Nikon D500 - Manual (Page 226)

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• Manual: Choose the flash level manually.
• Repeating flash: The flash fires repeatedly while the shutter is
open, producing a multiple-exposure effect. Choose the flash
level (Output), the number of times the units fire (Times), and
the number of times the flash fires per second (Frequency,
measured in Hertz). Note that the number of times the units
fire in total may vary depending on the options selected for
Output and Frequency; see the documentation provided with
the flash unit for details.