Nikon SB 20 - Manual (Page 52)

4) When shutter speed set on the FE or FM2 is faster
than the flash sync speed. Set the shutter speed to
the flash sync speed or slower one.
5) With the Nikon F-401/N4004, when the built-in flash
and SB-20 are OFF, and when the camera detects
flash is effective, the ready-light in the camera's
viewfinder blinks when the shutter release button is
lightly pressed.
With the SB-20 in TTL or A mode, if both viewfinder
ready-light and SB-20 ready-light blink after taking pic­
tures (to warn that lighting might have been insufficient
for correct exposure), confirm subject is within the
flash shooting distance range and, if necessary, use a
wider aperture or move closer to the subject.
With the Nikon F-401/N4004, for scenes where the
background is extremely bright, the ready-light may
not blink after the shot, even if flash is insufficient for
correct exposure.
With weak batteries, ready-lights blink to indicate in­
sufficient flash power, even if the subject is within
the shooting distance range; replace batteries with
a fresh set.
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