What are extension tubes and bellows for?
Extension tubes and bellows are used to allow a lens to focus closer than it can do with its own focusing mechanism. The extension rings in the Nikon system were originally designed for use with Micro Nikkor lenses that could focus to 1:2 unaided, but needed the addition of an extension tube to get to 1:1. The total extension needed to reach 1:1 is the same as the focal length of the lens, and to get to 1:2 you need an extension equal to half of the focal length of the lens. Hence, in the Nikon system you will find extension tubes of 22.5mm and 52.5mm in length, designed to be used with the 55mm and 105mm focal length Micro Nikkors. Increasing the extension by adding tubes or bellows to a lens will reduce the effective aperture of the lens. This can have a significant effect if you take a standard 50mm lens and add enough length of extension tube to give life-sized magnification, the effective F-stop is twice that indicated on the aperture ring of the lens i.e., you lose 2 stops of lig
