Canon EOS 6D Mark II Mode D'emploi page 151

Depth-of-Field PreviewN
The aperture opening (diaphragm) changes only at the moment when
the picture is taken. Otherwise, the aperture remains fully open.
Therefore, when you look at the scene through the viewfinder or on the
LCD monitor, the depth of field will look narrow (shallow).
Press the depth-of-field preview button
to stop down the lens to the current
aperture setting and check the depth of
field (range of acceptable focus).
f: Aperture-Priority AE
149