Originally Posted on Backscatter. By James Emery.
Photo: James Emery / Backscatter
Backscatter has published a how-to, and a short video, on high speed sync underwater. James Emery walks through when it is worth using, and when a normal flash is the better tool.
Most cameras sync a strobe only to about 1/200 or 1/250 second. Past that, a normal flash can leave a dark band because the shutter is only a slit. High speed sync, also called focal plane sync, pulses the strobe through the exposure so the whole frame is lit, on some cameras as fast as 1/8,000 second. In bright, shallow water that extra speed can turn a washed-out blue background black while the aperture stays wide.
The tradeoff is power. HSS spreads the strobe’s energy across those pulses, so the light is weaker and recycle times get longer. “High speed” means shutter speed, not burst rate. The piece treats this as a macro tool: the subject is close, so the lost power matters less, and the faster shutter is what knocks the background down. Wide-angle scenes and fast action still belong on a normal flash. With HSS, shutter speed also works as a dimmer. Slow it down if the subject is too dark. Speed it up if it is too bright.
High Speed Sync isn't a replacement for normal flash synchronization. It's a specialized tool for situations where your camera's normal sync speed can't give you the exposure you want.
— James Emery, Backscatter
Dark bands in the frame usually come from the camera’s shutter settings, not a failed strobe. On Sony, the article says to use the mechanical shutter and turn the electronic front curtain off. A global-shutter body such as the Sony a9 III can sync a normal flash at those high speeds without the HSS power loss. Both the trigger and the strobe have to support the mode. The piece lists compatible triggers from Backscatter, Turtle, and UW Technics, and strobes from AOI, Backscatter, Isotta, Retra, Seacam, and Sea & Sea.
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