After purchasing a Nikon D70S with the Sigma 18-200mm lens, I recently purchased a Nikon D200 and a Sigma 10-20mm ultra wide lens. This lens is one of the best I have ever used on any kind of camera. It not only has a good professional look and feel to it, but it works flawlessly with the smoothness of lenses in a much higher price range. Images are clean and crisp, and the autofocus is butter smooth. I paid a bit over $500 for mine and I doubt there is a nickels worth of difference, except in price, between the Sigma brand and a comparable lens bearing the Nikon name. This lens is a keeper for sure!! 

Question: Where is there a good contrast of colors?
Answer: Images are clean and crisp , and the autofocus is butter smooth