A lens with a longer focal length is telephoto lens. These lenses, depending on their focal length, cover a narrow angle of view compared to a normal or a wide angle lens. Object at a distance appears much closer with these lenses. Longer the focal length of a lens, narrower the angle it will cover, and it will bring a distanced object, more closer.
On a 35 mm film, or full frame digital camera format, 50 mm lens is consider as a normal lens. Anything beyond 70 mm is considered as medium tele-lens but after 135 mm threshold, actual group of tele-lens starts. This group may go up to 1200 mm and more.
Mirror lenses are tele-lens, ranging from 150 mm to 500 mm. Mirror lens has good advantage as the actual size and weight of the lens is very less as compared to same focal length regular tele-lens. However, mirror lens has a fix aperture, which is normally calibrated to f 8 or f 11.
We can increase the focal length of a lens by using a 1.4X or 2X tele-converter. A tele-converter is fixed between the SLR camera body and a lens. It multiplies the focal length of the lens by its factor. So if we put a 1.4X converter, a 300 mm lens will become 300 x 1.4 = 420 mm lens and if we put 2X converter, then the same lens will become 300 x 2 = 600 mm. However, a tele-converter will reduce the effective aperture of the lens and the lens will become slow.
Tele lenses are indispensable when we are shooting wild-life photographs or a sport. However, they are also used in urban landscape and architectural photography, as these lenses compress the perspective and minimise the distortion. Medium tele-lens shows better proportion of human face and is preferred for portrait shoot.
For given aperture, a tele-lens gives much shallow depth of field as compared to its normal or wide angle counter part. This effect is preferred, when we want to blur out the background in a portrait photography.
A sturdy tripod is must when we are using a telephoto lens. As a long focal length lens has a narrow angular coverage, it will record slightest of camera shake as a motion blur. So a firm camera support is required. We may rest the camera on some rock or a tree but a tripod is more easy to adjust the camera position and compose the image.
Most of long telephoto lenses are constructed in such a way, that the lens body is directly mounted on the tripod. The camera body hangs out as a cantilever and the whole camera is balanced on the tripod head. This reduces one sided load on the tripod. Lens with long body also has a provision to rotate the lens by loosening a grip. In this case, we can select horizontal or vertical composition without turning the base plate of the tripod. Again, this is given for reducing one sided load on the tripod.
Extreme tele-lens result is achieved with a telescope. Some telescope manufactures provide a mount, on which we can directly fix an SLR camera body (without lens) with an adapter. This is how astronomical images are shot.
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