Underwater Photography Settings Calculator

Without a strobe, exposure comes entirely from ambient light — the calculator tracks EV loss with depth and visibility, then solves shutter speed at a fixed f/5.6 for sharpness. With a strobe, the subject is lit primarily by the flash rather than ambient light, so the calculation switches to solving aperture from the strobe's guide number divided by subject distance, paired with a fixed 1/125 flash sync shutter speed — ambient EV is effectively ignored in that branch since the strobe dominates the subject's exposure at typical underwater working distances.

It divides a typical underwater strobe guide number (22) by your subject distance in meters, then snaps that raw f-stop to the nearest standard aperture from the list [2.8, 4, 5.6, 8, 11, 16, 22]. This means closer subjects call for a smaller aperture (higher f-number) to avoid overexposure from the strobe, while subjects farther than about 2m from a GN-22 strobe will push toward wider apertures since flash output can't reach as far.

Strobes emit balanced white light close to the subject, restoring natural color within their effective range, so wbShiftK is set to 0 whenever useStrobe is on. Without a strobe, water absorbs red and orange wavelengths progressively with depth, shifting the ambient scene blue-green, so the calculator estimates a warming correction of roughly 200K per meter of depth to compensate in-camera or in post.

Backscatter risk only appears when you're using a strobe AND visibility is poor or moderate (vizLevel 0 or 1) — it's rated high (2) in poor visibility and moderate (1) in moderate visibility, with no risk flagged in good or excellent visibility or when shooting ambient-only. This reflects that backscatter comes from the strobe lighting up suspended particles close to the lens; without a strobe firing, or with clean enough water that few particles are present, that specific problem doesn't occur.