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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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Why does my lens refocus when the f-stop is changed?

I have a prime Sigma f1.4 50mm lens on my Nikon D90 in P mode and focus mode on a set point of the field and notice that when I am using a tripod and change the f-stop that the camera changes the ...
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How to understand a quote for the correct exposure when shooting ColorChecker?

The problem concerns the examination of the correct exposure photographing a colorchecker... Manual mode, fixed ISO 100 (camera dafault), other values read from the lightmeter ... and now: Can you ...