f = ‘focae’ and indicates a ratio 1:2, 1:5.6, 1:16 etc, which we shorten to f2, f5.6 and f16, etc.
1:2 means that the diameter of the aperture opening of a given lens is half of the focal length. For a 50mm lens to open up to f2, the diameter of the opening will need to be 25mm.
f4 on a 50mm lens requires the aperture to open as wide as 12.5mm
f4 on an 80mm lens requires the aperture to open as wide as 20mm
f4 on a 28mm lens requires the aperture to open as wide as 7mm
Because the actual diameter opening of f4 varies (12.5mm, 20mm and 7mm) depending on the focal length (50mm, 80mm and 28mm) in the examples above – f4 lets the same ‘brightness’ through, whatever the lens or format.
The same goes for all the other aperture settings – they’re universal.
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