A long-focus lens is a lens that has a focal length longer than the diagonal measure of the film or sensor. A telephoto lens is a sub-category, whereby a lens contains a group of elements that allow the physical length of the lens to be shorter than the focal length. Therefore all telephotos are long-focus lenses, but not all long-focus lenses are telephotos.

The most important difference between a long-focus lens of conventional construction and a telephoto lens of the same focal length lies in the overall length of the lens. Thus, a conventional 400mm lens will be positioned roughly 400mm away from the film, and will be fitted in a lens barrel approaching 400mm in length. Many of the lenses produced by companies such as Tewe were long focus lenses. A telephoto with a focal length of 400mm, on the other hand, may be as little as 300mm in length, and will be usually much lighter in weight than the conventional lens.

The reduced length of a telephoto is derived by using a more “complex” optical design. One tremendous advantage of the telephoto construction is that it permits the use of lenses of very great focal length – lenses which would be impossibly heavy and inconvenient if the normal construction were used. Historically, long focus lenses, with focal lengths up to 2000mm, were often so big that it was customary to support the lens on a sturdy tripod.