Exposure lock is a term that applies to different kinds of exposure and how long you might want the exposure to be locked. EOS cameras have two independent metering systems, one for ambient light (AE-lock) and another for a camera-controlled Speedlite (FE-Lock). Some cameras combine the lock of both flash and ambient, while others feature individual lock buttons for flash and ambient.
If you’re using One-Shot AF, you can unwittingly lock the exposure; when focus is achieved in this mode, cameras automatically lock the exposure with Evaluative metering. This makes sense as Evaluative is using info from the focus system to determine the position of the subject, and recomposing might change the brightness of the scene by including more brighter or darker areas – but not change the exposure on the original subject.
