[BrianCoup]

It's interesting to see that NISI changed their design to include a filter adapter instead of their previous questionable slip-on design. Although now I don't really see the advantage compared to the cheap Chinese adapter and regular 49mm filters, especially if the Nisi vignettes with a regular filter as you say. I think those JJC stick on filters are a thoroughly bad idea. You are introducing reflections and another optical surface of questionable quality, jeopardizing the camera's great lens.

That's interesting you think the stick on filter will also cause issues. The screw on filter I used was an old one and about 5mm wide. I can try it again with a slimmer filter. Perhaps the tolerances are that fine that any additional filter will cause it to vignette. There were no issues with just the NiSi kit connected. Wondering if larger filter system and a step up ring are any better.

[gbl]

I don't think the stick on filter would cause vignetting. But I would be afraid of it deteriorating the image quality, especially by adding unwanted reflections. You will see those quickly in low light shots with artificial light sources.

Personally, I trust the "Dust-off" function of the GRIII, and I'm not worried about dust. I don't see the point of a permanent UV filter on the GRIII.

The cheap Chinese filter adapter does not vignette at all with a filter attached. I only use it with a polarizer and additional ND filters, though. It didn't vignette even when I stacked a ND filter and a polarizer.