Why is the standard lens focal length of 135 camera 50mm?

Because the lens with a 45-degree viewing angle is simple in optical structure and easy to manufacture, it is a lens with a focal length of 50mm on the frame of 135 (the focal length is equal to the diameter of the imaging ring, that is, the 45-degree viewing angle). This has nothing to do with the visual angle of human eyes. Macular area (the area where the human brain processes the main visual content at any time) has a viewing angle of less than 8 degrees, and the full-field viewing angle reaches about 120 degrees. A 45-degree angle of view has no special significance to human vision.

The standard lens is only available in Chinese mainland, because for a long time, the camera lens that people in China can see is only about 50mm fixed focus. The reason for saying "standard" may be that a small number of 42mm and 58mm lenses can be seen at the same time in China, but the number is far less than 50 mm. Under the ideology at that time, the obviously dominant 50 mm lens should be called the standard lens. There is no such name in the Soviet Union with similar ideology. It should be because the optical design of the Soviet Union was much stronger at that time. Photographers use at least 35mm and 50mm lenses, so they can't call either "standard".

For photography, there are at least three kinds of fixed focus that can be used to shoot "not particularly wide or very close": 35mm for humanities photography, 42mm for architecture photography and 50/58/60mm for still life photography. 50mm became the "standard" because Chinese mainland had a special market for a period of time, and it had no special significance.