D70 + 12-24 in Ikelite acrylic prot.

With +3 dioptries lens:

Without the +3 dioptries lens:

Can't really see any difference.

Any chance you have the same shot with and without the diopter?

I can't really tell at this resolution, but it looks like the "without the diopters" are a little mushy at the corners (or the cup coral has been smushed somewhat.) (I also suspect getting the same shot for with and without the diopter is tricky <since you have to open the housing.>)

I remeber that diopters were reccomended for making lenses focus close enough to the virtual image of the dome, but I don't know what the current wisdom is for the the super wide zooms. All the housing manufacturers are quick to say "ours works great for that lens, no diopter required", and I have to wonder if a +3 diopter on a 12-24mm lens is much different than zooming in to 18mm to just miss the mushy corners.

I've got a 10-22 on the canon 20d behind the aquatica with the big dome, and at 10mm the edges get mushy underwater (it also happens on land, but to a much lesser extent, so I'm hesitant to plop down the $66 a shot for various 77mm diopters to see if it's a dome issue.)