Bellows macro lenses with a short focal length (e.g. 12.5 mm, 20 mm or 38 mm) can provide magnifications from around ×3 to ×20. They do not incorporate a focusing mechanism and are not designed to be mounted directly on a camera; they need to be used with an extension bellows or a variable extension tube, and a macro stand is almost essential. They are designed for larger-than-life work, and so they should never be used reversed.
Short focal length means a short working distance, so they need a bellows unit that allows the front standard to be mounted at the front of the rail, with the camera body on the rear standard being moved to alter the extension.
Traditionally made with manual diaphragms and the same thread as microscope objectives, but the second versions of the Olympus OM 20 mm and 38 mm lenses have automatic diaphragms and the normal Olympus OM lens mount
Adapters to fit macro lenses (or microscope objectives) with an RMS thread to M42 (Pentax screw) cameras or T2 adapters are available from SRB Film Services in the UK or Beljan Manufacturing in the USA. The RMS (Royal Microscopical Society) thread is 0.8″ diameter, 36 tpi.
Bellows macro lenses with longer focal lengths (e.g. 80 mm, 100 mm or 135 mm) can provide magnifications from around ×0.25 to ×4. To allow lower magnifications (or even infinity focus), they may extend inside the bellows.
They may give better results at magnifications greater than life size if they are mounted on a reversing ring. Reversing the 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. SRB Film Service manufactures retrostep rings that allow a filter, lens hood or ring flash to be attached to the rear of a lens.