Bi-Prism Focusing
Purpose
Shows how a bi-prism in a 35 mm SLR camera is used to focus on an object. Two opposing prisms displace images in opposite directions; the images align only when the object is at the focal plane, demonstrating the split-image rangefinder focusing principle.
Figure 1:
A single prism on printed text displaces the underlying text sideways. Raising the prism from the page increases the offset.
Figure 2:
Two prisms with opposing slopes placed on the page. The text is aligned when the prisms contact the page; raising them causes opposite displacements.