KMZ Mir-20M 20mm f/3.5

You filter this lens from the back. The Mir-20M has no front thread at all; KMZ cut an M28x0.75 thread behind the rear element instead and packed three small filters in the box: a UV-1x, a yellow Zh-2x and an orange O-2.8x. Changing one means taking the lens off the Zenit, screwing the filter into the rear, and mounting it again. It is a slow ritual. The Arsenal versions for other bayonets show you the alternative, since they took 95mm filters on the front, and once you have priced a 95mm orange the little rear thread starts to look clever.

The optics come from Jena. GOI took the Carl Zeiss Jena Flektogon 2.8/20 as its starting point, and Volosov, Khmelnikova and Shamanina worked out a slower f/3.5 version for Soviet production. It has 9 elements in 8 groups, covers 96 degrees and focuses down to 0.18 m. Use that close focus. Put a flower almost against the front glass and the whole room still sits behind it, the near subject swollen and the walls leaning away in the exaggerated perspective a 20mm gives on full frame.

On an M42 Zenit the switchable automatic diaphragm earns its keep. In auto, the body presses the lens's pin and the blades close at the moment of exposure, so you compose at f/3.5 and the finder stays as bright as it can be. Flip to manual and the lens stops down as you turn the ring, which you need on bodies or adapters that never press the pin. A dim Zenit screen needs every bit of light, because judging focus on a 20mm by eye is hard even in good conditions. The easy way is to set a distance on the scale, stop to f/8 and rely on depth of field.

As with most ultra-wides of its period, the usual practice is to stop to f/8 or f/11 when detail has to reach the edges of a landscape or a cramped interior. This is the single-coated lens from before the MC Mir-20M of 1989, so treat the sun with care. Single coating is more prone to veiling flare and ghosts when a bright source sits in the frame or just outside it. A hood does a little. Moving your feet so the sun stays out of the corners does more.

Speed is the honest limit. At f/3.5 it is slow for anything past daylight, and the rear-filter routine means you cannot quickly swap a yellow for an orange halfway through a scene as you could with a front-threaded wide. People cross-shop it with the Flektogon 2.8/20, which is faster and often dearer, and with the later MC Mir-20M, which has the multicoating this one lacks. The MC Mir-20K, the Pentax K version KMZ made in a small batch in 1992, is rare enough that it turns up far less often than the M42 lens.

One exposure point is particular to this lens. A Zenit with a through-the-lens meter reads through the rear filter on its own. A handheld reading does not. The orange O-2.8x costs about a stop and a half and the yellow Zh-2x one stop, so enter the filter in Zone Light Meter and it will add the factor to the reading for you.