How to Use Slow Sync Flash for Creative Photography Lighting
User manual Sekonic SpectroMaster L-858D (English - 223 pages)
Awesome video explaining and showing flash sync speeds : r/photography
Amazon.com : Godox Lux Junior Retro Camera Flash, GN12 with 7 Levels Flash Power, CCT 6000K±200K with S1/S2 Optical Control for Fuji, for Canon, for Nikon, for Sony, for Olympus Camera :