Nikon D5100 Reference Manual page 4

When AF-Cis selected for viewfinder photography (_ 39), this option controls
whether photographs can be taken whenever the shutter-release button is pressed
(release priority) or only when the camera is in focus (focus priority).
Option'°_ ......... Description
Release Photos can be taken whenever the shutter-release button is pressed.
_'"'] Focus Photos can only be taken once the camera has focused.
Choose whether the built-in AF-assist illuminator lights to assist _\t
the focus operation when lighting is poor.
_-_O_p'tio°n° T_ i_:!_7: ................ Description
available when both of the following conditions are met:
1. AF-Sis selected for focus mode (_ 39) or single-servo autofocus is selected
On
when the viewfinder is used in AF-Amode.
2. Ira] (Auto-area AF) is chosen for AF-area mode (_ 42), or an option other than
Ira] is chosen and the center focus point is selected.
The AF-assist illuminator does not light to assist the focus operation. The camera
Off may not be able to focus using autofocus when lighting is poor.
[] See Also
See page 41 for information on using the AF-assist illuminator. Information on the shooting
modes in which AF-assist is available may be found on page 216. See page 201 for
restrictions on the lenses that can be used with AF assist.
158