SeaFrogs A6xxx 6" dome with Sigma 16mm distortion issue?

[DiveVenture]

I recently started experimenting with underwater photography, at the surface I'm a confident shooter, but below the surface I still have a lot to learn. I bought a second hand almost new (plastic still on it) underwater house from SeaFrogs for one of my cheaper Sony camera's, the A6400. It also came with a 6" dome.

The seller advised me that the Sigma 16mm lens would be a great choice for shooting with the dome, which I have laying around. So I took the combination on a trip but ran into an issue.

I seem to have huge distortion or blur on the sides of my images. I thought a dome was meant to avoid distortion. So naturally I started to look for the cause and checked if the lens was supported.

According to SeaFrogs the Sigma 16mm 1.4 is supported if you use the 6" dome:

6" Dry Dome Port for A6xxx / RX1xx Salted Line waterproof housings 40M/130FT

6 inch Wide Angle Dry Dome Port allows to use most popular wide-angle fisheye lenses while shooting underwater with Seafrogs Salted Line Underwater Housing

www.saltedlineeurope.com

So I compared my 6" dome with the current one on listing and it seems my extension ring is shorter then what is displayed on the website right now. Do I have an older dome version that does not support the Sigma 16mm lens?

Or... Is this normal for underwater photography and should I use a smaller aperture? I tried but it only helps a little bit. On the other side I don't want to use a aperture too small... I do (want do do) video mostly and do not have the luxury of the amount of light a flash provides.

Any tips, insight for me? SeaFrogs support does not even seem to understand the problem in the first place. They suggested to buy a 2cm extension ring and use that on top of what I already have, which cannot be locked in place... That seems like a no go for me.

Should I buy a newer 6" dome? Or is that the same one that I already have and will a 2cm extension ring help?

 

[Nemrod]

Sometimes you need a close up diopter on a lens behind a dome. In fact, I think this same question came up a few years ago and it was a diopter as the solution. The lens has to focus on the false image created by the dome which is often too close without a close up diopter.

Are you sure those are not air bubbles.Maybe I need to go find my glasses.

 

[DiveVenture]

Image quality is not great after uploading it here. But I think you can still see how the head of the fish is in focus, while the tail is not. This fish barley moves, so it is not motion blur, and the shot was taken at an almost 90 degree angle towards the fish, so it is not depth of field either.

I have many more images with the same blurriness towards the edges.