Canon EOS Rebel T6s EF-S 18-135mm IS STM Lens Kit Instruction Manual - Page 129

A Locking the ExposureK You can lock the exposure when the area of focus is to be different from the exposure metering area or when you want to take multiple shots at the same exposure setting. Press the button to lock the exposure, then recompose and take the picture. This is called AE lock. It is effective for backlit subjects, etc. 1 Focus on the subject.  Press the shutter button halfway.  The exposure setting will be displayed. 2 Press the button. (0)  The icon lights up in the viewfinder to indicate that the exposure setting is locked (AE lock).  Each time you press the button, the current autoexposure setting is locked. 3 Recompose and take the picture.  If you want to maintain the AE lock while taking more shots, hold down the button and press the shutter button to take another shot. 129