The focal length of the opaque projector's lens is determined by the radius of the curvature on the lens. To find the focal length of a lens, hold it in direct light beside a ruler. Move the lens up and down to find the place where the light makes its smallest point. The distance between the lens and the ground is its focal length.
Opaque projectors use double convex lenses, also known as magnifying lenses. Both sides of the lens curve out. This shape makes a picture or object look larger on screen than it is in reality. The rounder a lens is, the greater its magnification.
There are lenses similar to those in a opaque projector in LCD projectors, copy machines and overhead projectors due to their shape and average size. A person trying to make his own opaque projector can use a copy machine or overhead projector lens. The opaque projector was once common in schools, particularly in art rooms and libraries, but now found in few schools or workplaces.
In modern use, a document camera has taken the place of opaque projectors. Instead of lights and lenses, a document camera uses a video camera held in a fixed position that films objects or images to project onto a screen. Document cameras are compact and easier to use than opaque projectors.