Tips on using Autofocus

While Nikon DSLR cameras feature advanced autofocus systems, the autofocus system is not infallible. Autofocus may not always perform as expected under the following conditions listed below. The shutter release may be disabled if the camera is unable to focus under these conditions. Please note that the camera may believe focus has been acquired and still display the in-focus indicator and the camera may still make a beep indicating correct focus.

If there is little or no contrast between the subject and the background, for example when the subject is the same colour as the background. The camera will be unable to focus.

When the focus area contains areas of sharply contrasting brightness, for example if the subject is half in the shade. The camera may not focus correctly on the subject.

If the focus area contains objects at different distances from the camera, such as Subject is inside a cage. The two different distances read by the same sensor will confuse the AF system.

If the main subject in the focus brackets is relatively small, such as a person standing in front of a distant background. This may result in the background being in focus, while the subject is out of focus. This is more likely to occur when a wide angle lens is used.

If the subject is dominated by regular geometric patterns, like a row of windows in a skyscraper. The camera may not focus correctly on the subject.

When the main subject contains lots of fine details, such as a field covered in flowers, autofocus may be difficult to obtain. This is more likely to occur when a wide angle lens is used.

In the above situations:

Focus on a different subject located at the same distance from the camera, then use the focus lock and recompose the scene and shoot
With some cameras such as the D200, D80, D90 and D5000 there is the option to switch between "wide frame" and "normal frame" AF. Changing from "normal frame" to "wide frame" will give the camera more information to help achieve focus.
Set the camera's focus mode selector to M (manual) and focus manually on the subject.
On some cameras there is the option of an 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 during continuous-servo autofocus or manual focus, in shooting modes in which the AF-assist illuminator can not be used, or if Off has been selected in the Custom settings. The illuminator has a range of about 0.5 - 3.0 metres and when used with a lens of focal length of 18-200 mm it is recommended to remove the lens hood. There are some lenses that will not work with the AF-assist illuminator on certain cameras. For further details regarding camera and lens compatibility please click here.
In AF-C mode or when continuous-servo autofocus is selected in AF-A mode, the camera gives priority to shutter response and the shutter may be releases before the in-focus indicator is displayed. Changing to AF-S mode can help ensure the focus locks prior to releasing the shutter.