The Russian ZM-6A is a classic sample of Soviet-era engineering and workmanship. It is a 500 mm telephoto mirror lens based on the design of a known Soviet engineer Maksutov. This lens succeded the original MTO-500 lens, which was a heavy, brass unit. "MTO" stands for "Maksutov's Tele Ob'ektiv". ZM stands for Zerkal'nyi (Mirror) Maksutov's. The ZM-6A was much improved, it is a faster lens (1:6.3 vs. 1:8 in the MTO-500). It is also lighter, multicoated, and has a built-in sliding lens hood.

Lens' Specifications

Focal length: 500 mm
Aperture: 1:6.3 (fixed, this design does not allow for variable aperture)
Field of coverage: 5°
Focusing: 6 m - Infinity (the lenses of this design may focus past infinity, this is done to compensate for thermal expansion)
Filter size: M95x1 (4 filters are included)
Optical registration (back focus): 45.2± 0.02 mm
Mount: M42
Dimensions: Max diameter: 100mm (4"), Length: 165mm (6.5")

The glass is coated; the barrel is finished in tough black enamel
(Source: www.fedka.com)