Canon DSLRs with adapters have given a new lease on life to many classic lenses with the M42 (Pentax-Praktica) screw mount. Here "M42" means "metric, 42 mm" and has nothing to do with the Orion Nebula, Messier 42.
These lenses are mechanically sturdy and free of flexure in long exposures. Optical quality varies from brand to brand, but Pentax SMC Takumars and East German (Jena) Zeiss lenses are often quite good. In their own time, these Zeiss lenses were not marketed in the United States, and we Americans didn't know what we were missing.2 Today, these lenses are abundant on the secondhand market, which is fully international thanks to eBay.
Because of its simplicity and great ruggedness, M42 is a standard that won't die. It was introduced by East German Zeiss on the Contax S in 1949 and promoted by Pentax as a "universal mount" in the 1960s. Pentax switched to a bayonet mount in 1976, but Zenit brand M42-mount cameras and lenses were made in Russia until 2005, and to this day Cosina makes the M42-mount Voigtlander Bessaflex, marketing it as a living antique for people who prefer older cameras.
Figure 7.11 shows what an M42 lens mount looks like. Don't confuse it with the T-mount, whose threads are closer together. The pin enables the camera to stop the lens down to the selected aperture when taking a picture; the rest of the time, the lens is wide open so you can view and focus easily. When using an adapter, you'll want to disable this feature by switching the lens to manual (M). Older versions of the mount have only manual diaphragms and no pin.
Beware of M42 mounts with extra protrusions that may interfere with the use of an adapter. In the 1970s, Pentax, Olympus, Mamiya, and other manufacturers added aperture indexing so that the exposure meter could sense the selected
2 From 1945 to 1990, like Germany itself, Zeiss was partitioned into separate eastern and western companies, and in America only the western Zeiss could use the trademarks and product f -stop without stopping the lens down. Unfortunately, they each did it in their own way. Pentax added a small, retractable pin that usually does not cause problems; the others made larger modifications.
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