Nikon Df - Manual (Page 81)

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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 (I) may be displayed
and the camera may sound a beep, allowing the shutter to be released
even when the subject is not in focus. In these cases, focus manually
(0 68) or use focus lock (0 66) 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
: Blinds or 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.