Nikon D780 Reference Manual page 665

D
Fixing Shutter Speed at the Flash Sync Speed Limit
In modes S and M, shutter speed can be fixed at the value selected for
Custom Setting e1 [Flash sync speed] by selecting the next shutter speed
after the slowest possible shutter speed (30 s or % ). An X (flash sync
indicator) will be displayed in the viewfinder and control panel together
with the flash sync speed.
D
Auto FP High-Speed Sync
•
Auto FP high-speed sync allows the flash to be used at the highest
shutter speed supported by the camera. When [1/200 s (Auto FP)] is
selected, the camera automatically enables auto FP high-speed sync at
shutter speeds faster than the flash sync speed. This makes it possible to
choose the maximum aperture for reduced depth of field even when
the subject is backlit in bright sunlight.
•
The flash information display shows "FP" when auto [1/200 s (Auto FP)]
is selected.
Menu Guide > A Custom Settings
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