Extension tubes (or rings) are essentially hollow metal tubes of precise lengths which can be fitted between a compatible camera body and lens to increase the distance of the lens from the film or sensor. This greater distance has the effect of increasing the reproduction ratio (magnification). As the extension distance increases, so the reproduction ratio increases. Extension rings are available individually, and also sometimes supplied as sets of three or four rings of varying lengths. The lengths of the rings are calculated to provide a range of extension distances which provide an unbroken range of reproduction ratios. By selecting the correct extension tube, or combination of extension tubes, it is therefore possible to obtain any reproduction ratio within a wide range of values. When the amount of extension is equal the focal length of the lens, the reproduction ratio is 1:1.
Extension tubes contain no optical glass and, when fitted, prevent lenses from being focused at infinity. However, the greater the extension the closer the lens can be placed to the subject and the greater the reproduction ratio. The more sophisticated types of extension ring also extend the automatic focusing and metering and aperture functions of automatic cameras. These facilities are very useful because focusing becomes critical and exposure may be difficult to determine. In the past, complicated calculations were necessary, or a table of suggested values had to be used. Through-the-lens metering solves this problem to some extent. When using a digital camera, the histogram shows clearly whether or not the exposure is correct.
Extension bellows are essentially infinitely variable extension tubes which can be extended to about 200mm in length. This degree of extension provides reproduction ratios of up to 10:1 with an appropriate lens. Bellows extension units all have a blind spot in their focusing range caused by their collapsed length.