Extension bellows fit between the camera and the lens, and provide a variable extension; they are available for 35 mm and 120 cameras. The minimum extension is usually so great that only magnifications greater than ×1 can be obtained with a 50 mm standard lens on a 35 mm camera.
|Basic bellows unit||Bellows unit with focusing rail|
Special lenses (known as bellows macro lenses or lens heads) are available without focusing mounts, designed for use on bellows, and these will produce excellent results. They are available in a range of focal lengths from about 12.5 mm to 135 mm, including some with automatic diaphragms, and can provide magnifications up to about ×20. The longer focal length lenses will focus to infinity. Good quality enlarging lenses will also produce excellent results. Exposure needs to be adjusted for magnification unless you use a TTL meter. A good bellows unit will be fitted with a focusing rail, so that you can set the magnification you want and then focus by moving the whole bellows assembly.
Using an ordinary lens, better definition will normally be obtained if it is mounted back-to-front, with a reversing ring. If the lens that you are reversing incorporates floating elements for better performance at short distances (like some macro lenses and fast wide-angle lenses), then you should make sure that the lens is set for close focus, not for infinity focus.
Reversing a lens exposes the rear element and various coupling mechanisms that are protected when the lens is mounted normally. You can provide them with some protection by adding a short extension tube to act as a makeshift lens hood; an old manual tube will do. You can also buy retrostep rings from SRB Film Service that allow a filter, lens hood or ring flash to be attached to the rear of a lens.