4.5 glass dome or 6inch acrylic?

Hello!  So I have a Sony a7iii and I'm planning on getting the tokina 10-17 fisheye lens with an adapter.  Is a 4.5 inch glass dome a better choice than a 6inch acrylic?  The glass dome is smaller but costs about $100 more.  Any feedback would be great!  Thank you!

Fisheye lenses are less sensitive to dome size than rectilinear, but on full frame many people see advantages to slightly larger domes.  Which housing system is this for? 

You know the 10-17 is an APS-C lens so edges of frame won't be that great on full frame?  I think there have been posts about using it on full frame on here.  The 10-17 is generally regarded as versatility but perhaps not the sharpest knife in the drawer. 

Wesley, I’d be wary of using the 10-17 with a full frame sensor as Chris suggests.

I tried  this with the Nikon D800. Pretty horrible. If you had the lens already, fair enough but I’d recommend against buying one to go with a FF. 

16 hours ago, ChrisRoss said:

Fisheye lenses are less sensitive to dome size than rectilinear, but on full frame many people see advantages to slightly larger domes.  Which housing system is this for? 

You know the 10-17 is an APS-C lens so edges of frame won't be that great on full frame?  I think there have been posts about using it on full frame on here.  The 10-17 is generally regarded as versatility but perhaps not the sharpest knife in the drawer. 

Thank you, Chris.  It's for the Isotta housing.  I understand that it's for a crop sensor and I was just going to keep it at 15mm.  What do you think?

15 hours ago, TimG said:

Wesley, I’d be wary of using the 10-17 with a full frame sensor as Chris suggests.

I tried  this with the Nikon D800. Pretty horrible. If you had the lens already, fair enough but I’d recommend against buying one to go with a FF. 

What focal length were you at?  I was just going to keep it at 15.  I heard that is fine....  Now I'm having second thoughts?

51 minutes ago, WesleyD said:

What focal length were you at?  I was just going to keep it at 15.  I heard that is fine....  Now I'm having second thoughts?

I can't honestly recall now a specific focal length that was ok. The whole thing was just horrible. A FF high resolution sensor (eg the D8xx) shows up the flaws perfectly on the 10-17 which is designed for DX not FX.. 

The Tokina is a fantastic focal length on cropped sensor, but is not very good optically. This will become immediately apparent when used with your a7!

It may also not autofocus, depending on which adaptor you use...

I would recommend using a bigger dome too...

There is a bunch of information about this lens and using it with full frame here on Wetpixel. I would have a good search...

2 hours ago, adamhanlon said:

The Tokina is a fantastic focal length on cropped sensor, but is not very good optically. This will become immediately apparent when used with your a7!
 

Yeah, I just can't think of why you would buy one to go with a FF sensor. Maybe worth going back to whoever told you it was fine at 15mm and define "fine".

You've spent a lot of cash on a top rate camera body. Do you really want to then pair it with a lens that the FF will highlight its worst aspects? It seems a bit counter-intuitive.

5 hours ago, WesleyD said:

Thank you, Chris.  It's for the Isotta housing.  I understand that it's for a crop sensor and I was just going to keep it at 15mm.  What do you think?

Changing the focal length really won't change anything, lenses are designed to produce an image circle of a certain size and are optically corrected out to that size.  Beyond that the optics rapidly go off.  Tokina and other 3rd party lenses will fit and function on FF cameras but doesn't necessarily mean you'll be happy with the results.  I've seem some tests of the Tokina floating around and it certainly takes images you might be happy with if you were doing big animals in the blue, but......

If you're just going to leave it at 15mm why not use the Sigma 15mm fisheye which is designed for full frame.   Neither option is on the port charts for Isotta, however Nauticam lists Sigma and the Canon 8-15 which is on the port chart for Isotta.  From that chart the sigma uses no extension with the 140mm dome  and the Canon 8-15 uses 30mm.  So this implies the Sigma would  use no extension with the 4.5"dome and 10mm extension with the 6"dome.   I do note though that I have seen it mentioned that the hood on the Sigma is too large for a 4"dome.  You would need to check fit on the 4.5"dome and this may push you in the direction of the 6"dome. 

As the others have said you've spent all this money on a full frame camera and housing and why would you put a lens on it with questionable optics.  If you want to do the sigma I would ask Isotta about recommendations if you want to use it with the 6"dome as they don't have a 10mm extension.  The other alternative is the Canon 8-15 which has great optics but is pricey.

Just on the question of the dome, I would now choose acrylic every time, since scratches can be polished out and they are much lighter. I owned a 9" glass dome which was great except for the drawbacks of cost, weight and the scratch issue.