Originally posted by marcusBMG I don
I don't understand this. With a K mount lens, "K" or "M" series, a pentax dslr simply doesn't stop down at all in Av mode. This is because the "crippled" KAF mount doesn't have the second mechanical aperture connection that the film cameras have, this comunicates the f-stop set. Moving the aperture ring of a K/M lens on a dslr to a larger F number while in Av mode does nothing, the iris is held open by the camera. The only hack that can be done is to not fully mount the lens. In this case the lens aperture lever doesn't connect with the camera aperture actuator and so the iris does stop down. However you have to careful of your lens falling off!
Av mode works with M42 lenses because there is no connection between the camera and the iris, you have stop down aperture priority.
With the K30, if there was no electrical connection to the lensmount, the camera would try to stop down the lens as much as set in by the aperture ring. So normally the aperture is held open, but on pressing the shutter, the camera would stop down the lens to whatever the aperture ring was set to.
Now the interesting part is, the K30 would adjust exposure automatically for widest aperture (e.g. f1.7) by itself. Now if I set the aperture ring to f4, the K30 would still measure at f1.7 and stop down the lens (as far as it would go) to f4 prior to taking the shot. This results in a picture that is overexposed by exactly 2.5 steps (slight error in previous post).
Now these 2.5 steps can be set by using the cameras exposure compensation, thus eliminating the need for the green button.
This in turn means that you have to adjust EV comp. with changing the aperture, however this is much less trouble than the classic stop-down metering.
For simplification ev-steps for M 50mm f1.7:
1.7 -> +0
2.0 -> +0.5
2.8 -> +1.5
4 -> 2.5