M42 is a screw-type lens mount used to attach lenses to SLR cameras. M42 was originally designed by Carl Zeiss in 1938, but was adopted by a large number of companies throughout Europe and Asia. Some of the notable companies that used the M42 lens mounting system include Praktica, Pentax, Carl Zeiss, Fujifilm, and Zenit.
Because of the simple design of the M42 lens mount and its long flange distance, adapting them to modern-day SLR or mirrorless cameras is very easy. There have been a huge number of lenses produced in M42 mount, covering every focal length and level of optical quality.