The aperture is the opening that lets light into a camera. Aperture size is directly proportional to the depth of field and exposure of photos. The size of the aperture is often measured in stops, specifically f-stops, known as the f-number. Aperture stops are inverse, so a larger f-number results in a smaller entrance pupil, while a small f-number describes a wide open aperture.


The f-number N is given by:

\({\displaystyle N={\frac {f}{D}}}\)

where \(f\) is the focal length, and \(D\) is the diameter of the entrance pupil (effective aperture). It is customary to write f-numbers preceded by "f/", which forms a mathematical expression of the entrance pupil's diameter in terms of \(f\) and \(N\). For example, if a lens's focal length were 10 mm and its entrance pupil's diameter were 5 mm, the f-number would be 2. This would be expressed as "f/2" in a lens system. The aperture diameter would be equal to \(f/2\).

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