Originally posted by dragonfly For an M42 lens, doesn't Av mode work similar to P mode?
More or less - you don't need to change any settings on the camera to adjust exposure, just use the aperture ring on the lens to adjust that and the camera will take care of the rest.
One thing that I didn't see mentioned (unless I overlooked it) is to make sure that your M42 lenses are set to "M" or "Manual",
not "A" or "Auto". (This threw me for a little while until I realized it.) Auto mode only works on actual M42 cameras that can press the pin on the back of the lens. On a DSLR, leaving it in Auto mode will mean that you'll shoot at maximum aperture all the time, no matter where you turn the aperture ring.
I tend to prefer M42s to Pentax-M lenses. In Av mode, they behave very much like a modern lens, but the "M" lenses require you to do the extra step of stopping down before each exposure, unless you are shooting wide open in which case you can also use Av mode.