f-stop Number

This is the ratio between the focal length of a lens and diameter of its’ aperture (or light entrance pupil) and is expressed by the equation:

 

 

Where N is the f-number, f is the focal length, and D is the aperture diameter.

f-numbers are typically measure in full-stops, where each successively larger stop number halves the light gathering area of the lens, as in the series; f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11f/16

source: wikimedia.org

f-numbers in full-stop increments

f-stop numbers can further be divided by halves or thirds.

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