Vivitar DSLR AF Flash Pro 648 Power Zoom VIV-PRO-648 - Manual (Page 13)

Front and Rear Curtain Synchronization
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Front and rear curtain synchronization is supported, to use simply set into Front or Rear mode
when using.
For Pentax, you could select the synchronization mode as front curtain ( ) or rear curtain
( ). The flashgun will fire according to the mode selected to match with the camera shutter.
Canon, Sony Alpha, Konica Minolta, Nikon, Olympus and Panasonic cameras control front or
rear-curtain synchronization, therefore settings for the flashgun are not required.
Note: The rear curtain synchronization is only possible on cameras with this feature.
See your camera’s instruction manual for details.