AE Lock is an abbreviation for Auto Exposure Lock which functions to lock the exposure on the camera. If we use auto mode, the exposure will change depending on the metering of the object. Apart from that, it can be affected by unstable and changing lighting. This will be very annoying when you want to take several photos at once for panoramas and focus stacking.
This feature is owned by all Canon EOS and Canon PowerShot cameras which are marked with a star icon (*).
AE Lock does not function in manual mode, only in program mode (P), shutter priority (Tv), aperture priority (Av) and A-DEP mode. If the AE Lock feature is on, there will be an asterisk on the camera as below.
After you determine the exposure you will use, you can press the AE Lock button so that the exposure will be locked. You can try shifting the focus on the object and you will get the same exposure. This feature only lasts for a few seconds. If the star icon disappears, it means the AE Lock feature is no longer working and you need to reset it. If you want to use this feature for a long time, you can press continuously until you finish shooting.
There are tips for those of you who want to use this feature for a fairly long period of time, up to 30 minutes. But it can only be used in live view shooting, cannot use viewfinder shooting mode.
1. You can click menu button.
Source : https://support-my.canon-asia.com/contents/MY/EN/8202214500.html
2. Click the "Q" button select the camera icon
3. Turn the wheel at the top right of the camera, select the white boxes
4. Rotate the circle at the bottom of the camera, select Metering Timer, then click SET
5. Select the time you want, click SET
6. Press the shutter button 1/2 to get focus on the object, then press the star icon, then AE Lock will work according to the time you specified.
If you want to cancel the AE lock, you can change the ISO and the exposure will return to its original setting. If you continue to hold down the AE lock button, you can release it and wait about 4-6 seconds for the star to disappear. Alternatively, you can press the AF point button next to the AE lock button.
What you need to know is that when using autofocus in One Shot AF and Evaluative metering modes, you only need to press the shutter 1/2 (not fully) and the exposure will be locked automatically without having to press the AE lock button. Note that this feature is only available when using autofocus and metering modes, and not in other modes.