Canon MARK IV User Manual (en)

 Shooting Movies
When the shooting mode is <a>, you can manually set the ISO speed, 
shutter speed, and aperture for movie shooting.
Set the shooting mode to <a>.
Press the <W> button and turn 
the <6/5> dial to select <a>.
Set the ISO speed.
Press the <Z> button and look at 
the screen while turning the <6/
> dial to set the ISO speed.
For details on the ISO speed, see 
page 146.
Manual Exposure Shooting
During movie shooting, you can lock the exposure (AE lock) by pressing 
the <A> button (p.115). After applying AE lock during movie shooting, 
you can cancel it by pressing the <S> button.
If you set the power switch to <J> and turn the <5> dial, you can set 
the exposure compensation.
The ISO speed will be set automatically within ISO 100-12800. If the 
[Highest ISO speed] is set to [H1], [H2] or [H3] in [8C.Fn I -3: Set ISO 
speed range
] (p.208), the ISO speed will be set automatically within the 
expanded range up to the upper limit. However, setting the [Lowest ISO 
] to [L] will not expand the ISO speed range to the lower limit. Also, 
even if the ISO speed range is set to a narrower range than the default, 
the narrower range will not take effect.
Pressing the shutter button halfway displays the ISO speed, shutter 
speed, and aperture at the screen bottom. This is the exposure setting 
for taking a still photo (p.148). The exposure setting for movie shooting is 
not displayed. Note that the exposure setting for movie shooting may 
differ from that for still shooting.