Nikon D300 User Manual (en)

When an SC-series 17, 28, or 29 sync cable is used for off-camera flash 
photography, correct exposure may not be achieved in i-TTL mode.  We 
recommend that you choose spot metering to select standard i-TTL flash 
control.  Take a test shot and view the results in the monitor.
In i-TTL, use the flash panel or bounce adapter provided with the flash 
unit.  Do not use other panels such as diffusion panels, as this may 
produce incorrect exposure.
Use Only Nikon Flash Accessories
Use only Nikon Speedlights.  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 
Speedlight not listed in this section, contact a Nikon-authorized service 
representative for more information.