San Fransisco in 3D

The picture consists of two photographs of the same scene, taken with differently coloured filters. They have been displaced slightly relative to each other and the picture thus becomes fuzzy.

A depth effect arises when you use the coloured spectacles with red and blue filters. It is also important that you have approximately equal sight in your two eyes.

Illusions of this type can reveal whether young children are affected by squint. Certain types of squint lead to the test subject using only a single eye, in order to avoid double vision.