Viewfinder
The viewfinder is an essential part of most cameras for previewing what might be the image after exposure. Only a few camera types are regularly not equipped with viewfinder, for example repro cameras. In digital cameras the viewfinder might be omitted since the LCD display on such a camera's back might serve as provisional viewfinder. It's regularly ommitted on big plate cameras which are only used with ground glass focusing/previewing.
In modern cameras the optical zoom viewfinder is the most important version. This modern variant of the optical viewfinder is used in compact cameras with zoom lens.
waist level finders
In antique cameras the reflecting type viewfinder is the most common means of image preview. It is not very reliable but easily to add to a folding camera's front standard or folding bed, or into the housing of a box camera. The brilliant finder is the widest spread of such finders, a combination of a lens, a mirror in 45 degree angle behind that lens, and another lens in right angle position to the first one to view the mirrored image from top. An older version was the cubic small Watson finder with lens and mirror as in the brilliant finder, but with a matte screen for viewing the finder image. A special version was the Sellar finder wich consisted just of a concave mirror with targeting aid. Old SLR cameras have a bright reflecting type finder with matte screen that uses the same lens as the camera uses for exposures. Before exposure the mirror is lifted so that the light coming from the image subject through the lens can pass towards the image plane were the focal plane shutter allows the exposure of the film for an instant. TLR cameras have a bright reflecting type finder with an own lens, a "twin" of the camera lens. All these finders are to be viewed from above. Together they are the class of waist-level finders.
eye level finders
Other optical viewfinders of old cameras are placed upon the camera top as small rectangular "telescope". Since the 1950s these viewfinders were more and more integrated into the camera bodies. In older cameras the finders might consist of only the front lens and the ocular lens. Modern zoom finders might be more sophisticated. More sophisticated are also viewfinders with superimposed rangefinders, and those with parallax correction. Parallax correction means that an optical system positioned parallel to the camera optics never gets the same image as the camera lens for nearness exposures if the the ocular would not be lifted slightly, or other provisions would help to get near equality of finder image and image plane.
Finders of modern SLR cameras are completed with a pentaprism plus ocular or a mirror system plus ocular. With these additions the reflex finders become eye-level finders and enable the photographer to see the image like through a telescopic optical viewfinder.
A common old finder type is the frame finder, consisting of two frames, or one frame with or without crosshairs plus targeting aid. A big rectangular frame made of massive wire combined with targeting aid is called sports finder. A modern plastic variant of the sports finder can be bought as accessory for underwater cameras. A mixture of optical finder and frame finder is the Newton finder, with one lens and targeting aid.
A new type of the viewfinder is a combination of a miniature color LCD screen with an ocular lens. It's often found in compact digital cameras with a very wide zoom range.
