Canon EOS-1D X Mark III Advanced User's Manual page 480

Magnified View
You can magnify images displayed for [Set up processing→JPEG] or [Set up processing
→HEIF] by pressing the <
[Image quality] setting. With <
To cancel the magnified view, press the <
Caution
Results of processing with [Digital Lens Optimizer] set to [High] are only applied
in magnified view. Results are not applied in normal display.
> button. The magnification ratio varies depending on the
>, you can scroll around the magnified image.
> button again.
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