Focal Ratio (f-number) Calculator

About this calculator

The focal ratio (or f-number, written f/N) of a lens or telescope is its focal length divided by the diameter of its aperture: N = focal length / aperture diameter. A lower f-number means a wider relative aperture that gathers more light per unit of sensor area, while a higher f-number means a narrower aperture and greater depth of field.

Photographers use f-numbers on the standard scale (f/1.4, f/2, f/2.8, f/4, f/5.6...) to control exposure and depth of field, where each full stop halves or doubles the light reaching the sensor; astronomers use the same ratio to describe how "fast" a telescope is for imaging faint deep-sky objects — a low f-ratio scope captures dim nebulae in less exposure time.

Photographers choosing lenses and exposure settings, amateur and professional astronomers selecting telescopes for astrophotography, and optics students learning how aperture affects image brightness all use this calculation.