confused with focus & recompose

I thought depressing the shutter halfway should lock the exposure indefinitely but it doesn't seem that's the case.

In one shot, I focus by pressing the shutter button halfway, then keeping the shutter button held down I recompose and shoot. It seems that I only have a limited time to recompose and shoot or else the camera will try to focus again. Just through trial and error I've discovered that if while depressing the shutter halfway I hit the af-on button my focus will remain locked for an indefinite amount of time. But I feel like this isn't right, that the af-on button has another purpose. I don't want to get used to doing something the wrong way. Am I doing it wrong? What should I be doing?

seems you may want to check certain settings, like make sure your Af is on AI-Servo and on One-Shot. Or could be that your not holding down the shutter button hard enough and when you recompose your finger moves slightly and thus tries to re-focus when you re adjust your finger.

You need to go into your Custom Functions and set the AF/AE buttons to do what you need them to. They are customisable and it may be yours aren't set to work in default mode.

also, i dont think hitting the shutter halfway would lock exposure. i think you need to:
make sure AF mode is on OneShot
before you recompose, hit the AE button. that should lock your exposure should the camera meter the scene differently after you recompose

As long as the AF is set to a rear button as compared to the half press, you can leave it on servo and it will be fine. Hitting the shutter halfway won't lock exposure, but holding it will (assuming that CFn 4 option is selected).

Learn to use Exposure Lock button, and learn to use Focus on the back of the camera...that leaves the shutter button to ONLY take the photo, according to the focus setting and exposure setting which you choose before you recompose and shoot.