TalenLenses: Focal length
The key feature of any lens is its focal length. In optical terms, this is the distance between the lens and the film or sensor when you have focused on a distant subject. In photographic terms it gives you a guide to the scale of the image in relation to the subject.
A lens with a short focal length, such as 20mm, gives very low magnification (actually, it’s a reduction, since the image is much smaller than the subject, but photographers use the word magnification to mean both reduction and magnification). The field-of-view covered by a short focal length lens is wide, so they are called wide-angle lenses.
A lens with a long focal length, such as 400mm, gives greater magnification, making the subject appear much closer than it really is. These lenses are called telephotos (the effect is similar to viewing through a telescope).
A lens with just a single focal length is called a prime lens. However, these days most photographers prefer a lens that offers a range of different focal lengths in a single unit. These are called zoom lenses.
