Canon EOS R3 Advanced User's Manual page 133

Note
Aperture value display
The higher the value, the smaller the aperture opening will be. The aperture value
displayed varies depending on the lens. If no lens is attached to the camera, "F00"
will be displayed for the aperture.
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.
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