>>3019354The numbers represent the ratio of the focal length of the lens to the diameter of the lens diaphragm opening. That's why it's called an f(ocal)/number. The designation "f/2" means that the diameter of the aperture is 1/2 the focal length of the lens. The designation f/32 means that the diameter of the aperture is 1/32 the focal length of the lens.
all lenses let in the same amount of light at f/2, but the longer the lens is, the wider the aperture has to be to let in the same amount of light.
this is why fast telephoto lenses have to be fucking huge.
