Visual System - Chromostereopsis

The cornea and lens refract/bend different wavelengths of light at different distances/depths within the eye. The eye focuses medium wavelengths (green) best. Long wavelengths (red) are focused beyond the retina. Short wavelengths (blue) are refracted more strongly and, as a result, are focused in front of the retina. The lens of the eye must become more convex to focus on "red" and less convex to focus on "blue." This chromatic aberration of the eye can create depth or stereoscopic effects, as can be seen in the example below.

For most individuals viewing the above example, "red" will appear closer. However, thirty percent (30%) will perceive "blue" as closer, and ten percent (10%) will perceive both as being in the same plane. This example illustrates why "red" and "blue" primaries should never be used one on the other in displays. This also reinforces why pure "blue" should not be used for text or fine details. Especially to be avoided are pure "blue" text, lines, or symbols on a "black" background, particularly, if there is "red" or "white" nearby to create focusing conflicts.