To help ensure a proper exposure, your Rebel T6i/750D camera continually meters the light until the moment you press the shutter button fully to shoot the picture. In autoexposure modes — that is, any mode but M — the camera also keeps adjusting exposure settings as needed.
For most situations, this approach works great, resulting in the right settings for the light that's striking your subject when you capture the image. But on occasion, you may want to lock in a certain combination of exposure settings. For example, perhaps you want your subject to appear at the far edge of the frame. If you were to use the normal shooting technique, you would place the subject under a focus point, press the shutter button halfway to lock focus and set the initial exposure, and then reframe to your desired composition to take the shot. The problem is that exposure is then recalculated based on the new framing, which can leave your subject under‐ or overexposed.
The easiest way to lock in exposure settings is to switch to M (manual exposure) mode and use the same f‐stop, shutter speed, and ISO settings for each shot. In manual exposure mode, the camera never overrides your exposure decisions; they're locked until you change them.
But if you prefer to stay in P, Tv, or Av mode, you can lock the current autoexposure settings by using the AE (autoexposure) Lock function. Here's how to do it:
Press the shutter button halfway.
If you're using autofocusing, focus is locked at this point.
Press the AE Lock button.
Exposure is now locked and remains locked for 4 seconds, even if you release the AE Lock button and the shutter button.
To remind you that AE Lock is in force, the camera displays a little asterisk at the left end of the viewfinder or, in Live View mode, in the lower‐left corner of the display. If you need to relock exposure, just press the AE Lock button again.
Note: If your goal is to use the same exposure settings for multiple shots, you must keep the AE Lock button pressed during the entire series of pictures. Every time you let up on the button and press it again, you lock exposure anew based on the light that's in the frame.
One other critical point to remember about using AE Lock: The camera establishes and locks exposure differently depending on the metering mode, the focusing mode (automatic or manual), and on an autofocusing setting called AF Point Selection mode. Here's the scoop:
Evaluative metering and automatic AF Point Selection: Exposure is locked on the focusing point that achieved focus.
Evaluative metering and manual AF Point Selection: Exposure is locked on the selected autofocus point.
All other metering modes: Exposure is based on the center autofocus point, regardless of the AF Point Selection mode.
Manual focusing: Exposure is based on the center autofocus point.