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Notes on how to lock the exposure for backlit subjects on Canon 80D

Exposure is related to metering and metering is tied to focusing. But using the Exposure Locking, called AE, through the AE * function (at the back of the camera) one can detach focus/picture taking from metering.  So now it becomes a three-step process, viz., 1) Exposure Lock, 2) Focus, 3) take the picture.

To understand the exposure lock, first look at how pressing the focus button locks the exposure but as soon as you refocus on a different subject, it will meter/determine the exposure again.  Try this: 

• Lock focus using the back button focus, and move the camera around — the speed and aperture displays in the viewfinder won’t change, even if the lighting does. And, there’s no asterisk icon in the finder to indicate exposure is locked. Now release the back button focus and press it again and you will see that the speed and aperture displays in the viewfinder change.  This means that refocusing the subject changes the metering (if there is a difference in lighting condition)

So in order to "lock" metering on a subject and then take picture of some other composition do this: 

• AE Lock is accessed via the * button that is near the back button focus
• Press the back button to focus on where you want metering to be done. 
• Press the * button for AE. 
• Note there is a * icon now in the viewfinder to indicate exposure is locked. 
• Move the camera around, and refocus — the speed and aperture displays in the viewfinder won’t change, even if the lighting does. This is AE Locking. 

Note that if you want to set the metering mode (evaluative, to spot, or something else), it can only be done through the LCD. Unfortunately, the metering mode is not an available option on the 80D viewfinder.