What are the typical situations where you should use AE for locking exposure?
AE Lock is for situations where you want to use metered exposure later than you metered it:
- locking exposure capturing parts of a panorama - for seamless stitching, frames with similar exposure will work better than differently exposed ones;
- when you meter from a gray card, lock exposure, then remove the gray card and compose;
- you point your camera on a surface that should be exposed as 18% gray, lock exposure, recompose
- as @mattdm commented - when there are areas that would confuse matrix metering, compose so they are excluded, lock exposure, recompose
AE Lock can be thought of as a "quick version of temporary M mode". As such, you can quickly lock metered exposure and take a few shots with it. Most cameras will unlock AE after some idle time, so manual mode is more suitable when you work slowly and want to tweak things little by little.
Many people use "focus and recompose" to both focus and meter a specific location, then recompose the image without changing the settings. AE lock is a specific part of that puzzle, the metering. If there is a particular part of the composition you feel you need to meter for, then you can do that and set AE lock.
I don't personally use AE lock because I normally shoot on M anyhow. When I'm moving fast, I find my Canon cameras do fine if I focus/meter and keep the AF button down until I've recomposed. But it's all about personal preference. If you like locking in an exposure, or as previously mentioned, are shooting an 18% gray card, then AE lock makes sense.
I often use it when spot metering. I can press the "fn" button on the front of the camera to turn spot metering on, take a reading from the specific point I want, press the AE-L button, then recompose and refocus using the AF-On button before finally pressing the shutter to make the shot.
When you have the metering, focusing and taking the shot all as separate functions and not combined all into one with the shutter button you'll find you can work faster, smarter and better!
I tend to use it in situations in dark situations with lots of shadows (like a dark room with a just a spot of light from a small window or lamp). I measure the exposure where I want and then recompose the way I want.
I have used AE-L when taking sunset pictures. I would aim the camera so that the sun is just out of the shot, use AE-L to lock the exposure then recompose with the sun in the frame where I wanted it keeping the exposure locked in at the previous spot.
I do not normally shoot in manual so this allows me to get a correct exposure without too much effort on my part since I am still learning how to shoot in non-auto modes.
If you are using a Nikon DSLR here is a quick tutorial from the Nikon site: