Slains Castle

by pakman

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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 ...

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Low f/stop = fast lens? [duplicate]

I have heard of lenses being described as fast that have f/stop values of f1.2 and f/2.8. My question is why are such low f/stop lenses called fast?