Nikon D780 DSLR Camera (Body Only) - Manual (Page 551)

505 Remote Flash Photography
7
C / f : Take the photograph.
Take the photograph after confirming that the flash-ready
lights for all flash units are lit.
D Optical AWL
Position the sensor windows on the remote flash units to pick up the light
from the master flash (particular care is required if the camera is not
mounted on 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 (
q A mode), as this may interfere
with exposure. To prevent low-intensity timing flashes emitted by the
master flash from appearing in photographs taken at short range, choose
low ISO sensitivities or small apertures (high f-numbers). After positioning
the remote flash units, take a test shot and view the results in the camera
display.