Focal length
lastmod 2021-01-28created 2021-01-283 edits
Focal length is the distance from the principal point to the focal point in optics.
Focal length in cameras
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| [1] Lens secondary principal point [2] Focal length [3] Angle of view (measured diagonally) [4] Focal plane (image sensor plane) |
Focal length (or focal length range, for zooms) is usually the most important consideration when choosing a lens for a particular photo or type of shooting. A lens's focal length determines two characteristics that matter greatly to the photographer: magnification and angle of view.
A longer focal length means higher magnification, and vice versa. A wide-angle lens with a short focal length has low magnification, which means you have to get physically close to an average-sized subject to fill the frame. But it also means you can fit a large subject in the frame without shooting from a distant position. A telephoto lens with a long focal length has high magnification, so you can fill the frame with a subject far from the camera.
Technical definition
The focal length of a lens is defined as the distance from the secondary principal point to the rear focal point when the lens is focused at infinity.
The secondary principal point is one of the six "cardinal points" used as reference points in an optical lens (front and rear focal points, primary and secondary nodal points, primary and secondary principal points). The position of the secondary principal point of a compound lens is not predefined. Depending on the lens design, it can be at a particular point inside or outside the lens barrel, so it is difficult for users to measure a lens's focal length accurately themselves.