Canon PowerShot SX400 IS Black Manual (en)

Locking Image Brightness / Exposure (AE Lock)
 Still Images 
Before shooting, you can lock the exposure, or you can specify 
focus and exposure separately.
Lock the exposure.
Aim the camera at the subject to 
shoot with the exposure locked. 
With the shutter button pressed 
halfway, press the [
] button.
] is displayed, and the 
exposure is locked.
To unlock AE, release the shutter 
button and press the [
] button 
again. In this case, [
] is no 
longer displayed.
Compose the shot and shoot.
AE: Auto Exposure