How to Lock the Flash Exposure on Your Canon EOS 6D
A handy lighting option on your Canon EOS 6D is locking the flash exposure to a certain part of the frame. This option is handy when your main subject isn't in the center of the frame or you want to throw some extra light on a specific part of the scene you’re photographing. To lock flash exposure:
Insert a supported flash unit into your camera’s hot shoe.
Move your camera until the center autofocus point is centered over the part of the scene where you want to lock flash exposure.
Press the AE/FE Lock button.
The camera fires a pre-flash to calculate exposure for the area over which you pointed the autofocus point. The flash icon in the viewfinder flashes on and off, and the FE lock icon appears.
Recompose the scene through the viewfinder and then press the Shutter button halfway to achieve focus.
A green dot appears in the right side of the viewfinder when the camera achieves focus.
Press the Shutter button fully to take the picture.
The flash unit fires, properly exposing the area over which you locked flash exposure.
If you’re really into controlling your Canon Speedlite through the camera, check out the External Flash C.Fn (Custom Function) Setting menu. This menu has options for custom functions that you can use to gain further control over your Speedlite. The available functions vary depending on the Speedlite you own. Refer to your Speedlite manual for more information.