What is AE Lock?

The AE Lock function allows you to lock your exposure settings, and continue shooting without your ISO, Aperture, and Shutter Speeds constantly readjusting as you recompose your shot. The AE Lock button on Canon EOS cameras is denoted with a * symbol and is placed within easy reach of your right-hand thumb.

Popular photographic composition conventions – such as the Rule of Thirds and the Golden Ratio – suggest that your main subject should not be placed in the middle of your shot. The centre of the frame, however, is where the most effective AF point is, and is also the starting point for where AE is metered.

A common technique therefore is to press the shutter button half way down to focus your subject at the centre point, before recomposing your image (See AF Lock). When shooting in One Shot AF Mode, this will also lock the exposure, giving the photographer a quick and easy way to get the right focus and exposure, particularly for portrait shots.  

On the other hand, you might not want this behaviour to occur. A good example here is with landscape photography. 

Say you’re shooting outdoors in a city square. It’s sunny and you want to capture the rich blues of the sky, together with the action in the square below. If you focus your camera on the action in the city square, chances are that the sky will likely be extremely bright and lose its blues. By pointing to the sky and locking the exposure, then re-composing your frame to shoot the square, you will retain those sky blues while capturing the action below.

AE Lock is also useful where you want to achieve a consistent exposure setting over a number of sequentially captured images. All you need to is keep holding down the AE Lock button as you continue to take your shots.