Conventionally, the focal length of a camera's “normal” lens is equal to the length of the diagonal of the image on the negative. The lengths of the diagonals of some common formats are:
| Format | Length of diagonal (millimeters) |
|---|---|
| 35mm | 43 |
| 4.5 cm × 6 cm | 75 |
| 2¼″ × 2¼″ (6 cm × 6 cm) |
81 |
| 6 cm × 7 cm | 92 |
| 6 cm × 9 cm | 108 |
| 2¼″ × 3¼″ | 100 |
| 4″ × 5″ | 151 |
| 5″ × 7″ | 207 |
| 8″ × 10″ | 311 |
| 11″ × 14″ | 436 |
In practice, though, the normal lens is often a little longer: 50 mm in 35mm photography (inherited from the Leica) and 210 mm in 4 × 5 (up from 150 mm in the days when press photographers used 4 × 5, probably because most of today's 4 × 5 photographers use the front and back movements and want a lens that covers a bigger area in the film plane).
Lenses with focal lengths shorter than normal are called “wide angle.” A common misconception is that wide angle lenses have more depth of field than normal lenses. With minor exceptions, all focal lengths give the same depth of field at the same magnification and aperture.
“Telephoto,” which originally referred to a long focus lens physically shorter than its focal length (thanks to a particular optical design), now generally refers to any lens with a significantly longer-than-normal focal length.
Click on a film format in the list below for a table showing the angle of view for the longer side of the image, the shorter side and the diagonal, for the focal lengths in which interchangeable lenses have been commonly available.
Particularly in the large formats, the image formed by the lens may cover a much wider area than the film itself.
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Last revised: 11 August 2004.