Formats & focal length
The focal length of a lens, wide-angle, standard, telephoto, depends on the film format. The term "standard" for example, is a lens that gives a normal looking perspective which is close to what human eye sees. This occurs when the length of the frame diagonal is approximately the same as the focal length. The diagonal of a 6X7 format for example, is about 100mm which makes this lens standard for 6X7 formats. The following chart shows the focal lengths that would give similar perspective to those lenses in 35mm photography.
24mm = 35mm in 645, 40mm in 6X6, 50mm in
6X7  24mm equivalent shot |
| 35mm = 50mm in 645, 60mm in 6X6, 75mm in
6X7
|
| 50mm = 75mm in 645, 80mm in 6X6, 100mm in
6X7
|
85mm = 100mm in 645, 120mm in 6X6, 150mm in
6X7  85mm equivalent shot |
100mm = 150mm in 645, 160mm in 6X6, 200mm
in 6X7  100mm equivalent shot |
200mm = 300mm in 645, 350mm in 6X6, 400mm
in 6X7  200mm equivalent shot |
300mm = 400mm in 645, 500mm in 6X6, 600mm
in 6x7  300mm equivalent shot |
