Quote Originally Posted by Seele
First of all, I think it is instructive to get the terminology right. The manual focus, focal-plane-shuttered Minolta SLRs have a mount called the "SR"; "MC mount" is actually short for "SR mount with additional Meter Coupling", and "MD" adds another coupling which enables the aperture to be set by the camera body, necessary for programmed or shutter-speed priority exposure automation; but the mount is still SR mount.

However, the SR mount also includes an internal iris diaphragm actuator, so some lenses without automatic diaphragm, even when in SR mount, cannot utilise its full functionality, and require the iris to be stopped-down manually, which also means stop-down metering is required if the camera has TTL metering.

Minolta supplied adapters for using M42 and Exakta lenses on its cameras for quite some time, it is possible to find original ones but perhaps possible to find aftermarket ones, although it is not known if that for the Exakta mount was ever made by aftermarket makers.

Since the aperture stop-down actuation system works in a radial direction, and the M42 arrangement works in a direction parallel to the optical axis, using an adapter to fit a M42 lens to a Minolta camera means that the diaphragm will never be closed down to its selected value automatically by the camera. So the lens has to be made to work as a fully manual lens where the aperture has to be manually closed down.

Many early M42 lenses (and even some later ones) have pre-set manual diaphragms, like those in T-mount which, obviously, has no auto-manual switch. But for a M42 lens with automatic diaphragm, the mechanism has to be disengaged or you will be shooting at full aperture no matter which aperture setting you have selected. In that sense, if you have a lens with automatic diaphragm but without a switch to disengage it, you would not be able to use it correctly via this adapter: the first lens that comes to mind would be Hugo Meyer's Domiplan standard lens.

Different adapters are designed differently, however. When Praktica introduced the B-series, the adapter for using M42 lenses were designed concurrently to ensure maximum compatibility; therefore, inside the adapter is an additional flange, so when the lens is screwed into the adapter, the flange pushes in the actuator pin all the way home, so even without an auto-manual switch, the lens is forced to work as one with manual diaphragm. Pentax should have done that with the adapter when it switched from M42 to the K-mount, but as all Pentax M42 lenses with automatic diaphragm were equipped with the auto-manual switch anyway, the adapter does not incorporate that flange to force the lens to work as manual, which might have caused some grossly over-exposed negatives.
Thanks, that was a great education, not that I know, that it actually applies to real life shooting situations, but filled in some of the blanks that I have found over the 25 years I have been shooting Minolta gear..

I am glad someone had the time to learn this amount of information and pass it along to the rest of us.

Thanks again.

Dave