Yesterday I started trying to use the AE Lock button on the 5D mk 2. The primary purpose (for now) is to prevent skies from being blown out when the main subject is in a darker portion of the frame. I will say that the function works marvelously - but only when it works!!
What I mean by that is that when I get the exposure locked (shown by the * icon in the viewfinder), most of the time it unlocks itself before I can get the image recomposed and re-focused. Sometimes it works but most of the time it will unlock.
Am I just not fast enough? It's not so bad when handholding but if on a tripod it's very hard to meter the sky, recompose and refocus on your subject, all in about 3-4 seconds. It's further complicated by the fact that I like to use the two second timer so I don't shake the camera; and even more so when I'm on my knees and trying to look up through the viewfinder. It gets real tiring, real fast; especially for these old joints .
In some places I've read, it says to keep the shutter button halfway down after you've metered the sky (or where ever), but it seems to me that if you've pointed the camera at the sky and pressed the shutter button halfway down, and never re-focus, then your subject will be out of focus, so I don't see how that helps. As I said, I just started this adventure yesterday and so I don't have a lot of images to look over - just trying to use (gasp ... ) LOGIC.
By googling around, I did find one solution that seems to work. In Av (my normal mode), meter the sky and remember the f stop and shutter speed. Then switch to manual mode and apply those same settings, focus and frame normally and take the shot. All the shots I've taken using this method seem to have turned out wonderfully, and I must say it's much more user friendly than trying to make AE Lock work for me.
But I know I'm missing some REALLY important here, and I'd love to know what it is.
Sleepless near Seattle ...