Originally posted by Marc Sabatella And K-mount lenses with no "A" position on the aperture ring will only work in manual ("M") exposure mode, but the camera *will* meter with them and set an appropriate shutter speed for the selected aperture when you press the green button.
I think non-A and screwmount lenses will even work in Av mode, as long as you use them wide open. If you stop down at the aperture ring, all you have to do is dial in an exposure compensation of + 1 EV per stop. That way, you wont have to push the green button (which I keep forgetting all the time ...).
And there is a focus assist function in the viewfinder for manual focus lenses. It displays a green hexagon as soon as the central AF point is sharp. If the camera is set to AF, it wont even fire before it thinks the center is sharp
You can keep the shutter pressed and turn the focus ring on your manual lens and it will fire as soon as the center is in focus (look for "focus trap").
One potential issues is that TTL flash is not supported with non-A and screwmount lenses AFAIK. Flash will work but fire at full power which I find quite useless when using fill-in flash in bright conditions.
Except for that, old lenses work very nicely