Nikon D300 User Manual (en)

The Flash Sync Mode Display
M does not appear in the control panel flash sync mode display when [– –] 
is selected for [Built-in flash] > [Mode].
Flash Compensation
The flash compensation value selected with the 
Y (M) button and sub-
command dial is added to the flash compensation values selected for the 
built-in flash, group A, and group B in the [Commander mode] menu. A 
Y icon is displayed in the control panel and viewfinder when a flash 
compensation value other than ±0 is selected for [Built-in flash] > [TTL]. 
Y icon flashes when the built-in flash is in mode [M].
Commander Mode
Position the sensor windows on the remote flash units to pick up the 
monitor preflashes from the built-in flash (particular care is required when 
not using a tripod). Be sure that direct light or strong reflections from the 
remote flash units do not enter the camera lens (in TTL mode) or the 
photocells on the remote flash units (AA mode), as this may interfere with 
exposure. To prevent timing flashes emitted by the built-in flash from 
appearing in photographs taken at short range, choose low ISO 
sensitivities or small apertures (large f-numbers) or use an optional SG-3IR 
infrared panel for the built-in flash. An SG-3IR is required for best results 
with rear-curtain sync, which produces brighter timing flashes. After 
positioning the remote flash units, take a test shot and view the results in 
the camera monitor.
Although there is no limit on the number of remote flash units that may 
be used, the practical maximum is three. With more than this number, the 
light emitted by the remote flash units will interfere with performance.