focal length of lenses

for photography

“normal” focal length

The focal length of a camera's “normal” lens is conventionally equal to the length of the diagonal of the image on the negative. The lengths of the diagonals of some common formats are shown in the table below. The formats shown with a color background are digital sensors; examples of their dimensions are here.

Format Length
of diagonal
(millimeters)
Actual
usual
16mm 12.7 25
super 16mm 14.5  
Four-thirds 22  
APS-C 25-28 ~24
APS-H 32-34  
35mm
film or digital
43 50
4.5 cm × 6 cm 75  
2¼″ × 2¼″
(6 cm × 6 cm)
81 80
6 cm × 7 cm 92  
6 cm × 9 cm 108  
2¼″ × 3¼″ 100  
4″ × 5″ 151 210
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.

Angle of View

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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