For use with both film and digital SLR cameras. These fit between the lens and the camera body. They are supplied in sets of  tubes of different lengths, giving a combination of several different extensions. The longer the extension, the greater the magnification. Calculate the magnification by dividing the focal length of the lens into the total extension being used. EG 100mm of extension with a 50mm lens will give 2:1. 50mm of extension on a 100mm lens will give 1:2. Wide angle lenses give greater magnification than standard or telephoto lenses but with shorter working distances.