When should I use spot metering?
Metering the picture as if it is all equally important (matrix) or biasing the metering towards the middle (center weighted) is all well and good, but sometimes you really are only interested in one tiny part of the frame.
In these situations you need a metering mode that ignores most of the frame and only measures a tiny area – spot or partial metering is for you. Spot metering measures a spot 3% or 4% of the frame, does all the light metering there and ignores the rest of the frame. Partial metering (found on certain Canon cameras) is similar but has a bigger spot – about 8% of the frame but still ignores the other 92%. The spot has a hard edge, different to the fuzzy edge of the big spot of centre-weighed. (Sorry about the American spelling, it’s just easier and it’s what it’s called in the menus!)
Let’s imagine that you have someone on stage lit by stage lighting that you want to get the exposure exactly right, and you don’t care about the background. Some Canon literature uses a picture of Lady Gaga, believe it or not!
But it’s not just rock gods that can be completely different to the rest of the shot – flowers can also be very different in their reflectivity to the rest of the shot:
Another good use of this type of metering is when photographing stained-glass windows. The stain glass is always difficult as it is transmitted light – so the trick is to spot meter on the reveal of the window just inside. This is reflected light, so tends to meter about right. If you are fanatical about metering you might want to check out the work of Jim Zuckerman – he is the master of this kind of stuff – photographing black panthers in a coal shed – that kind of thing.