Canon EOS-1D X Mark III Advanced User's Manual page 112

Depth-of-Field Preview
Press the depth-of-field preview button to stop down the lens to the current aperture setting
and check the area in focus (depth of field).
Note
The larger the aperture value, the wider the area (from the foreground to the
background) in focus, but the darker the viewfinder display.
The depth-of-field effect is readily apparent on the Live View image as you change
the aperture value and press the depth-of-field preview button (
Exposure is locked (AE lock) as you hold down the depth-of-field preview button.
Pressing the depth-of-field preview button when a Speedlite 470EX-AI is attached
with the <
measurement.
> switch set to < > (full-auto) starts AI.B full-auto distance
112
).