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 succeeded 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, with multi-coated glass, and has a built-in sliding lens hood.