Barmaglot

Retra Prime+ is 90W/s and Pro is 150W/s - no need to guess there, as those figures are quoted in specs. True, but you can tighten the beam with reflectors or widen it with diffusers - changing the actual power output is considerably more difficult.

I've seen a figure of 107 W/s quoted for Z-330, whereas, at least by published numbers (550 full-power flashes off a pair of 6000mAh Li-Ion batteries), Retra Maxi is somewhere around 300-350 W/s.

It's possible that the AOI trigger does not properly support the RC mode required to trigger HSS flash in this scenario - check with Backscatter to be certain. You could probably replace it with a Turtle trigger, which does support HSS, but that's a fairly expensive way to access a very niche feature.

High Speed Sync is not 1/250s; it's a completely different mode of operation. In regular flash x-sync, the camera shutter opens, the flash fires a pulse, then the camera shutter closes. However, the shutter blades (or, in older cameras, fabric curtains, which is where the terms 'front curtain' and 'rear curtain' come from) can move only so fast, so above a certain shutter speed, the rear curtain starts closing before the front curtain is fully open, creating a strip of exposure that races across the frame - firing a monopulse flash in this mode will produce a partially exposed image (usually dark top or bottom), hence the camera-dependant shutter speed limitation of flash shooting. High Speed Sync overcomes it by rapidly flickering the strobe (I've seen a figure of 40kHz quoted) instead of firing a single continuous pulse, which allows it to sync at pretty much arbitrary speeds, but significantly reduces overall power.

The allowable bend radius for fiber optic strands is directly proportional to the fiber diameter - i.e. the thicker the fiber, the less it can bend before experiencing significant transmission losses. The strands in multi-core fiber cables are extremely fine, and thus can tolerate smaller bend radii.

Wouldn't trapped bubbles get distorted by the surface they're trapped against? These look perfectly circular, and not on the same plane either.

No personal experience, but my best guess would be manual focus, possibly from a tripod, a buddy blowing a thin stream of bubbles with a second stage between the lens and the subject, and lots of attempts. The effect looks more like lensing than a reflection.

That's splitting some very fine hairs. Vignetting refers to the effect itself, not to its cause. The question was "Will a 1.4x TC show some black vignetting on the Canon 8-15 Fisheye @ 8mm ?", and your answer to this, "No, the 8-15 + 1.4x (or 2x for that matter) doesn't vignette by itself." is quite misleading, as shooting the 8-15mm @ 8mm with 1.4x TC will produce significant "black vignetting", whereas using a 2x TC won't.

It doesn't? I was under impression that 8-15mm + 1.4x TC vignettes when set wider than 11mm, and only 2x TC gets you the full zoom range.

FWIW, Kraken and Weefine are the same product, just different badges.

When I moved from a Sony a6300, which had a pop-up flash that I used as a backup to my UWT trigger on several occasions, to a Sony a6700 which doesn't have that anymore, I bought a cheap SeaFrogs trigger as a backup unit. It's basically a tiny flash that sits on the hot shoe, with a small xenon tube rather than LEDs, so it blasts out a lot of light, comparatively. The downsides are poor battery life (lasts just one dive, and consumes battery while on standby, which lasts no more than a couple hours), no TTL, and fairly slow (about half a second) recycle time, plus it will only fit housings with the optical bulkheads in front of the hot shoe and enough room to fit it, but it can save a trip if the main flash trigger fails for whatever reason.

One 'soft stat' that doesn't appear on any spec sheet and is difficult to measure without very specialized equipment is TTL response range. For what it's worth, I've tried shooting my 2nd gen Retra Pros in TTL and got very consistent deep underexposure no matter what settings I tried. Oddly, I got better results triggering with a pop-up flash than with my UWT converter, but still, not stellar. According to Pavel, the 2nd gen strobes have a very limited TTL range (and the 1st gen ones are worse still), while the latter generations are improved in that regard.

To be fair, the main value of the reflector, at least as I find it, is in creating a fairly narrow beam with very sharp edges, which allows for lighting up skittish subjects from a moderate distance (up to several meters) without catching all the back-scattering particles between them and the lens. It's fairly trivial to mount it on other strobes though.

Who are those competitors? Sea & Sea YS-D1 launched at $750 - I see YS-D3 Mark II listed at $800 and marked down to $750 on both Backscatter and Bluewater Photo; Divervision has it at $600. SUPE D-Pro is $758, Marelux Apollo III is $1199, Ikelite DS230 is $1295, Backscatter HF-1 is $899. Only ONEUW 160X II is comparable in price to Retras, listing at $2260, including a battery pack, whereas the $1600 Retra Pro Max comes without batteries, and you get to spend almost $500 more for a power vault. Even the Seacam 160D is not too far off at this point. While I've used the HSS mode on my Retras on occasion, I have never had it set to less than max power - even that is weak enough already. I don't know about 'countless users'; strobe floods are not that common IME. I have had the detector go off once - that was an Eneloop Pro deciding to leak electrolyte. I'll grant you the light-modifying accessories, mainly the reflectors which are quite unique, but it's easy enough to print an adapter to mount them on a different strobe. Regardless, all of that was present on 2nd gen strobes announced in 2018 and released in 2020 and doesn't in any way explain the doubling of the price that occurred since then.

Other than the massive price difference? I mean, I paid (checks email) €1478 for a pair of Retra Pros in December 2018. At current EUR/USD exchange rate, that's $1610 - almost exactly the price of a single current-gen Retra Pro Max. Yes, that price included a ~20% discount (IIRC the full price was €900 per strobe) and I had to wait over a year for them to ship (February 2020), but even so, the price hike from €900 to $1600 (~€1460) over a few years is massive and difficult to justify, especially in the presence of credible alternatives from Backscatter, Marelux, Ikelite, Sea & Sea, etc.