Focal Length

 

Focal length is the distance from the film to the optical centre of the lens when the lens is focused on infinity. On most adjustable cameras it is marked in millimetres on the lens mount. On 35mm-format cameras, lenses with a focal length of 50mm are called normal or standard lenses. Lenses of 35mm or less are called wide angle lenses and lenses of 85mm or more are called telephoto lenses. Lenses which allow varying focal lengths without changing focus are called zoom lenses.

Some confusion has arisen since the advent of digital cameras and a conversion factor has to be used to make a comparison between film cameras and digital cameras. For example, the conversion factor is around 1.6 for Canon and 1.5 for Nikon. Hence lenses of 23mm, 32mm, and 55mm on digital cameras are approximately equivalent to 35mm, 50mm and 85mm on 35mm film cameras.

Focal length also has an effect on the depth of field – the distance between the nearest and furthest sharp objects in a photograph. A wide-angle (or short) lens will provide a greater depth of field than a long lens.


20-mm.jpg (131698 Byte) focal length picture photo 20 mm 20 mm
28-mm.jpg (141225 Byte) focal length picture photo 28 mm 28 mm
35-mm.jpg (149840 Byte) focal length picture photo 35 mm 35 mm
50-mm.jpg (146054 Byte) focal length picture photo 50 mm 50 mm
70-mm.jpg (144113 Byte) focal length picture photo 70 mm 70 mm
85-mm.jpg (145897 Byte) focal length picture photo 85 mm 85 mm
105-mm.jpg (151153 Byte) focal length picture photo 105 mm
135-mm.jpg (166845 Byte) focal length picture photo 135 mm 135 mm
200-mm.jpg (156333 Byte) focal length picture photo 200 mm 200 mm
300-mm.jpg (131531 Byte) focal length picture photo 300 mm 300 mm