Olympus's OM high-ratio macro — the Zuiko Auto-Macro 20mm f/3.5 for bellows close-up work.
The Olympus OM-System Zuiko Auto-Macro 20mm f/3.5 is a high-magnification bellows macro lens for the Olympus OM mount, part of Olympus's specialist macro system. It was designed for extreme close-up photography at magnifications beyond life size, serving scientific, technical and nature photographers rather than general use. It sat alongside the faster 20mm f/2 as a slower, more affordable high-ratio option.
This is a manual-focus Olympus OM macro lens with a 20mm focal length and a maximum aperture of f/3.5, designated Auto-Macro. Like other short-focal macro optics it is used on a bellows or extension system to achieve very high reproduction ratios rather than focusing conventionally. The verifiable specifications are the 20mm focal length, f/3.5 maximum aperture and manual-focus OM macro design; magnification range, mount thread and weight are omitted rather than guessed.
A 20mm macro is a specialist tool for imaging tiny subjects at greater than life size, resolving fine structure with a flat field. It is deployed with bellows and rails for entomology, mineralogy, small components and laboratory subjects. This is a controlled, tripod-based instrument for high-magnification work rather than anything handheld or general purpose.
Dedicated OM macro lenses like this were produced in limited quantities and are uncommon, so they attract specialist prices when they appear. Inspect the elements for haze and fungus, confirm the aperture operates cleanly, and check that the correct bellows or adapter hardware accompanies the lens since it does not mount like a standard optic. Verify the intended mounting method before buying. It can be used on mirrorless for extreme close-up given a suitable bellows or tube arrangement.