I can't address the vignetting issue directly but would caution against doing any sweeping scenics with a lens as wide as 20mm while using a polarizer. The variability of sky darkening can become quite objectionable even at 28mm! While I love polarized skies I did not like this aspect to them.
Terry Danks Wildlife and Nature Photography http://www3.ns.sympatico.ca/danksta/home/htm * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
