Viewfinder

The exact image (although upside-down and reversed left-right) was viewed on a ground glass installed either in a replaceable plateholder, or in a spring back where springs hold the ground glass at the focus plane until a photographic plateholder is slid in front of it.

A single divergent (plano-concave) lens, in front of a frame, when close to the eye, acts as a viewfinder.

Adding a convergent (plano-convex) lens makes a very short reversed Galilean (upright image) telescope.

With a mirror, of similar size to the film, held at 45°, it projects an upright image onto a focusing ground glass screen viewable from above.

TLR viewfinders do not have the interrupted viewing and shutter lag of the SLR type and so is preferred for dance photography.

[2] Reinhold Heidecke cited his experience with periscope focusing from the German trenches in 1916 as the inspiration for the Rolleiflex line in 1929.

Early SLRs were plate cameras, with a mechanism to insert a mirror between the lens and the film which reflected the light upwards, where it could be seen at waist level on a ground glass screen.

In addition to its primary purpose, an electronic viewfinder can be used to replay previously captured material, and as an on-screen display to browse through menus.

A still camera's optical viewfinder typically has one or more small supplementary LED displays surrounding the view of the scene.

Digital still cameras will typically also display information such as the current ISO setting and the number of remaining shots which can be taken in a burst.

For those cameras, the electronic image is shown on a small accessory screen for composition and focusing purposes.