If you’re using Evaluative exposure metering, exposure is also locked with your press of the AF-ON button; in other metering modes, exposure is set at the time you fully depress the shutter button, just as when you use the shutter button to set focus. You’ll notice that when you do go to take the shot, the shutter button has already lowered its head halfway as if by magic.
Using the AF-ON button can also save time when you’re shooting a series of images of the same subject. If you use the shutter button to set focus, you have to press halfway to set focus for each shot. But if you keep pressing the AF-ON button, you can take as many shots as you want, and the camera will use the same focusing distance.You also have the option of using the AF-ON button to set focus only and the shutter button to set exposure only, or vice versa. Some photographers like to use this setup so that exposure isn’t locked along with focus in the Evaluative metering mode.