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There are very different views on the gudie number values underwater: some use a factor of 2, some even 4(!) to estimate underwater GN from land values. Which is correct for clean blue seawater or 50m vis pool? I suppose color temperature also has a significant effect here, the 4200-4800K strobes getting punished for their nice warm light... E.g how do the inon z240's compare to the subtronic novas in terms of underwater working distance? (iso, f-stop being the same) Coverage might be the cause of confusion here, "flat port" strobes, like the inons, squeeze their beam underwater, e.g. the 100°-s of the z240 becomes 70°, approximately doubling the power/surface ratio. I guess a factor of 2 is sufficient for "squeezed angle" strobes, factors of 3+are for dome-port units, like the subtronics, seacam.