Focal Length

Definition

How strongly a lens or mirror bends light — the distance from the lens to the point where parallel rays meet.

$\frac{1}{f} = \frac{1}{d_o} + \frac{1}{d_i}$

Symbol: $f$ · Unit: meter (m); often cm or mm

Technical definition

The distance from a lens's (or mirror's) center to its focal point, where initially parallel rays converge (converging optics) or appear to diverge from (diverging optics). Related to object and image distances by the thin-lens equation.

Example

A 50 mm camera lens has a focal length of 50 mm — short enough to fit in a pocket, gathering a wide view. A 400 mm telephoto lens magnifies distant birds because its long focal length bends rays more gently over a longer distance.

Common confusion

Focal length is not zoom, and longer is not 'stronger.' A short focal length bends light sharply (wide-angle, strong converging power); a long focal length bends it gently (telephoto). Eyeglass prescriptions use the reciprocal — optical power in diopters (1/f in meters) — so a −2.00 diopter lens has a 0.5 m focal length, and the minus sign marks it as diverging.