scuba1974

send a personal message direct please if you have anything, thanks

No, the radius of the dome is the curvature of the dome. It's completely independent of how close the lens is to the dome. Sure a lens close to the dome sees a smaller area of the dome, but this doesn't alter the radius, which is fixed when the dome is manufactured.

Oh thanks, I really wasn't aware of that. In all my tests the closer the dome port is the further the new close focus distance is pushed out from the actual one. I thought this was the general rule. More tests I guess.....

My (limited) understanding is that the dioptre strength should depend on the radius of the dome, not the distance of the dome from the lens. I have used +2.4 dioptre for 8" domes and +3.3 for 6" domes in the past - planoconvex lenses made specially, but now I use B&W +2 with a Nauticam 230mm dome on the Nikon 12-24. Sure you lose angle of view, but without it the corners are very soft. (Particularly compared with Alex Mustard's Zen 230 and 16-35 combination which looks stunning.)
The extension should place the centre of curvature of the dome at the front entrance pupil of the lens. Who will build the first adjustable length extension so we can get this right for each dome/lens combination?

I agree about the radius for comparisons of say a 6" to an 8" dome if the distances are the identical but when a lens is closer to the dome optic then it uses a much smaller part of the dome so a much smaller radius

What Pete said.

The front optical node of the lens should be at the radius of the curve. With zoom lenses, the node may move. Hence a problem.

I have a large number of domes inc. Subal, Nexus, Seacam and have found the sharpest to be the Sea & Sea (optical glass) port in conjunction with the DX Tokina 10-17. I was duplicating a trip to Truk so I put it on my Hugyfot housing with my 15mm FX lens as a test but got slightly less good results than with the original Hugyfot acrylic dome. The reason? Obviously the front node was not exactly in the right spot.

However, I have found that the best way to get the best results is to get optically clear water to shoot in.

I completely agree about the water! Interesting you you rate the sea & sea optical glass as one of the best domes, isn't this only about 6-7" diameter? I have read the large sea & sea fisheye dome is optically better than their glass offering. I do prefer glass domes though for durability. I did plan to get a zen 9" dome and an adapter made for my sea and sea rig but on arrival of the sea&sea fisheye dome I was quite impressed so it would be silly not to give it a good trial hence asking the questions here

Thanks I will have a read.

Ideally I want the best balance but compromising the lens is not an option, the combination needs to maintain the correct lens close focus distance and have the ability to focus on infinity so it just needs to be balanced with the correct dioptre to counteract the close focus shift. If a shorter extension with a stronger dioptre will yield better results than a longer extension without a dioptre then I will happily go this route at the sacrifice of split shots. I am just trying to find out the best possible solution for this lens as sea and sea have it so wrong on their chart. Personally from tests years ago a dioptre does not always sharpen the corners although in most cases there usually is an improvement, it varied from dome to dome

thanks for your input though, if anyone else has experience with this lens/dome port combination please chip in here and we can work out the very best solution for anyone wanting to use this lens