Chromatic Aberration

Chromatic aberration happens because glass bends different colors of light by slightly different amounts, so red, green, and blue rays do not all land on the same focal point. The visible result is a fringe of color, often purple or green, along high-contrast edges such as a dark target against a bright sky. This fringing robs the image of crispness and makes it harder to resolve fine detail at the limits of your magnification.

Optical designers fight the problem with multi-element lens groups and with low-dispersion glass, where an ED-glass element corrects much of the color error. The treatment on the objective lens and other surfaces also matters, since a good lens coating improves contrast and transmission so the residual fringing is less distracting. When you compare one scope to another in harsh light, the amount of color fringing is one honest measure of the glass quality.