Nikon D3000 User Manual

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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.
Small aperture (f/22)
Large aperture (f/5.6)
Mode dial
Command dial