Canon EOS-1D X Mark III Advanced User's Manual page 225

4.
Set [HDR assist disp: playback].
Converted images are displayed on the screen for playback, giving an
impression similar to how they would appear on an HDR display
device.
Select [Exposure prior. (mid-tones)] or [Tones prior. (highlights)].
[ ] Exposure prior. (mid-tones)
Provides reference display that emphasizes exposure of subjects (such
as people) with intermediate brightness.
[ ] Tones prior. (highlights)
Provides reference display that emphasizes gradation of bright subjects
(such as the sky).
5.
Set the image quality.
For details, see
Note
Settings for [HDR assist disp: shooting] also apply to images displayed on the
screen immediately after you shoot.
Caution
HEIF images are captured at L size.
Expanded ISO speeds (L, H1, H2, H3) are not available in HDR shooting.
Some scenes may look different from how they appear on an HDR display device.
Histograms of images captured with [HDR shooting
not based on images as converted for HDR assist display. Image areas shown in
gray in the histogram roughly indicate signal values that are not used.
Image Quality.
225
] set to [Enable] are