Aperture is a “hole” or an opening through which light travels. The aperture of an optical system is the opening that determines the cone angle of a bundle of rays that come to a focus in the image plane.
The f-number (sometimes called focal ratio, f-ratio, f-stop, or relative aperture) of an optical system is the ratio of the lens’s focal length to the diameter of the entrance pupil.
The focal length of an optical system is a measure of how strongly the system converges or diverges light.
The amount of light captured by a lens is proportional to the area of the aperture.
|A||Aperture area (cm2)|
|f||Focal length of the lens (cm)|
|N||Focal number (f-number) (dimensionless)|