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The f-stop of an optical system expresses the diameter of the entrance pupil in terms of the focal length of the lens; in simpler terms, the f-stop is the focal length divided by the "effective" aperture diameter. It is a dimensionless number that is a quantitative measure of lens speed, an important concept in photography.

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How is the number of stops of benefit provided by VR determined?

I don't know anyone who calculate those kind of thing, people just pull numbers out of their .. hemm .. I mean people produce approximate numbers based on their rich experience. If you did want to ca …
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Why are only certain shutter speeds or apertures available?

Most DSLRs let you choose shutter speed and aperture at 1/3 of a stop difference (3 clicks of the dial to double or halve the amount of light), I'm not a camera designer but I would guess that since 1 …
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