Focal Length Calculator

About this calculator

The thin lens equation, 1/f = 1/dₒ + 1/dᵢ, relates a lens's focal length f to the object distance dₒ and image distance dᵢ. A lens's optical power in diopters is simply P = 1/f (with f in metres) — a common unit on eyeglass prescriptions and camera/telescope specs.

Opticians and optometrists use this relationship (and optical power) when prescribing corrective lenses; photographers and cinematographers rely on focal length to understand field of view and depth of field for a given lens; and telescope and microscope builders use it to design optical systems, positioning lenses and mirrors to achieve a target magnification. Enter the object and image distances to find the focal length and power.