Hello Suzanne, and welcome to the friendly Camera Labs forum!
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For ultra-wide on a Nikon DX body I'd suggest the 10-24mm! I once had a Sigma 10-20mm and the difference between 10mm or 12mm on the short end can clearly be seen.
As to your filter-Q: someone else has to jump in, because I'm not a filter-guy...
Thomas (beware: Nikon-fanboy and moderator!) My Lens Reviews
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