The M42 lens mount is a screw thread mounting standard for attaching lenses to 35 mm cameras, primarily single-lens reflex models. It is more accurately known as the M42 × 1 mm standard, which means that it is a metric screw thread of 42 mm diameter and 1 mm thread pitch. The M42 lens mount should not be confused with the T-mount, which shares the 42 mm throat diameter, but differs by having a 0.75 mm thread pitch. It was first used by the East German brands VEB Zeiss Ikon in the Contax S of 1949, and KW in the Praktica of the same year. VEB Zeiss Ikon and KW were merged into the Pentacon brand in 1959, along with several other East German camera makers. M42 thread mount cameras first became well known under the Praktica brand, and thus the M42 mount is known as the Praktica thread mount. Since there were no proprietary elements to the M42 mount, many other manufacturers used it, leading to it being called the Universal thread mount or Universal screw mount by many. The M42 mount was also used by Pentax; thus, it is also commonly known as the Pentax thread mount, although Pentax did not originate it.
Evolution and automation The M42 mount was first developed by Carl Zeiss at their Jena plant in 1938 at the request of the KW camera company for their Praktica line, which had previously used M40 (40 mm by 1 mm DIN). The first lenses were plain stop-down design, but many manufacturers extended the M42 lens mount to provide extra features. The first innovation was the pre-select type, which allowed an aperture value to be pre-selected without actually closing the aperture, with a separate ring to close down the aperture quickly to the chosen value. This gave the user the benefit of comfortable framing and focusing with a bright viewfinder and clear focus separation, and then closing the aperture without the need to remove the eye from the eyepiece. A further development followed with "auto" lenses, which have a pin in the mount which closes the aperture against a spring to the chosen setting when it is pushed. This was adopted as a common standard by virtually all lens manufacturers. Cameras designed for these lenses have a bar in the bottom of the mount which depresses the pin when the shutter is released. The first cameras, such as the Praktica Nova range, used physical finger pressure on the shutter button to operate the bar and close the aperture, allowing a stopped-down preview of the depth of field before the shutter fired. However, this function was removed in later Praktica models because some users found it was possible, with longer exposure times, to release the shutter button and open the aperture before the shutter had closed. The bar on Pentax Spotmatic cameras is operated by spring pressure with timing linked to the shutter, but these cameras also had a separate switch for the light meter circuit which closed the aperture and gave the depth of field preview in this way. To allow auto lenses to be used on earlier cameras without the bar, many lenses were provided with a switch or button to put the lens into stop-down mode, commonly referred to as the Auto-Manual or A/M switch. The last development of M42 lenses was the introduction of a link between camera and lens to transmit the lens aperture setting, which allowed light metering with the aperture remaining wide open. The means of doing this was not standardised, Praktica's method was to use an electrical connection which transmitted a variable resistance value to the camera's metering circuit. Asahi Pentax developed an additional lever on the lens that operated a variable resistor in the camera mount. These mechanisms spurred the use of electronic shutters linked to the metering circuit, allowing for automated shutter speed selection by the camera (aperture priority). M42 cameras with wide open metering facility include:
Praktica PLC2, PLC3, EE 2 and EE 3 (1975, 1977, 1979) using Pentacon Electric lenses Fujica ST705, ST801, ST901 Olympus FTL (1970) Pentax Spotmatic F, Electro Spotmatic (Japan only), ES and ESII (1975, 1971-3) Yashica Electro AX (1972) Zenit 18 (with Zenitar ME1 lens only) GAF L-ES Hanimex Praktica Nova 1B Mamiya/Sekor 1000 DTL or automated selection of aperture (shutter priority) for
… excerpt ends here. Continue reading the full article.
