Everybody knows, by now, that there are two steps to a shutter button:

  1. Pressing the shutter half-way activates the auto-focus, if the focus is still associated to the shutter button, and sets the automatic exposure combination in Av, Tv, or P mode.
  2. Pressing completely the shutter takes the photo.

What most people do not realize is that when you are in Evaluative Metering and Single Shot AF mode, pressing the shutter half way locks the exposure for 4 seconds. This is the equivalent of pressing the *.

This doesn’t apply to the Evaluative Metering and the AI Servo AF. The exposure will change as the focus tracks. To lock in the exposure to a maximum of 4 seconds, you must use the * button at the back. Each time you press the *, the camera forgets about the previous exposure combination and re-locks the new exposure combination for up to 4 seconds.

This applies not only to the Canon 7D, but to all the current Canon cameras: T3i, 70D, 7D, 1Dx, 5DMk3, 7DMk2 …