Extension tube
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An extension tube is an accessory for cameras with interchangeable lenses, used primarily for macro photography. The tube contains no optical elements; its sole purpose is to move the lens farther from the film or digital sensor. 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 extends this movement. When a lens is focused at infinity, its maximum magnification is the length of the extension divided by the focal length of the lens.
Extension tubes without electrical contacts will not allow the camera to control the lens disabling autofocus and forcing a user to shoot wide open. More expensive extension tubes contain electrical contacts allowing the user to use autofocus and control the aperture of the attached lens.
Other items like lens adapters may unintentionally have an effect similar to an extension tube.
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