Focal length is the distance from the optical center of a lens to the sensor when the lens is focused at infinity. It is measured in millimeters, and it determines the angle of view and the magnification of the lens. A short focal length, like 24mm, captures a wide view. A long focal length, like 200mm, magnifies distant subjects. Focal length is not the same as zoom. A zoom lens has a variable focal length. A prime lens has a fixed one.

Focal length affects more than field of view. It changes perspective. A wide lens exaggerates depth, making close objects look larger and distant objects look smaller. A telephoto lens compresses depth, making distant objects look closer together. That is why portraits are often shot with short telephoto lenses, around 85mm to 135mm. The compression flatters facial features. Focal length also affects depth of field. Longer lenses produce shallower depth of field at the same aperture and distance. Understanding focal length helps you choose the right lens for the shot.

Focal length ranges

• Wide. 14mm to 35mm, broad view.
• Standard. 40mm to 60mm, similar to human vision.
• Telephoto. 70mm to 200mm, magnifies distant subjects.
• Super telephoto. 300mm and up, for sports and wildlife.

Focal length is not the same as field of view. Field of view depends on focal length and sensor size together.