I'm afraid you have just identified the one weakness of DX. There just aren't any really wide, fast primes. I used a Nikon 12-24 when I shot DX and I think it was really good. But not terribly fast at f/4. Fast enough with the newer sensors, but not exceptional like what you can get in the FX world where both really wide and very fast are achievable (you can use the same lenses on DX, but they aren't wide - just sort of wide). It's what finally drove me to FX. But I do miss the lighter weight and smaller size that DX offers.