How does mm convert to lens magnification?

Answer 1

With 35 mm film cameras, the 35 mm refers to the film size, not to be confused with the lens size. A 50 mm lens is usually considered the normal or neutral size, with a magnification factor of one. A 50 mm lens has no magnification. So, as the size (millimeters) of the lens become larger, the magnification of the image becomes greater. A 100 mm lens gives 2 times magnification, and makes the subject appear 2 times closer and gives less viewing area; a 200 mm lens gives 4 times magnification, and so on.

A 25 mm lens would make the subject look farther away by a factor of 2 and give a wider viewing area. Fisheye lenses, usually about 8 to 15 mm, give a very wide view and negative magnification. The increased area appears as a distorted "fisheye" view.

Digital cameras have electronic image sensors which are usually smaller than the area of film exposed in a 35 mm camera, and lenses will have a different magnifing effect. It is not a problem, though, as the manufacturer almost always gives a calculation of the equivalence of the lens with the digital camera compared to a 35 mm film camera, and calculates the magnification of the lens of the digital camera.