Canon EOS 6D Mark II Basic Instruction Manual - Page 151

f: Aperture-Priority AE 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). 149