Canon EOS-1DX Instruction Manual page 388

Troubleshooting Guide
The maximum burst during continuous shooting is lower.
If you shoot something that has fine detail (such as a field of grass),
the file size will be larger and the actual maximum burst may be lower
than the number mentioned on page 126.
ISO 100 cannot be set/ISO speed expansion cannot be
selected.
If [z2: Highlight tone priority] is set to [Enable], the settable ISO
speed range will be ISO 200 - 51200 (ISO 200 - 25600 for movie shooting).
Even if you expand the settable ISO speed range in [ISO speed range],
you cannot set expanded ISO speeds (L, H, H1, or H2). If [z2: Highlight
tone priority] is set to [Disable], ISO 100/125/160 can be set (p.156).
The Auto Lighting Optimizer cannot be set.
If [z2: Highlight tone priority] is set to [Enable], the Auto Lighting
Optimizer cannot be set. If [z2: Highlight tone priority] is set to
[Disable], then the Auto Lighting Optimizer can be set (p.156).
Even though I set a decreased exposure compensation, the
image comes out bright.
Set [z2: Auto Lighting Optimizer] to [Disable]. When [Standard/
Low/High] is set, even if you set a decreased exposure compensation or
flash exposure compensation, the image may come out bright (p.152).
I cannot set the exposure compensation while both manual
exposure and Auto ISO are set.
See page 178 to set the exposure compensation.
Even if an exposure compensation is performed, it will not be applied
for flash photography.
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