Long-focus Lens is gradually getting more importance with the advent of digital SLR cameras. The photographer with a DSLR now can get high magnification. By definition, a long-focus lens is a lens which has a focal length longer than the diagonal measurement of its sensor.
Basics of Long-focus Lens
For the sake of lowering the manufacturing cost, the sensors of most digital SLR cameras are smaller than the film SLR. In most brands, it is necessary to apply a multiplier of 1.5 to find the focal length of a lens that would offer the same framing for the 24x36mm film. At Canon such multiplier is 1.6 in most cases, although Canon also sells very expensive Long-focus lens for the cameras with sensors measuring 24 x 36 mm. For Sigma, the coefficient is 1.7. The smallest digital SLR sensors are those of Olympus has a multiplier of 2. As a result, the photographer with a digital camera can get high magnification with lighter and less expensive (because of shorter focal length) lens than he/she otherwise would need to buy to get the same picture with a film camera. However, as the small sensor enlarges the center of the image, it is necessary that it is of good quality, which is not always the fact in case for zooms when used to their maximum focal length.
Uses of of long-focus lenses
Best example in everyday life is the Sports photographers, who almost always seen to be using long-focus lenses. Second usage is by the professional wildlife photographers. Third and quite important segment now is the advanced amateurs.