I have a Nikon FM3a, which only has center-weighted metering.
I know the zone system, and I know how to use a spot meter, but I am not sure how to use the center-weighted meter.
It's easy to use a spot meter, because you always know exactly what you are measuring, and you can decide what zone to place it in, etc, but with center-weighted metering I am never sure what I am metering exactly.
For example, if I try to measure the shadows, I'll put them in the center disc, and they will account for most (60%) of my exposure reading, but the meter will also read some light from the edge of the frame.
Is that weighted enough to invalidate my measurement? Who knows? I can't tell!
In fact, the meter is not even weighted evenly 60% in the circle, and 40% outside the circle. The weight varies continuously across the frame. Let's say I want to meter off a bark of a tree, which I plan to place in Zone V. I get close to the bark, and make my exposure. I am very close to the tree, so that the bark is much larger than the 60% circle in the viewfinder.
However, some tiny amount of sky is visible just at the edge of the frame. If I move my camera just a tiny bit (the bark is within the 60% circle at all times), the reading varies because the light from the edge of the frame changes (even though that light is weighted relatively little).
What's the best way to use this metering mode? I've only shot b&w on this camera so far, but I plan to shoot slides, where correct exposure is much more important.
For the level of control you desire with a camera that only offers center weighted average metering and no instant histogram on the non-existent LCD that is not on the back of the camera, you're going to need to go the route of a handheld light meter.