Originally posted by davids8560 Where's an explanation of stop down metering?
Not sure what you mean.
How it works or how you do it?
Stopping down a lens in general means not using the widest aperture.
The (modern) camera always measures the exposure with the lens wide open (or what it thinks is wide open).
With an A lens (or newer), the camera knows which aperture is set (and the widest aperture of the lens) and can adjust the shutter speed / ISO according to the selected aperture.
With a M42 / K / M lens, the camera has no knowledge of the selected aperture or the widest aperture, so can't adjust the shutter speed / ISO automatically in Av mode. Therefore in Av mode, the camera assumes that what it measured is what will be used and does not adjust anything; for that reason it will also not attempt to close the aperture while taking the photo.
With M42 lenses this is no problem in Av mode, because if you change the aperture it will physically change immediately, the camera will detect it and adjust shutter speed/ ISO thinking that it's wide open.
In M mode, you can use the green button (or optical DOF preview) to close the lens to its stopped down aperture and the camera will measure the exposure while the lens is stopped down. So now it can adjust shutter speed / ISO for correct exposure.
Note that the camera will never have a means to close a M42 lens; in M mode (just like in Av mode) it does not matter as the lens is already closed down and the camera thinks it has closed the lens down when it does the exposure measurement.
Hope this clears it up a a bit.