Focus

Focusing in a stereoscope is the adjustment of the distance between the lenses and the stereoview. This adjustment allows compensation for differences in eyesights.

Focusing can be achieved in different ways. One method uses a rack-and-pinion mechanism that moves the oculars with the lenses of the stereoscope forward or backward. Another method adjusts the position of the stereoview holder itself, as used in the Holmes–Bates stereoscope.