My good friend @petitmew asked me on twitter today how to cope with a tricky situation for getting exposure right. She's shooting a scene with lots of spotlights in it and they're throwing off the camera's ability to correctly meter the scene and choose an appropriate aperture and shutter speed.
Spotlights can confuse the camera's metering system
To understand what's going wrong we need to know how the camera meters the scene. By default the camera looks at our picture and tries to figure out where the subject is based on an internal database of picture types. Then it looks at how bright everything is, giving extra importance to the subject and chooses the aperture and shutter speed that will make the average brightness "mid grey". So there will be some stuff darker and some brighter but the average will be mid grey.
However, your camera has more than one way of metering the scene. The default mode is intelligent and that database of picture types gets probably 99% of pictures right. But sometimes the camera can be fooled and that's what's going wrong with those spotlights. So it's time to choose a different metering mode. Most cameras have a "spot" or "partial" metering mode. Instead of looking at the whole picture, "spot" metering will just look at the very middle of the picture and try to make that part mid grey. "Partial" metering is the same but the spot in the middle of the picture is a little bigger. Sometimes there will be a circle in the middle of your viewfinder showing the partial metering area.
Spot metering is useful because it lets you control exactly what parts of the scene the camera will use to make its decision about aperture and shutter speed. You'll typically want to pick a face if you're shooting people or something that should be about mid-grey in the scene. For extra credit you might want to add +2/3 of a stop exposure compensation if you're metering off a face. Once you're metering off your subject instead of the whole scene you will, of course, find that the spotlights are blown out and that's as it should be. You want your subject to be well exposed and the camera can only handle a small range of brightnesses above and below that midpoint.
The down side of using spot or partial metering is that you have to be very careful and steady handed while shooting to make sure the spot is in the right place. And if you forget you're in spot metering mode you'll get some very strange pictures until you figure out what's wrong. So if the light in your scene isn't changing I'd suggest using spot metering to get a good exposure then make a note of the aperture and shutter speed that the camera chose. Switch to manual mode, dial in those numbers and shoot away without worrying about metering any more.
Have fun and let me know if it works for you!