Any vignetting in the corners?
I've not noticed any vignetting.
According to the Seafrogs website the port has Optical Glass, quite what that means I'm not 100% sure, but I've used the set up I mentioned above (with two Sea&Sea strobes) on about 25 dives now and optically speaking it's been fantastic. Clear, sharp, no distortion or noticeable vignetting.
However, I have noticed that when I screw the WWL-1 on if you screw it all the way around the in built "hood" covers the the corners when shooting at 28mm. It's easily fixed by unscrewing it slightly and aligning the top of the WWL-1 with the top of the housing.
But for the price I don't have any complaints about the Seafrogs set up. It's all plastic and I did feel a bit nervous on the first dive but so far have had no problems with it. A good pre dive check in the pool / water tanks always helps calm the nerves!
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