Canon EOS-1DX Instruction Manual page 178

a: Manual Exposure
Exposure Compensation with Auto ISO
If the ISO speed has been set to A (AUTO), you can set exposure
compensation (p.181) as follows.
Under [85: Custom Controls], use [s: Expo comp (hold btn,
turn )] (p.353).
Quick Control (p.51).
If Auto ISO is set, the ISO speed setting will change to obtain a standard
exposure given the shutter speed and aperture set. Therefore, you may
not obtain the desired exposure effect. In this case, set the exposure
compensation.
If flash is used when Auto ISO is set, exposure compensation will not be
applied even if an exposure compensation amount is set.
In [z2: Auto Lighting Optimizer], if the checkmark <X> for [Disable
during man expo] is removed, it can be set in the <a> mode (p.152).
When Auto ISO is set, you can press the <A> button to lock the ISO speed.
After recomposing the picture, you can see the exposure level difference
on the exposure level indicator (p.22) compared to when you pressed the
<A> button.
If exposure compensation (p.181) was applied in <d/s/f> mode,
and then the shooting mode is switched to <a> and Auto ISO is set, the
exposure compensation amount already set will be maintained.
178
5
Set the exposure.
Check the exposure level and set the
desired shutter speed and aperture.
If the difference with the standard
exposure exceeds ±3 stops, the end
of the exposure level indicator will
display <
6 Take the picture.
> or < >.