Originally posted by Sandy Hancock Extremely low. Negligible. Exceptionally so for a full frame wide angle lens, and even less of an issue on a crop camera.
If you want to get really close you need the 35. If you want a WR macro there's the DFA100.
If you want a weather resistant lens for stitching, I go back to my earlier recommendation of the DA* 55/1.4 - its resolving power is outstanding, colours and micro-contrast are exceptional and distortion is (again) a non-issue.
DA* 55 panorama. Follow the link to Flickr and download the full size file if you're interested.
As always, Dr. Hancock nails it. I'm not an enormous fan of stitching 55mm shots on crop sensor, but i've stitched the 43 and liked it. If I'm going to stitch, I prefer to start a bit wider and use fewer frames.
As far as distortion, I think that's not much of an issue. I always use the lightroom profile to fix distortion ahead of time, and it always works pretty well. I actually stitch the 16-50, 18-135 and 16-85 all fairly frequently and they turn out fine. Don't over think it; stitch what you have!
Here is some of my stitch work. Full res files on my flickr
The "lowly" 18-135 (works pretty well!)
16-85 @35
16-50
DA15 Ltd
FA31
The old DA40 Ltd. stitches well:
And my largest, favorite stitch of all time, with the FA43; the original is almost 39,000 pixels wide.