M42 to A mount

Blame wrote: Woodrim Good advice. Explanation might be a little off. It is not just an exposure problem. Think is that the adapter tells the camera that it is f/1.4 wide open. So if you set the aperture to anything higher the camera attempts to part close the iris when you take a picture. Catch is that it can't. The M42 adapter won't let the lever move so the camera thinks it's broken and locks up with an error message. At least that is what happens with my a900.

Well, I could be wrong, but so could you. In my experience, but with an A200, I will get over-exposures if I leave the aperture setting above f/1.4 by up to a few stops, but if too more than that,and I don't know the limit, it will get an error. The fact that the error only comes with drastic overexposure and not a stop or three is what leads me to believe it is an overwhelming of the sensor that causes the error. Try setting your camera at f/2.8 and see if you don't get locked up, but only overexpose.

EssexUK wrote: I take it the metering system will still work and set appropriate shutter speed.

Don't get too confident    Some lenses overexpose on my camera requiring me to set a full stop exposure compensation. Then when stopping down the opposite occurs - the further stopped down, the more the underexposure. However, this is my camera and others may perform differently.

The camera indeed presents an error if it cannot physically move the aperture actuator when requested (by setting an aperture value other than the maximum of the adapter). Whether or not the aperture actuator can move depends on the specific camera, lens, and adapter you have (and luck, how you're holding the lens and camera, etc). If you're unconvinced, try this: remove the lens from the adapter and release the shutter with any aperture value; it probably won't lock up anymore (or then your adapter is different than mine). Put the lens back on and it will probably lock up again because it's hitting the lens (or the weight of the lens causes the adapter to shift or whatever).

Personally I bent the aperture actuator on my A100 by an imperceptible fraction of a millimetre and now it never locks up with any aperture value… I didn't bother to modify the A900 thus as it hasn't been locking up as much to begin with and the only time I set an aperture value other than the maximum is by accident.