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.
Questions tagged [f-stop]
86 questions
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Why are 1/3 stop apertures uneven numbers apart?
Why do 1/3 stop apertures go like 8, 9, 10, 11, 13, 14, 16, 18?
There's a difference of 2 between 11 and 13, it goes back to 1 between 13 and 14, and it goes back up to 2.
clickbait
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Why are f-stops not linear?
Why don't we use a linear scale for f-stops, rather than one based on square roots? A linear scale will be easy to remember (in fact, there will be nothing to remember) and to calculate how much brighter one stop is over another. That is, f/6 will…
Kartick Vaddadi
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Can a lens exist with an f-stop of less than 1?
As I understand it, the F-number of a lens is equal to the aperture diameter divided by the focal length. So f/1 would mean that the aperture is as wide as the lens is long.
Question: Is it physically possible for the aperture to somehow be wider…
MathematicalOrchid
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Low f/stop = fast lens?
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?
Sachin Kainth
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