[emalvick]

I have both these lenses, and the comparison images posted show what I've observed myself.

I bought the Tamron first, to supplement my Tamron 17-50 on a trip to Iceland, where images wider than 17 mm were necessary, especially with the northern lights. What I remember noticing, and this may only be true at its longer end, is that images from 17-24 mm end of that wide Tamron were always inferior to those coming from the 17-50 lens. I purchased the 15 mm specifically to replace the 10-24 mm once I get some practice with it and confidence that I won't miss the 10-14 mm side of the Tamron. The 15 mm seems to be about even in image quality to the Tamron 17-50 zoom under similar conditions.