Nikon Camera F3AF - Manual (Page 80)

Ready-Light
A built-in LED ready-light is provided in the upper right-hand
portion of the viewfinder outside the picture area. When a Nikon
dedicated flash unit (or a TTL sensor cord) is attached to the
accessory shoe ®, the ready-light comes on as soon as the
flash is recycled, indicating that the unit is ready to fire. So, you
never have to remove your eye from the viewfinder. Moreover,
in fhe TTL mode, whenever the flash unit fires at its maximum
output, the ready-light starts blinking for approx, two seconds,
warning you that the light was probably insufficient for correct
exposure. As an additional warning, it blinks if the flash is not
connected properly or if the ASA/ISO setting on the camera is
outside the dedicated flash unit’s range of ASA/ISO 25~400.
For more detailed information, refer to the flash unit’s instruc­
tion manual.