The 35mm film format has a image (or frame) size of 24 x 36mm with a diagonal of 43.267mm. This has been the standard since Oskar Barnack created the first Leica prototype using cheap 35mm movie film running lengthwise in the camera body. Although the film strip is 35mm wide, 11 mm is used up by the sprocket holes on either side, which are needed for the film transport mechanism.
Technically speaking, although most manufacturers market the 50mm lens as the normal lens, the correct “normal” lens for a 35mm camera should be as close to 43mm as possible. A lot of rangefinders and other fixed lens cameras were made with 40mm or 42mm lens back in the 60’s and 70’s, and for a very good reason. They were trying to stick to the “true” normal focal length. This focal length is extremely good for people and general photography. Try one and see the difference for yourself.
The 50mm focal length probably became the de facto standard because a lens with that focal length sees the world like the human eye sees it – with no PERSPECTIVE distortion. However, the human field of vision is a lot more than the 46 degrees afforded by a 50mm lens – it’s probably closer to what a 28mm lens sees (72 degrees) or even the 24mm lens (84 degrees). The 28 and 24mm lenses have edge-barrel distortion, however – something that our brain compensates for when looking at objects at the edge of our field of view (in our peripheral vision).
The 35mm focal length lens with a viewing angle of 63 degrees is a good compromise, and that’s why some professional photographers use the 35mm f/1.4 or f/2.8 as their standard ‘normal’ lens
An interesting side note – for the 50mm to be really “normal”, the image area or frame size would have to be 35mm x 36mm, instead of 24mm x 36mm. This would be the case if the 35mm film strip was completely utilized, including the area currently used up by the sprocket holes.
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