Chromatic Aberration

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Chromatic aberration is probably most easily understood as a prism effect. When light shines through a prism, you see that different colors of light bend differently and create a rainbow effect. When light shines through any lens, including glasses and the natural lens of your eye, the same occurs to a lesser degree. The effect of this is that the Focal length of the lens is not exactly the same for all colors of light.

Duochrome test

KH HK
EPO OPE

The chromatic aberration expected during a normal eye exam is about a half diopter difference between red and green. This can be a handy test to dial in a final prescription by looking at text on red and green backgrounds and seeing which is clearer. If the green side is clearer, a quarter diopter of sphere is added, if the red side is clearer, a quarter diopter is removed. If the optometrist is trying to introduce a small amount of Myopic defocus he/she wants the red side clear.

External Sources