Nikon 10-24mm vs Sigma 10-20mm – First, thanks to esfotoclix.com for a superb side-by-side image of the Nikon 10-24mm f/3.5-4.5G ED AF-S DX Nikkor, Nikon 16-85mm f/3.5-5.6G AF-S DX ED VR Nikkor Wide Angle Telephoto Zoom and Sigma 10-20mm f/4-5.6 EX DC HSM lenses.
The conclusion places the Nikon 10-24mm as the overall winner over the Sigma 10-20, citing more consistent exposures, better image frame edges and corners, a more accurate focusing mechanism and a better zoom ring as the reasons.
In a related post, eNo gives the following reasons for justifying the higher price of the Nikkor:
1) The 4mm will come in handy. Personally, ~$100 per mm is worth it for me.
2) In addition, the Nikkor is a slightly faster lens (f/3.5-4.5 vs. f/4-5.6) while covering a longer range (see #1).
In addition, it appears to be a better performer at 10mm (where I use these lenses the most) in 3 aspects:
1) Chromatic aberration wide open.
2) Consistent light (exposure).3) Consistent focusing. (Though I have loved my Sigma, lately I've been noticing that one shot will come in sharp, and the next, taken at the same aperture will not… This was my motivation to try the Nikkor. Now I suspect the focusing is letting me down.)