Nikon D3X - Manual (Page 108)

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Getting Good Results with Autofocus
Autofocus does not perform well under the conditions listed
below. The shutter release may be disabled if the camera is unable
to focus under these conditions, or the in-focus indicator (
●) may
be displayed, allowing the shutter to be released even when the
subject is not in focus. In these cases, use manual focus (pg. 83) or
use focus lock (pg. 80) to focus on another subject at the same
distance and then recompose the photograph.
There is little or no contrast between the subject
and the background
Example : subject is the same color as the background.
The focus point contains objects at different
distances from the camera
Example : subject is inside a cage.
The subject is dominated by regular geometric
patterns
Example : a row of windows in a skyscraper.
The focus point contains areas of sharply
contrasting brightness
Example : subject is half in the shade.
Background objects appear larger than the subject
Example : a building is in the frame behind the subject.
The subject contains many fine details
Example : a field of flowers or other subjects that are
small or lack variation in brightness.