Zoom lenses continue to be improved. At the ultra-wide end, there are zooms [DA12-24mm, Sigma 8-16mm, and Nikkor 14-24mm] that outperform available primes. I was impressed by the MTF results in Photozone's test of the relatively new Sigma AF 17-50mm f2.8 on a Canon EOS 50D, and wonder if zooms are now matching the performance of primes at longer focal lengths. By comparing pairs of lenses tested on the same camera with one of the same lenses tested on another camera and repeating this process, it appears that this Sigma zoom betters the MTF of any Pentax zoom and equals the MTF of Pentax primes and available third party primes, including the Zeiss 35mm f2 ZF.

I did not do comparisons of primes of 40-50mm, where I suspect some primes, especially macro lenses, would outperform the zoom lens.

So, I wonder if users of the Sigma 17-50mm f2.8 have found its performance from 17-35mm matches the best prime lenses they have used. That is in resolution and contrast, not speed, bokeh, distortion, or abberrations.