Definitions for "Relative aperture"
Keywords:  focal, aperture, diameter, lens, length
Numerical expression of effective aperture, also known as f-number. Obtained by dividing focal length by diameter of effective aperture.
For a photographic or telescopic lens, the ratio of the equivalent focal length to the diameter of the entrance pupil. Expressed as f/4.5 or f.4:5, also called f number or speed of lens.
The f number of a camera (eg. f/2.8). This number expresses the effective aperture and is calculated by dividing the focal length by the diameter of the effective aperture.