Canon EOS 5D Mark III Instruction Manual (en)

: 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 PreviewN
If the “30"” shutter speed blinks, it indicates 
Turn the <6> dial to set a larger aperture (lower 
f/number) until the shutter speed blinking stops or set a 
higher ISO speed.
If the “8000” shutter speed blinks, it indicates 
Turn the <6> dial to set a smaller aperture (higher 
aperture f/number) until the shutter speed blinking stops 
or set a lower ISO speed.
Aperture Display
The higher the f/number, the smaller the aperture opening will be. The 
apertures displayed will differ depending on the lens. If no lens is attached to 
the camera, “00” will be displayed for the aperture.
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 
The exposure will be locked (AE lock) while the depth-of-field preview 
button is pressed.