12-24 extension ring

I've been using my 12-24 with the Aquatica 8in dome port with no extension ring.

Whats the point of having an extension ring for this lens?

The proper extension ring for your lens and dome with put the image at the lenses posterior nodal point. The with give you the optimum image sharpness and color reproduction.

A quick and dirty way to determine if the lens is at the correct point is to open the lens all the way up. Then look at your image in the aperture. No matter what angle you look from your mug should stay centered in the middle of the lenses aperture opening.

People tend to set the lens closer to the dome than is optimal. This lets you focus closer but degrades image quality.

I've been using my 12-24 quite a lot lately and I use the 50mm extension ring with my Subal and a +4 diopter, now the edges are not the sharpest I've see but ok, then the other day I forgot to put the diopter on, when I uploaded the images to my computer the first thing I noticed was how sharp the edges were today?? then later I saw that I had forgot the diopter, also I was focusing right up to the dome.. So for there next few times no diopter so I can really see the difference. Anyway back to the extension ring thing, I tried 3 rings and the 50 was without doubt the best at keeping things sharper, right to the edges.