Basics
Originally, big unwieldy cameras---now called view cameras---were made with a ground-glass focusing screen on the end where the photographer looked through and a lens on another board at the opposite end. These two boards were connected by a bellows that could be moved back and forth, bringing the lens nearer to or farther from the ground-glass focusing screen. While the bellows have disappeared in all but the view camera, the principle remains the same.
The Lens
A lens on a 35mm-style or medium-format-style camera is made up of several glass elements in a barrel, usually made of metal or plastic. While the rear lens element tends to be stationary, the other elements move as the user twists the focusing ring of the lens barrel. The lens then projects the image through itself to a mirror that is at a 45-degree angle.
The Camera
The image on the mirror is projected up through a ground-glass focusing screen into a pentaprism that corrects the image from left to right and turns it right-side up. This image goes to the viewfinder eyepiece where the photographer sees the image. In older medium-format-style cameras, the image comes through the mirror and the focusing screen, but the photographer looks down through the waist-level viewfinder. The image in this case is correct to right-side up, but not left to right. When the photographer adjusts the focusing ring on the lens, he moves the lens elements until the image is sharp on the ground glass and in the viewfinder, then snaps the shutter.
Tags: focusing screen, ground-glass focusing, ground-glass focusing screen, focusing ring, focusing ring lens, left right
