In photography we measure film and focal length in millimeters. But it’s important to not get the two confused. Read focal length to become familiar with it’s measurement. In film the measurement of 35 mm that we typically hear is the whole height of the film coming out of the cartridge.
We compare the size of the film and the focal length so that we can predict if the photography will be wide angle, telephoto or “normal”. Normal is what we would perceive if we were looking at the same scene with our human eye rather than through the viewfinder. The measurement we take to do this comparison is not the same “whole height” of the cartridge, but rather the cross section of the film plane, the approximate 43mm in the case of 35 mm film.
When the diagonal measurement of the film plane is greater than the focal length. On a standard 35mm camera, lenses with the focal lengths below 40mm would be wide angle. These wide angle lenses are also called “fisheye” lenses because of the effects they create of displaying more than the human eye would see.
When the diagonal measurement of the film plane is equal to the focal length. On a standard 35 mm camera, lenses with the focal length between 40mm and 50mm are considered “normal”. These lenses create about a 47º field of view, the same as your eye.
Digital Focal Length Note
For the digital photographers out there, you should know that your CCD sensor is much smaller than film, but don’t worry. Digital Lens manufactures have made it standard to have 50mm lenses look “normal” although the focal lengths on those lens’s isn’t really 50mm. The CCD diagonal measurement is closer to 10mm, and each one is different. That’s why you see that your 35mm aren’t typically compatible with your digital SLR and digital SLR lens’ are made for specific camera models.
When the diagonal measurement of the film plane is less than the focal length.
In this case 50 mm was called normal because the picture looks neither zoomed in or zoomed out. It looks just as it would if you were standing there, a field of view similar to human vision. You’re question is really so good it requires a whole new article that helps us understand focal length using a 35 mm camera. Just like the one that took these pictures.