Focal Length Calculator
Solve the thin-lens equation for focal length, object, or image distance
Frequently Asked Questions
1/f = 1/do + 1/di, where f is focal length, do is the object distance from the lens, and di is the image distance from the lens to the sensor/film. All three share the same unit.
Magnification m = -di / do. A negative sign means the image is inverted. If |m| is greater than 1, the image is enlarged; if less than 1, it's reduced.
Under 35mm is generally wide-angle, 35-70mm is standard/normal, 70-135mm is short telephoto, and beyond 300mm is super telephoto (on a full-frame sensor).
The thin-lens equation only works when do, di, and f are expressed in the same unit (usually mm). Mixing units gives an incorrect result.