Super-Multi-Coated and SMC TAKUMARs are the most sophisticated lenses, both mechanically and optically, in the Asahi lineup for 35 mm SLR cameras with M42 thread mount. One of the distinguishing features of these lenses are the MAN. — AUTO diaphragm mode switch located on the lens barrel near the mount. In order to use these lenses with contemporary digital SLR or mirrorless cameras which does not support automatic aperture control with M42 lenses, you have first to select manual aperture control mode by setting the switch to MAN. setting. In this mode the rotation of the aperture ring leads to immediate closing of the aperture to the selected value (in AUTO mode it happens only after the mechanism of the camera pushes the protruding aperture pin located at the rear part of the lens mount).
However you may probably tried to switch to AUTO mode and after that discovered that the switch is stuck and can’t be returned back to the MAN. setting (or it was only possible when the rear lens cap was put on) and you may only shoot at the fully open aperture. So, what’s the problem with that switch and why it happens?
TAKUMAR M42 lenses are compatible with all Asahi 35 mm SLR cameras with M42 thread mount. However the company constantly updated its cameras and provided them with new features. In 1973 saw the light Pentax Spotmatic F which supported the open-aperture exposure metering (its predecessor, Pentax Spotmatic II, was produced at the same time and did not have such feature).
When the user mounted Super-TAKUMAR or earlier lenses, the Pentax Spotmatic F knew that they do not support the open-aperture exposure metering and performed the stop-down metering. The mounts of newer Super-Multi-Coated and SMC lenses have a special linkage which tells the camera that the lens supports the open-aperture exposure metering. However when the photographer tried to close down the aperture manually without using the mechanism of the camera, it resulted in serious exposure metering errors because the camera thought that the closed aperture is the maximum (fully open) aperture of the lens and underexposed the image. For example, if the photographer set the aperture F/8 and the camera thought that it meters at F/1.8, the exposure error was 4 and a half of stop (that is really too much). In order to eliminate such situation all Super-Multi-Coated and SMC lenses have an additional pin at the rear part of the mount:
When mounting a Super-Multi-Coated or SMC lens, the pin was fit into the special groove on the mount of Pentax Spotmatic F. When the pin was allowed to push out, the MAN. — AUTO switch was locked at the AUTO setting and can’t be switched back to the MAN. setting. With older Asahi cameras and third part M42 cameras which did not support the open-aperture exposure metering and did not have a special groove on their mounts, the pin was always pushed in and the MAN. — AUTO switch worked as usual.
The conventional Asahi TAKUMAR rear lens caps for Super-TAKUMAR or older lenses do not push the pin in. So, if the MAN. — AUTO switch is blocked and you need to switch back to the manual aperture control mode, just push the pin in by your fingernail and then switch to MAN. mode. Now you can use your Super-Multi-Coated or SMC TAKUMAR lens with your digital SLR or mirrorless camera via the corresponding adapter, control the aperture by the corresponding ring located on the lens barrel and perform the stop-down exposure metering.