Focal length

Focal length

The focal length of a lens determines its angle of view, and thus also how much the subject will be magnified for a given photographic position.

Focus lens perspective

Perspective distortion takes two forms: extension distortion and compression distortion, also called wide-angle distortion and long-lens or telephoto distortion.

When talking about images with the same field size. Extension or wide-angle distortion can be seen in images shot from close using a wide-angle lens (with an angle of view wider than a normal lens).

Object close to the lens appears abnormally large relative to more distant objects, and distant objects appear abnormally small and hence more distant – distances are extended.

Compression, long-lens, or telephoto distortion can be seen in images shot from a distant using a long focus lens or the more common telephoto sub-type (with an angle of view narrower than a normal lens).

Distant objects look approximately the same size – closer objects are abnormally small, and more distant objects are abnormally large, and hence the viewer cannot discern relative distances between distant objects – distances are compressed.