I recently read that you calculate f-stops by dividing the lens focal length with the aperture optical diameter. Meaning, f/2 = 100mm focal length/ 50mm aperture diameter
What I dun understand is that how come, for example, a 28mm lens with a 10mm aperture diameter has a f/2.8 but 80mm lens with the same 10mm aperture diameter yield a f-stop of 8. Cos I thought both supposedly have the same aperture diameter that allows same amount of light in.
Can someone explain the theory why? Thanks.