Nikon D90 User Manual page 258

Flash Contacts
The D90 is equipped with an accessory shoe for attaching optional flash units directly
to the camera and a sync terminal that allows flash units to be connected via a sync
cable.
II The Accessory Shoe
Use the accessory shoe to mount optional flash units directly
on the camera without a sync cable (pg. 234). The accessory
shoe is equipped with a safety lock for 5peedlights with a
locking pin, such as the 5B-900, 5B-800, 5B-600 and 5B-400.
Accessory shoe
PI
The AS-15 Accessory Shoe Adapter
When the AS-15 accessory shoe adapter (available separately) is mounted on the camera
accessory shoe, flash accessories can be connected via a sync cable.
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