Extension tube

A set of extension tubes with a pen illustrating the lack of internal lenses

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 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 imposes this movement.

Extension tubes without electrical contacts will not allow an electronic automatic camera to control the lens, thus disabling autofocus and in some cases forcing a user to shoot wide open unless the lens offers manual aperture control. More expensive extension tubes contain electrical contacts allowing the user to use autofocus and electronically control the aperture of the attached lens. A great advantage to the non-electrical tubes is their low price. Since no electronics or optics is involved, it is usual to find rings for as cheap as $4 USD.

Other items like lens adaptors may unintentionally have an effect similar to an extension tube.

See also

• List of Nikon compatible lenses with integrated autofocus-motor

External links

• Canon EF 25mm Extension Tube II review at www.the-digital-picture.com
• Extension Tubes for Macro Photos at www.shutterfreaks.com
• Kenko Extension Tube Set review at www.the-digital-picture.com
• Experiments with Close-up Lenses and Extension Tubes
• Canon Extension Tubes M Booklet for Canon FD lens mount, at www.canonfd.com

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