focallength

Focal length is a measure of how strongly an optical system converges light. It is defined as the distance from the lens’s optical center to the focal plane, typically the image sensor in a camera, when the system is arranged for distant objects. In practical terms, focal length determines how much of a scene is captured and how large subjects appear in the image.

The focal length relates to the lens equation 1/f = 1/do + 1/di, where f is the focal length, do is the object distance, and di is the image distance. For objects at infinity, di is approximately equal to f. The focal length therefore influences magnification and field of view: shorter focal lengths yield wider fields of view and less magnification, while longer focal lengths provide greater magnification and a narrower field of view. The apparent size of subjects also depends on the distance to the subject, which interacts with the lens’s focal length to determine framing.

Focal length is commonly categorized as wide-angle (short), standard (around 35–70 mm in 35 mm-format terms), and telephoto (long). In cameras with crop sensors, the effective focal length is the actual focal length multiplied by the sensor’s crop factor, altering the field of view without changing the lens’s physical properties. While related, focal length is distinct from the aperture, which controls light transmission and depth of field.

Beyond photography, focal length is a core concept in optics, used in microscopes, telescopes, and other imaging systems.