Nikon D800 User Manual (en)

g: Aperture-Priority Auto
In aperture-priority auto, you choose the aperture while the camera 
automatically selects the shutter speed that will produce the 
optimal exposure. Large apertures (low f-numbers) increase flash 
range (
0 187) and reduce depth of field, blurring objects behind 
and in front of the main subject. Small apertures (high f-numbers) 
increase depth of field, bringing out details in the background and 
foreground. Short field depths are generally used in portraits to 
blur background details, long field depths in landscape 
photographs to bring the foreground and background into focus.
To choose an aperture 
between the minimum 
and maximum values 
for the lens, rotate the 
sub-command dial 
while the exposure 
meters are on. Aperture 
can be locked at the selected setting (
Small aperture (f/36)
Large aperture (f/2.8)
Sub-command dial