused at different points, either further from the axis (positive coma) or closer to the axis (negative coma). In general, a bundle of parallel rays passing through the lens at a fixed distance from the centre of the lens are focused to a ring-shaped image in the focal plane, known as a ''comatic circle'' (see each circle of the image in the below figure). The sum of all these circles results in a V-shaped or comet-like flare. As with spherical aberration, coma can be minimised (and in some cases eliminated) by choosing the curvature of the two lens surfaces to match the application. Lenses in which both spherical aberration and coma are minimised are called ''bestform'' lenses.

[[File:lens-coma.svg|frameless]]