Unfortunately it's a rather deep scratch caused by a barnacle in a challenging shore exit. I will try polishing but I am not optimistic. If I use a wide aperture it's better but still noticeable. I wish I could find a replacement dome.

12 hours ago, Grantmac said:

Unfortunately it's a rather deep scratch caused by a barnacle in a challenging shore exit. I will try polishing but I am not optimistic. If I use a wide aperture it's better but still noticeable. I wish I could find a replacement dome.

@Grantmac:
I am corroborating @FOTOSUB, that dome is, fortunately, very reparable. I was in the exact same situation as you, with a very badly and deeply scratched front element from a metal flag mount inside a float that I hastily threw my housing into without cover. Very deep scratches I could stick fingernail deep into.

I felt I had nothing to lose by attempting the polishing, lens was unusable otherwise anyway. Used the same kit I've always relied on for acrylic dome ports, 400 -> 12000 grit (https://micro-surface.com/product/nc-78-1-acrylic-restoral-kit/). The 12mm element is so soft it doesn't take that long at all. That kit starts with 1500, so I found some rougher grades down to 360 I believe, just to remove those deepest scratches.

I was so impressed with the job I decided to do the same on the six other 12mm lenses I had, all had minor scuffs and I was trying to sell them all. I made them all look mint with the polishing (starting with 1500 grade on these) and sold them all successfully, receiving nothing but positive feedback from all buyers.

You're never gonna find any spare parts for that lens I suspect. Japan is where I acquired every single one of my 12mm's.

@OlDirtyDiver

I believe this exact lens is one I bought from you and before my accident had only minor imperfections.

Did you focus on the entire dome to maintain shape or just the affected areas?

11 hours ago, Grantmac said:

@OlDirtyDiver

I believe this exact lens is one I bought from you and before my accident had only minor imperfections.

Did you focus on the entire dome to maintain shape or just the affected areas?

Ideally you'd only focus on the specific area of element. But, the job requires expanding outward in concentric rings with each finer grit (each new grade applied 90º compared to previous), so with the small amount of real estate on that element you'd end up sanding most of it by the time you get to finishing grade. And that was the greatest challenge of that job; after wiping and drying the element you'd see residual haze on the outermost 1cm of the element, because that's hardest part to sand uniformly. Meanwhile, middle of element very easy to restore to mint. So I had to put some extra effort into the outer edge. But also knew that this would not be in middle of photo, and edge sharpness also not always expected to be as sharp as center anyway with domes.

Just know that you can't really ruin the element attempting this. If you end up with the scratches still being slightly noticeable after drying element, or haze at edge, you'd just do the job again and know that it gets better every time. I did a single lens up to 5 times in trying to restore to mint (and each time just takes ~15min).

10 hours ago, OlDirtyDiver said:

Ideally you'd only focus on the specific area of element.

Removing more material from one spot on a dome will result in an optical distortion. Ideally you should remove the same amount of material from the whole dome.