Update: according to DZOptics, the normal focal length, the one that allows to get closer to human vision, is determined by the diagonal of the negative used: 43mm in 24x36 for example. In practice, we generally speak of a 50mm. Cameras with a smaller focal length are called wide angle, the others are telephoto lenses. The value of the minimum and maximum focal length is written on the front of the lens. These distances differ completely between a film camera and a digital camera, since the size of the sensor of the digital camera is relatively small. The lenses are then very close to the sensor. That's why we usually find the correspondence with a classic 35 mm that characterizes film cameras.