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I don't quite understand why most lenses seem to have approximately the same diameter aperture regardless of focal length.
F-stop is, by definition, a ratio of focal length to diameter, so a 20mm lens with a 40mm aperture should be f/0.5 and a 400mm lens with a 40mm aperture should be f/10.
However, what we actually see in lenses is roughly the same distribution of f-stops for both wide angles and teles, yet the actual diameters aren't largely different. So is my understanding of f-stop flawed?
Note: The question has been substantially reworded as the point I was trying to get at is not covered in the answers to the 'duplicated' question. I hope my reworded question will be reconsidered. Thanks.