Nikon 5T vs 2+ Diopter for 8" dome

For my wide angle I have been using a B&H 2+ diopter with my Nikkor 12-24 DX in a 8" Ikelite port. I just bought a Nikon 5T and 6T at a garage sale ($3 for the pair ). Would using the 5T with a step-ring, instead of the 2+ diopter be better with the 8" port? Also, I would appreciate any pointers for using the 5T and 6T on my 105 VR for macro.

I cant comment on your first question as I have no experience with the two element diopters and that lens (or any wide angle lens for that matter). I personally use the related 3T and 4T on the older 105mm macro lens and have had great success with both filters. I prefer the 3T as it has a bit more working room and really only take the 4T down if Im doing the same dive a second time and know exactly what Im after as well as knowing the very close working distance is not going to be a problem. Often Ill shoot the images with the 3T and then if I feel I can get closer for a better composition, Ill change out to the 4T and have a go with it on the next dive.

I have not tested it, but the single element dioptre should be better because it will introduce more focus field curvature, which is what is needed to help the corners. The double element dioptres are actually designed to minimise field curvature compared with a single element dioptre. Field curvature is usually undesirable, but is rather a handy side effect of dioptres for us underwater photographers.

Alex