M42 is a threaded mount with an outer diameter of 42mm, a thread pitch of 1mm and a flange focal distance of 45.5mm, used to mount interchangeable lenses on 35mm SLR cameras. It was developed by VEB Kamera-Werkstätten Niedersedlitz for the Praktica 35mm SLR camera released in October 1949. The mount was also used from September 1949 in the Contax S 35mm SLR camera manufactured by Mechanik Zeiss Ikon VEB Dresden. In 1959, the above companies, along with VEB Altissa-Camera-Werke, VEB Aspecta Dresden and VEB Welta-Kamera-Werke Freital/Sa., were merged into VEB Kamera- und Kinowerke Dresden (since 1964 - VEB Pentacon Dresden), and the production of cameras continued under the Praktica brand.
Since the M42 mount first became known by Praktica cameras, it is sometimes referred to as the Praktica thread mount. After Asahi Optical Co., Ltd. chose this mount for its Asahi Pentax family of 35mm SLR cameras, it became known as the Pentax thread mount. Finally, after the mount became widespread and many manufacturers began to produce M42 lenses, it was also called the Universal Thread Mount.
Initially, the mount was designed for stop-down metering, but in 1956, when the Contax F and Praktica FX 2 cameras were released, it received an automatic aperture: a pin at the rear of the mount, pressed by the camera mechanism after the shutter was released, closed the aperture to the value selected with the aperture ring on the lens barrel. This method was adopted by other manufacturers and became standard. To ensure that auto diaphragm lenses could be used with older cameras, lenses were often equipped with AUTO - MAN. (A - M) switch. In MAN. (M) mode, rotation of the aperture ring immediately closed the aperture to the selected value, while in AUTO (A) mode, the aperture remained fully open until it was closed by the camera mechanism. Fully open diaphragm ensures maximum brightness of the image in the viewfinder which is very important for viewing and focusing.
In the 1970s, at the end of the popularity of the mount, a number of manufacturers began to modify it so that the lens transmitted the selected aperture value to the camera, which allowed exposure metering to be performed at full aperture. However, the implementation of aperture indexing differed from system to system, resulting in compatibility issues.
In the 1970s, all major camera manufacturers began transition to bayonet mounts, which had a significant advantage over screw mounts in providing precise orientation of the lens with respect to the camera, which is especially important for mechanical aperture indexing, auto diaphragm and alignment of electrical contacts (if available). Technical limitations of the M42 screw mount meant that cameras and lenses with this mount were no longer produced by the major manufacturers in the late 1970s / early 1980s.