KMZ MTO-500 500mm f/8 (mirror)

Five hundred millimetres in a barrel short enough to stand upright in a camera bag. D.D. Maksutov, already known to astronomers for his meniscus telescope, calculated the MTO-500, and KMZ put it into production in 1956, cut to the M39x1 thread and 45.2mm flange of the early Zenits. The design is catadioptric: a meniscus corrector up front, mirrors behind it, and a light path folded back on itself so the focal length no longer dictates the length of the tube. The result for Zenit owners was a 500mm option without half a metre of glass hanging off the camera.

Two years later the MTO-500 was among the lenses in the set that took the Grand Prix at the 1958 Brussels World's Fair. KMZ shipped it as a kit with a hood and four filters: yellow, dense yellow, orange and neutral. The coloured ones were for black and white film pointed at haze and distant water. The neutral filter mattered more, since a lens with no diaphragm has no other way to tame a bright day. The lens itself weighs about 1.5 kg, takes 77mm (M77x0.75) filters at the front, and focuses down to 4 m.

The central mirror blocks the middle of the light cone, so out-of-focus highlights render as rings rather than discs. Glitter on a river becomes a scatter of doughnuts. Choose a background with no specular points and the effect disappears; leave a single streetlight in the frame and it takes over the picture. Contrast runs lower than a good refractive telephoto, and flat grey light makes that worse, which is one reason the orange filter earns its place in the kit. Colour fringing is the pleasant surprise. Because the mirrors do most of the focusing, it stays low, and backlit branches show far less purple edging than a cheap long refractor gives.

Exposure is simpler than on any other lens you own, because f/8 is the only setting there is. Shutter speed, film speed and whatever is screwed onto the front do all the work. In Zone Light Meter, lock the aperture at f/8, let the reading pick the shutter speed, and enter the factor for the orange or neutral filter if one is mounted. Mirror lenses often expose a touch darker than the marked f/8 suggests, so bracket the first roll before trusting the numbers.

Handling is where it fights back. On an early Zenit the screen goes dim at 500mm and f/8, and focusing becomes a matter of rocking the ring back and forth until the image snaps. Depth of field is a sliver. Every tremor of your hands shows up at that magnification, and with slow film you will be at shutter speeds that call for a solid tripod and a cable release. Check the thread before you buy, too. This is the M39 Zenit fitting, not M42, and it will not screw straight onto an M42 body.

Copies still turn up with the original filters and hood. The later Soviet and Japanese 500mm f/8 mirror lenses are easier to live with day to day. Buy this one because it is the 1956 original, part of the Brussels Grand Prix set, and still a way to get an early Zenit to the far side of a harbour, or to the moon, from a lens that fits in a bag.