What are f stops?
Dial through the f-stops in Aperture Priority mode and the sequence of numbers that appears in the viewfinder can appear quite random.
Why does f/5.6 jump to f/8 when you close the aperture by one full stop, then from f/8 to f/11 to f/16 and so on? There seems to be no logic to it.
How these numbers are calculated is actually pretty complicated, but the thing to bear in mind is that an f-stop isn’t a measurement of the diameter of the aperture, but an expression of the ratio between the diameter of the entrance pupil and the focal length.
This means that aperture values are constant no matter what lens is in use.
For instance, an 80mm lens with an aperture setting of f/4 will have a pupil diameter of 20mm (80÷4), whereas a 400mm lens with an aperture setting of f/4 will have a pupil diameter of 100mm (400÷4).
While the 400mm has a considerably larger pupil, it lets in exactly the same amount of light as the same f/4 aperture does on the 80mm lens, because the light is much dimmer by the time it has travelled the length of the lens.
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