Caveat - I-ts a long time since I used a 20mm f/2.8D Nikkor underwater but I used to and I liked it well enough and it should still work well enough if situated correctly behind a reasonably sized dome port (Seacam, which I supply, certainly suggest that it is ok behind both the WidePort and Superdome, and I'd guess it will work ok behind the CompatPort too). Basically a lens with a 90 degree field of view is about the limit of what used to be considered viable - in the (film) days when smaller domes were more the norm - although often they were used with diopters behind smaller ports. Today we are more critical but you should be able to get it to work well enough. If its not 'supported'- I assume that simply means that there is no recommended port/extender combination - this is possibly because fewer people use fixed focal length lenses these days and little data is available (I'm surprised nobody who uses one has posted). On the Seacam it simply sits behind the port with no extension and this is where I would start. Personally I prefer fixed focal length lenses and would happily use this 20mm lens if I shot on Nikon - Canon's offering has not impressed me above, let alone below water!
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