Canon EOS-1D X Mark II DSLR Camera (Body Only) - Manual (Page 235)

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f : Aperture-Priority AE
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.
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).
Depth-of-Field Preview
A higher f/number will make more of the foreground and background fall
within acceptable focus. However, the viewfinder will look darker.
The depth-of-field effect can be clearly seen on the Live View image as
you change the aperture and press the depth-of-field preview button
(p.272).
The exposure will be locked (AE lock) while the depth-of-field preview
button is being pressed.