Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red-blue or red-green colors, but can also be perceived with red-grey or blue-grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration.
Usually it is observed by using a target with red and blue bars and an achromatic background. Positive chromostereopsis is exhibited when the red bars are perceived in front of the blue and negative chromostereopsis is exhibited when the red bars are perceived behind the blue.
Studies have determined that all wavelengths of light are focused on the retina, the difference being that the shorter wavelengths are focused more toward the nose while the longer ones are focused more toward the ear. This mimics ordinary depth perception, in which closer objects are focused more toward the ear and farther ones more toward the nose.
In addition to causing a depth effect, chromostereopsis can also be annoying and hard on the eyes. It is fatiguing.