Nikon D90 - Manual (Page 75)

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A 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
(
J) 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, rotate the focus mode selector to M and
use manual focus (pg. 59), or use focus lock (pg. 57) to focus on another subject at the same
distance and then recompose the photograph.
D The AF-Assist Illuminator
If the subject is poorly lit, the AF-assist illuminator will light
automatically to assist the autofocus operation when the shutter-
release button is pressed halfway. The AF-assist illuminator will not
light in
l, or m modes or if Off has been selected for Custom Setting
a3 (Built-in AF-assist illuminator; pg. 174). The illuminator has a
range of about 0.5–3.0 m (1 ft. 8 in.–9 ft. 10 in.); when using the
illuminator, use a lens with a focal length of 24–200 mm and remove
the lens hood.
A See Also
Custom Setting d1 (Beep; pg. 180) can be used to turn the beep speaker on or off.
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
areas of sharply contrasting
brightness.
Example
: Subject is half in
the shade.
The focus point contains
objects at different distances
from the camera.
Example
: Subject is inside a
cage.
Background objects appear
larger than the subject.
Example
: a building is in the
frame behind the subject.
The subject is dominated by
regular geometric patterns.
Example
: A row of windows
in a skyscraper.
The subject contains many
fine details.
Example
: A field of flowers or
other subjects that are
small or lack variation in
brightness.
AF-assist illuminator