Thin Lens & Mirror Equation Calculator

Kaava

How this is calculated

The thin lens (and spherical mirror) equation relates the focal length f, the object distance do, and the image distance di by 1/f = 1/do + 1/di. Enter any two of these three quantities in centimeters and choose which one to solve for. Rearranging gives di = 1/(1/f - 1/do), f = 1/(1/do + 1/di), and do = 1/(1/f - 1/di).

The magnification is m = -di/do. A negative magnification means the image is inverted (a real image for a single converging lens), while a positive magnification means it is upright. The magnitude |m| tells you how many times larger or smaller the image is than the object. With the convention used here, a positive focal length describes a converging lens or concave mirror.

Sign convention matters: for a lens a positive image distance (di > 0) indicates a real image formed on the opposite side from the object, while a negative di indicates a virtual image on the same side. Distances are in centimeters, but any consistent length unit works since the equation is scale-free. Watch for the edge case where 1/f - 1/do = 0 (object at the focal point), which sends the image to infinity and makes the result undefined.