ABSTRACT
The simplest and most accurate viewfinder is also the oldest type, where the camera lens projects an image directly on to a ground glass screen, which is replaced by a sheet of film for exposure. The screen is traditionally of ground glass with the textured surface positioned accurately in the true focal plane or an equivalent focal plane. The screen may have various engraved markings, such as a grid pattern, central circles or rectangles to show a metering or focusing area, a millimetre scale to determine magnification, or an alternative format area. An oft-quoted advantage of screen focusing is that viewfinder accuracy is exact, but in practice this is seldom true. The pentaprism viewfinder, not to be confused with a pentagonal prism, is a roof prism device whose role is to give an erect, unreversed image from the laterally reversed image on the focusing screen, turned through 90° for convenient eye-level viewing facing the subject.
