ikelite 6" dome + old canon lens = no focus

Is there anyway to make an old canon ef lens 35-70 (shooting on a 5d mark ii) work on an ikelite 6" dome port (5503.55)??
The lens is very old, and of course it's not on the official ikelite port chart.
I'm not a physician and I can't understand why I cant focus at any distance (near or infinite) while with other cameras it seems that no matter what dome or lens I use I can get focus everytime. [the focus is perfect outside of the water, of course]
I was thinking that maybe a diopter should contrast the dome "reverse-lens-effect". Will a diopter work or should I stick to a compatible lens for my dome? I don't want to believe that I have to stick to just 3 compatible lenses for that dome.. But maybe this is all just physics I don't understand and there is nothing I can do. Please enlight me..
Also: what would happen if I buy a flat port? Will every lens work with it? I have a few lenses and I would like to use them all.

Thanks for your help

You could make it work with a diopter, the minimum focusing distance is 40cm so it can't focus on the dome virtual image, which will be about 20.8 cm from the dome surface for an object at infinity and probably around 14cm from the dome at 0.5m object distance. The object distance is measured from the sensor when talking about minimum focus distance. This assumes the dome is a 6" hemisphere. If you don't understand dome port optics, the short explanation is that due to the being in water the lens does not focus on the object itself but on a virtual image of the object which is very close to the dome. SO lenses have to focus down to around 30cm to be usable without a diopter with this dome. Larger domes have the virtual image further away so more lenses are compatible

Looking at the ikelite port chart, the 35-80 lens is listed as requiring a +4 diopter with an 8" dome, so that might be a good starting point. You can test the diopter by checking that it focuses on objects between 14 cm and 21 cm from the dome surface on land. If you look at the ikelite website is says here: https://www.ikelite.com/products/fl-6-inch-dome-for-lenses-up-to-4-inches that :" Zoom lenses that do not focus to 12 inches (30 cm) require a +4 diopter close-up lens."

A flat port doesn't allow you to go very wide and will degrade images a little at the edges more so as you go wider. The 35-70 would be equivalent to about a 45-90mm lens through a flat port due to the magnifying effect that a flat port has.

There's nothing magic about lenses listed, they are just the ones that ikelite has tested. if they only focus down to 40cm, just add a diopter. The other requirement is not to be more than 100mm long according to Ikelite website. If they meet those criteria, you could try them. The other issue is whether you can find zoom gears for the lenses.

I don't know what you are planning on shooting with the 35-70, it's not very wide and doesn't focus very close so even with the diopter macro is difficult, and a bit narrow for scenic. I would suggest a 24-85 might be more generally useful and would do all the 35-70 does plus being able to shoot wider for scenic, fish schools etc. It would probably be available quite cheaply second hand. I would guess the 35-70 may work well for fish portraits. It all depends on what you planning on shooting with it.

Thank you very much for your explanation. It was very helpful.

About the 35-70: nothing special to do with it, it's just the first lens I was trying. I have other lenses, some slightly wider, some other narrower. I just wanted to find a solution without spending loads of money for a compatible lens!

Again thank you

If you have any sort of macro lens a flat port would be appropriate for that.