Originally posted by zekewhipper RioRico: What does the green button have to do with metering?
I use old M42 lenses on my K20D all the time, and I never have to touch that button for any reason. The camera automatically adjusts the exposure readings in real-time as you turn the aperture ring.
If you put an M42 lens with adapter on the K20D and set any Auto mode, it defaults to Av and yes, shutter speed is automatically set, as is ISO within its Auto range. This works fine on many occasions. But there are times when manual metering and exposing are desirable. So except with A-type lenses, I normally leave my K20D in TAv, right next to M, and quick-switch to M when I want manual shooting.
One such situation: The subject is severely backlit, beyond the +2 EV compensation range. So move in close, meter the subject manually, then move back to position and shoot. And chimp, and adjust and reshoot as needed. Another situation: Shooting a series in a varied scene, like a desertscape with blue skies, white clouds, bright hills, dark vegetation. If you shoot in Av, every exposure differs, depending on just where the lens points. So meter manually, find a good exposure, and stick with it until the light changes.
When I want control over the exposure, I meter manually. If I'm in a hurry to grab a shot and consistency doesn't matter, I shoot Auto.