A teleconverter (sometimes called tele extender) is a secondary lens mounted between a camera and a photographic lens which enlarges the central part of an image obtained by the objective lens. A 2× teleconverter for a 35 mm camera would enlarge the central 12×18 mm part of an image to the size of 24×36 mm in the standard 35 mm film format.
Teleconverters are typically made in 1.4×, 1.7×, 2× and 3× variants, with 1.4× and 2× being the most common. A 2× teleconverter effectively doubles the focal length of a given lens. Teleconverters also decrease the intensity of the light that reaches the film plane (or sensor) by a factor of 4—equivalent to doubling the focal ratio—and decrease the resolution of an image by a factor of 2.
An extension tube - also called extension ring - is used with interchangeable lenses to focus closer, useful in macro photography.
The tube contains no optical elements; its sole purpose is to move the lens farther from the image plane. The farther away the lens is, the closer the focus, the greater the magnification, and also the greater the loss of light (requiring a longer exposure time). Lenses classically focus closer than infinity by moving all optical elements farther from the film or sensor; an extension tube simply enables this movement.
Because extension tubes do not have optics, they don't affect the optical quality of a lens. Because of their function, there are other effects: decrease of light; shallower depth of field; and loss of ability to focus at infinity. The longer the extension tube, the closer the lens can focus. The amount of light and depth of field will be equally reduced. On modern cameras using auto exposure this is all corrected for by the camera, but when not using auto exposure this has to be calculated and taken into account when setting exposure.
The precise reduction of the amount of light reaching the imaging plane can be calculated using the inverse square law formula.
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