Canon EOS R5 C VR Creator Kit with RF 5.2mm f/2.8 Dual Fisheye Lens - Manual (Page 112)

Depth-of-Field Preview
Press the depth-of-field preview button to stop down the lens to the current aperture value
setting and check the area in focus (depth of field).
Note
The larger the aperture value, the wider the area in focus, from the foreground to
the background.
The depth-of-field effect is readily apparent on images 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.
112