The name "ZM" stands for "Maksutov Mirror" or "Mirror-Meniscus lens."
The use of the mirror-meniscus design allowed for a significant reduction in the length and weight of the lenses compared to traditional lens designs.
Mirror-lens ("ML") lenses with the "ZM" designation do not have diaphragm control mechanisms, so the relative aperture value is fixed.
The focus ring is designed with "overrun," which is necessary to maintain focus at extreme distance scale values (infinity) during significant temperature changes relative to "normal." A positive side effect of this design solution is the ability to mount the lenses on cameras with a slightly larger flange distance than 45.5 mm without losing focus at "infinity" (for example, on cameras with a "Nikon F" mount, where the flange distance is 46.5 mm).
In lens designations, "MC" indicates multi-coating; "A" indicates the use of a removable adapter.
