Originally posted by no694terry Extremely good point. I was just comparing the two on pixel-peepers.
I would imagine you need to stick to very slim filters to avoid vignetting on an UWA?
I've never had a problem with the 10-20. The filter threads are much wider than front element. I think you could probably even stack a couple. I'm pretty sure on my nikon version I had a polarizing filter and either an ND or a UV or both stacked at some point or another.
The 15mm limited, since it's built so compact, you get one filter before you get vignetting, and you can't use anything that's wider than the standard ring (i.e., something with a bit of extra grip or the extra width slim filters often seem to have) because the built-in hood is very tight to the lens. I've used it with a 10 stop ND filter and a polarizing filter stacked -- definitely some dark corners.
And of course, if you use composition adjustment (the setting that lets you shift the sensor so it's like you're using a shift lens) you'll get vignetting much more often. When I use it to stitch a larger image with my 8mm fisheye, I have to take the hood off or I can see a clear outline of the hood.