QUESTION: I'm curious, how often do people use exposure lock on their DSLR's?
And when they do, what's the best (or recommended) technique get proper exposure, in particular using a Canon 5D Mk 2? -Mark
ANSWER: Mark, there are some good questions here.
In simplest terms, when you press the exposure lock button, your camera will retain that measured lighting information for a short period of time. It's "locked in," so to speak.
On Canons, that's usually around 16 seconds. This allows you enough time to recompose and take the shot. Exposure lock is handy in mixed lighting conditions that might fool the camera's default metering.
In terms of what is measured, that depends on the metering pattern. Evaluative (or Matrix on Nikons) breaks the entire scene into several parts and compares the gathered information to profiles stored in the camera. If, for example, the parts at the top of the frame are very bright, the camera will guess that's a bright sky and compensate accordingly. In spot metering mode, the center of the frame is used to measure the light. Of course this can vary from camera to camera.
I think using exposure lock is easiest in spot metering mode. Just find a small neutral toned area that you want to use for your metering, point the camera there, and lock it in.
One thing to understand is that proper exposure is what you, the photographer, want it to be. Camera manufacturers decided a long time ago that the world should be metered at 18% gray. This means that no matter what metering mode you use, or how you meter, your camera is trying to balance the scene to 18% gray. It is up to us to decide if that's what we want.
For example a scene at the beach in my home town will be under-exposed by the camera trying to clamp down on the very bright sand. In this case, I would want to over expose the scene using exposure compensation or y adjusting my settings manually. More and more, the cameras these days are getting better at recognizing the lighting situation in front of them and compensating for it. But the default is still 18% gray. We've got a couple of articles on Exposure Compensation on this site like this one by Derrick.
Adds Derrick: I sometimes use exposure lock on my Canons as a quick way to set exposure for outdoor portraits. Instead of metering on the subject, I point the camera down to the grass (that's receiving the same light as the subject) or up to the blue sky, press exposure lock, then recompose and take the picture.
As Jeremy noted, the camera is calibrated for 18 percent gray. Lucky enough, blue skies and foliage fall in that range. This technique works great!