Exposure Lock

A very handy button to know is the 'Exposure Lock' button. When you have focused on your subject you can press the exposure lock button. This will then lock the exposure that the camera has determined until the shutter button is pressed (shutter speed in Av mode, aperture in Tv mode).

The 'Exposure Lock' button

But why is this useful? Let's say for example that your subject is in shadow, but the shadow only fills a small fraction of the frame. You take your shot with partial metering, but it's still over exposed. This is when to use the exposure lock button. Focus the camera on a shadow that fills the frame. Press the exposure lock button, recompose your original shot and shoot. The camera will have locked the settings it decided that were best to use for the shadow and used them on the shot. Let's look at an example.

Left: Partial Metering, Right: Exposure Lock

The first shot I took (left) the camel was under-exposed; the camera had calculated the exposure based on the brighter background. For the shot on the right I focused on the shadow (which extended close to me), pressed the exposure lock button and re-composed on the camel; the camel is now exposed correctly. The background highlights are over-exposed, but it is not a huge issue and is easily fixed in Photoshop (especially if you have taken the shot using RAW). This technique is a simple and easy to use method to get the correct exposure of a subject. If you have a zoom lens it is even easier; you can zoom in on your subject, exposure lock and then zoom out to recompose.