Nikon D5100 Reference Manual page 10

Choose the flash mode for the builtqn flash in P,S,A, and M modes.
[] Manual
A _ icon blinks in the viewfinder and information display when
Manual is selected and the flash is raised.
[] The SB-400
When an optional SB-400 flash unit (_ 202) is attached and turned
on, Flash cntrl for built-in flash changes to Optional flash, allowing
the flash control mode for the SB-400 to be selected from TTL and
Manual.
[] Flash Control
The following types of flash control are supported when a CPU lens is used in combination
with the built-in flash or optional SB-900, SB-800, SB-700, SB-600, and SB-400 flash units.
• i-fTL BalancedFill-Flashfor DigitalSIR: Information from the 420-pixel RGB sensor is used to
adjust flash output for a natural balance between the main subject and the background.
• Standard i-fTL Fill-Flashfor DigitalSIR: Flash output is adjusted for the main subject; the
brightness of the background is not taken into account. Recommended for shots in which
the main subject is emphasized at the expense of background details, or when exposure
compensation is used.
Standard i-TTL flash control is used with spot metering or when selected with the optional
flash unit. i-TTL balanced fill-flash for digital SLR is used in all other cases.
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