Nikon D5100 - Manual (Page 82)

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Mode A (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) 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 take photographs in aperture-priority auto:
1
Rotate the mode dial to A.
2
Choose an aperture.
Aperture is shown in the viewfinder and
information display. Rotate the command
dial to choose the desired aperture from
values between the minimum and
maximum for the lens.
3
Frame a photograph, focus, and shoot.
Large aperture (f/5.6) Small aperture (f/22)
Mode dial
Command dial