Here's my results on a quick test of Canon's new 16-35L f/2.8 vs. the 20-35 f/3.5-4.5 and for fun, a Canon G1 (at 35mm focal length equivalent). All shot in RAW ISO100 (G1 ISO50) aperture priority. Converted 8bit TIFFs and web JPEGs created by BreezeBrowser. Cropped in PS6.

100% magnification crops here

16-35L 16mm@f/2.8

 

16-35L 20mm@f/2.8

20-35 20mm@f/3.5 (max aperture)

16-35L 16mm@f/8

 

16-35L 20mm@f/8

20-35 20mm@f/8

16-35L 35mm@f/2.8

20-35 35mm@f/4.5 (max aperture)

16-35L 35mm@f/8

20-35 35mm@f/8

 

G1 35mm@f/5 ("best part of lens" f/8 equivalent as seen in other examples)