Nikon Z 8 Reference Manual page 612

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Caution: Auto FP High-Speed Sync
Depending on the shutter speed, horizontal lines may appear in pictures taken using auto FP high-
speed sync when [1/250 s (Auto FP)] or [1/200 s (Auto FP)] is selected. This effect can be mitigated
by:
choosing a slower shutter speed or
adjusting flash output.
Tip: Auto FP High-Speed Sync
Auto FP high-speed sync allows the flash to be used at shutter speeds as fast as ¹⁄₈₀₀₀ s. When [1/250 s
(Auto FP)] or [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.
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e1: Flash Sync Speed