Metering with old lenses on lower bodies.
There are a lot of threads about using AI and AIS lenses on the bodies that can't meter.
I just discovered there is an app for that. There is a light meter app for iPhone. It's free. It has three dials -- iso, aperture, shutter. You set ISO, then move either of the other two and the app calculates the third. Pretty neat. I'm not sure if it's averaging the whole image or center weighted. It's called "light meter". Probably something similar on android.
I still think it's easier to just shoot a test frame.