I don't yet have a proper mount for the internal lens of the corrector port. So I have to stick it to my camera lens with blue-tac. When I do this carelessly (for the pool tests) I get some vignetting in the extreme corners. I have commissioned two replacement lenses which are bigger and won't have this problem. The simple solution was ready for this trip, but does not work that well. So I am still using the original lens, which is a bit small and therefore vignettes if I fit it carelessly.
Thanks for the plausible explanation, Alex!