F-Number of an Optical System

The F-number (or f-stop) of an optical system such as a camera lens is the ratio of the system's focal length to the diameter of the entrance pupil (effective aperture). It is a measure of the lens's light-gathering ability, and it also determines the depth of field. A lower F-number corresponds to a larger aperture, allowing more light to reach the film or sensor, which is ideal for low-light conditions. Higher F-numbers give less light-gathering capacity, but a larger depth of field. This is useful in situations where keeping more of the scene in sharp focus is desired.

The F-number is crucial in both photography and optics, as it affects exposure and depth of field. In the formula, focalLength is the distance between the lens and the image sensor when the subject is in focus, and apertureDiameter is the size of the entrance pupil (effective aperture). The results are dimensionless, as both the focal length and aperture diameter should be measured in the same units.