Nikon D5100 Reference Manual page 48

The camera supports the Nikon Creative Lighting System (CLS) and can be used with
CLS-compatible flash units. Optional flash units can be attached directly to the
camera accessory shoe as described below. The accessory shoe is equipped with a
safety lock for flash units with a locking pin, such as the SB-900, SB-800, SB-700,
SB-600, and SB-400.
1 Remove the accessory shoe cover.
2
Mount the flash unit on the accessory shoe.
See the manual provided with the flash unit for details.
The built-in flash will not fire when an optional flash unit is
attached.
[] The AS-15 Sync Terminal Adapter
When the AS-15 sync terminal adapter (available separately) is mounted on the camera
accessory shoe, flash accessories can be connected via a sync cable.
[_ Use Only Nikon Flash Accessories
Use only Nikon flash units. Negative voltages or voltages over 250 V applied to the accessory
shoe could not only prevent normal operation, but damage the sync circuitry of the camera
or flash. Before using a Nikon flash unit not listed in this section, contact a Nikon-authorized
service representative for more information.
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