A teleconverter is an attachment to a camera’s lens to change its focal length, always expressed in millimeter (mm). It is often called a tele extender because most of these secondary camera lenses extend, or multiply, the focal length of its coupled host. With this accessory, a 100mm lens for portrait photography can easily convert to a 300mm for sports photography. Optically, it crops and enlarges the center of an image captured by the host lens.
Different models of these converter lenses are specified by their multiplying factor — commonly 1.4x, 2x or 3x. A 35mm lens attached to a 2x converter will produce an image enlargement equivalent to one taken by a 70mm lens, twice its focal length. It is also often called a telephoto converter lens. A standard lens approximating normal human vision can economically serve double-duty as a lens capable of imaging further objects. A separate, larger and heavier, usually very expensive telephoto lens need not be purchased.
If a standard lens was cross-sectioned for an analytic trace of the path of light bending through its series of concave and convex optical elements, the point of focus will be behind the lens where a sheet of film would be. A teleconverter attached to its back end intercepts a portion of that light prior to convergence and passes it through additional optical elements to re-bend it to a focus point just behind the unit. The total distance light travels to come to focus will have been lengthened, or extended. Most models are accordingly mounted between the camera and a lens.
There are several important characteristics of all teleconverters. Modern cameras have sophisticated electronic controls such as auto-focus and exposure; a converter model must be compatible with both camera and lens if these features are to be retained. The quality of a converter’s glass elements is important, but so is the fidelity of its coupled host lens. The teleconverter will also magnify a sub-quality lens’s optical shortcomings. The most concerning characteristics of are reduction in image resolution and decrease in the amount of light reaching the film or circuit chip.
For cameras with fixed lenses, there is a type of teleconverter called a teleside converter which is attached to the front of the lens. These may be alternatives for video cameras and compact digital cameras without a detachable and interchangeable lens. They will effectively convert the lens into a farther imaging telephoto. Most models, however, are a simple combination of afocal, or non-focusing, glass lenses. Even good quality ones will often introduce unusual aberrations, such as darkened edges, to the image.
Another type of teleside converter can reduce a fixed standard lens into a wide angle lens of shorter focal length. It is, however, technically impossible to construct an equivalent teleconverter such as a 0.5x multiplier designed to attach to the back of a lens. Another accessory connecting a camera to its lens are hollow extension tubes. They have no optical elements, and therefore do not convert a lens’s focal length. The extension decreases the focusing distance of a lens, and increases magnification; it is used for macro, or close-up photography.