The focal length of a lens controls not only how much of a scene is captured and which elements of a scene are in focus, but also perspective. It is therefore a consideration of fundamental importance when planning an image, and one which a photographer must understand.

The focal length of a lens is defined, for a subject at infinity, as the distance between the image plane and the point from which the image is projected. This can be readily understood by cutting a rectangular hole in a piece of card, and then viewing a particular subject through the hole while holding the card at various distances from the eye. When the card is close to the eye it is possible to see a wide angle of view, but as it is moved further away so the angle of view decreases. If the latter narrow-angle scene is then enlarged to the same size as the wide-angle view, the subject becomes larger.

The all important apparent difference in perspective exhibited by wide-angle and telephoto lenses comes about because of the change in reproduction ratio or magnification. A subject which fills the viewfinder when using a telephoto lens must be approached must more closely to capture a similar image with a wide-angle lens. The two views therefore appear subtly different and the apparent relative positions of objects with an image may change. A telephoto lens seems to have the effect of compressing distances, making background detail appear closer to a foreground subject than was actually the case. Wide-angle lenses seem to have the opposite effect, and tend to exaggerate distances between objects. In fact these effects are nothing to do with the lenses themselves. Standard lenses are designed to produce a field of view and perspective close to those of the human eye. The perceived differences do not actually exist in a strict optical sense, as all lenses obey the same basic rules. The telephoto view of a subject can be cut from the centre of a wide-angle view taken from the same location. It is the change of viewpoint that makes all the difference.