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Why are lenses with very large apertures so expensive? For example, generally speaking, there is a significant price difference between a f/1.8, f/1.4 and f/1.2 lens of the same focal length. Doesn't the larger aperture lens just have a larger aperture hole to let more light in, and maybe a little more glass. In short, is it actually significantly more expensive for the manufacturers to produce a larger aperture lens, or is the higher price just for the perceived value of the larger aperture?
I hope I've been able to make the question clear.