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What is chromatic aberration?
Answer provided by Brian Koch. Photo by Gene Tatroe.
Chromatic aberration is a distortion caused by a lens that does not focus all of the different light colors to the same point. This is due to the lens having different refractive indices for the different light wavelengths:
Figure 1: Chromatic aberration occurs when different wavelengths do not focus at a single point.
Sometimes this is referred to as "purple fringing." However, not all purple fringing is the result of chromatic aberration, sometimes it is due to sensor problems in digital cameras. Figure 2 shows areas in a photograph where chromatic aberration
is evident:
Figure 2: Example photograph that shows chromatic aberration.
To reduce the effects of chromatic aberration, several different pieces of glass with varying refractive indexes are used.