Canon EOS 5D Mark III Instruction Manual (en)

k Shooting Movies
Notes for Autoexposure, Shutter-priority AE, and Aperture-priority 
During movie shooting in the 
A, d, s, f or F mode,  this camera 
supports the function that turns the Speedlite’s LED light automatically 
in low-light conditions. For details, refer to the Speedlite’s 
instruction manual.
Using an EX-series Speedlite (Sold Separately) Equipped 
with an LED Light
You can lock the exposure (AE lock) by pressing the <A> button 
(except in the 
A mode, p.173). After applying AE lock during movie 
shooting, you can cancel it by pressing the <S> button. (AE lock setting 
is retained until you press <S>)
You can set exposure compensation by setting the <R> switch to 
the left and turning the <5> dial (except in the <
A> mode).
Pressing the shutter button halfway displays the ISO speed and shutter 
speed at the screen bottom. This is the exposure setting for taking a still 
photo (p.227). The exposure setting for movie shooting is not displayed. 
Note that the exposure setting for movie shooting may differ from that for 
still photo shooting.