Eric, you use the green button to meter if in M mode. It took me a while to realize it was working because I don't hear the characteristic
click-click of the aperture actuator thingy moving when it does it.
It is too early to say definitively, but last night just before bedtime I played around with two or three lenses to see how they metered in Av mode, as that is my preferred method. As we all know, there has long been a problem of lenses underexposing by different amounts at different apertures, which for many has made Av shooting a non-starter. But I tried shots wide open and at f8 and was shocked to see they exposed
identically. The histogram revealed underexposure, but by
the same amount, meaning a single EV adjustment should work across a wide range of apertures. I tried one lens wide open, at f8, and at f22 and the f22 didn't match up in exposure, but the f2 shot and the f8 shot did.
This is going to require more looking into, so don't hold me to the accuracy of that information just yet, but it does look promising.
Pentax K-5
Auto-Takumar 55/2