INFLUENCE OF LENS FOCAL LENGTH

INFLUENCE OF LENS

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. Wide angle lenses have small focal lengths, while telephoto lenses have larger corresponding focal lengths.
Note: The location where light rays cross is not necessarily equal to the focal length, as shown above, but is instead roughly proportional to this distance. Therefore longer focal lengths still result in narrower angles of view, as depicted.

Many will say that focal length also determines the perspective of an image, but strictly speaking, perspective only changes with one's location relative to their subject. If one tries to achieve the same subjects filling the frame with both a wide angle and telephoto lens, then perspective does indeed change because one is forced to move closer or further from their subject. For these scenarios only, the wide angle lens exaggerates or stretches perspective, whereas the telephoto lens compresses or flattens perspective. Perspective control can be a powerful compositional tool in photography, and often determines one's choice in focal length (when one can photograph from any position). Move your mouse over the above image to view an exaggerated perspective due to a wider angle lens. Note how the subjects within the frame remain nearly identical-- therefore requiring a closer position for the wider angle lens. The relative sizes of objects change such that the distant doorway becomes smaller relative to the nearby lamps.

Other factors may also be influenced by lens focal length. Telephoto lenses are more susceptible to camera shake since small hand movements become magnified within the image, similar to the shakiness experience while trying to look through binoculars with a large zoom. Wide angle lenses are generally more resistant to flare, partially because the designers assume that the sun is more likely to be within the frame for a wider angle of view. A final consideration is that medium and telephoto lenses generally yield better optical quality for similar price ranges.