Hi,
I recently visited Sichuan in China and we went to a mountain there at about 3100 metres. I used a 15 mm lens and a EOS 10D. Because the lens does not take any filters. I was worried about the high UV content affecting the pic quality. My worry comes true and all the pics taken were of poor quality. I also took some pics with a rangefinder and a lens with UV filter. The pics were normal.
yoyo