Two problems, two solutions
- Slow sync — the background is too dark. Hold the shutter open so ambient light registers; the flash still freezes the subject.
- High-speed sync (HSS) — there is too much light for a wide aperture. Fire the flash across a moving slit at speeds above the camera's sync speed.
Slow sync
Choose a shutter/aperture/ISO combination that lets ambient light in, while the much shorter flash burst pins the subject sharp. The background blurs but is properly lit; the subject stays crisp. Perfect for night portraits, light trails and dynamic street work.
Camera set-up (a7R III)
- Mode dial → Manual (M) (Shutter Priority (S) also works).
- MENU → Camera Settings 1 → Flash Mode → Slow Sync (flash at the start) or Rear Sync (flash at the end).
- Rear Sync is the default choice — trails run behind the subject and look natural. Front-curtain lights the subject first and then records the ambient over the top.
- Stabilise: the shutter is open a long time. Tripod or monopod, and a remote or Self-timer.
Starting points
| Setting | Start | Controls |
|---|
| Shutter | 1/5 s → 1/30 s | Motion blur & ambient exposure |
| Aperture | f/1.4–f/4 | Depth of field + flash exposure |
| ISO | 100–400 | Noise |
Reliable opening move: 1/15 s, f/2, ISO 200, then adjust.
Three worked set-ups (Godox V1S)
- Night portrait with ambient: M, 1/10 s, f/2, ISO 200, flash TTL, Rear Sync. Subject sharp, background lights visible, natural motion blur.
- Light trails behind the subject: M, 1/2 s, f/4, ISO 100, flash Manual @ 1/32, Rear Sync. Streaks from cars, cyclists, people.
- Indoor ambient fill: A, f/2.8, ISO 400, flash TTL, Slow Sync. Balanced subject against warm room light.
The V1S follows the camera for rear-curtain — nothing to set on the flash. Pan with the subject for trails; dial flash power down to let ambient dominate for a cinematic feel; try coloured gels.
High-speed sync
Native sync speed on the a7R III is 1/250 s (a6400: 1/160 s). Above that, the rear curtain starts closing before the front curtain has fully opened, so only a moving slit of sensor is exposed — HSS fires the flash for the whole traverse instead of one short burst.
- The classic use: outdoor portrait in bright sun. TTL says f/16 at 1/125 s — far too much depth of field. You want f/2, which is six stops, so the shutter has to reach about 1/5000 s. HSS makes that possible: sharp subject, soft background, full sun.
- Also useful with telephoto lenses, fast action and sport, where you need both a fast shutter and flash.
- Cost: HSS burns power — the flash fires far longer than in normal mode, so effective output drops. Expect roughly 1/2 power for outdoor fill at 1/1000 s, f/2.8, ISO 100.
- Turn HSS on at both the flash and the trigger.
Consolidated from references: Slow Sync Flash Cheatsheet (a7R III + Godox V1S); Slow/High Synch Flash notes; Godox V1S Manual Flash Cheat Sheet.