I got my 20-40 LTD yesterday. I did some quick optical tests today with my K5IIs using the 20-40 and the Tamron 17-50. The test pics were images of my shed from about ten feet away, tripod mounted, taken at f4.0 and f5.6.
Tamron: 16.5 ounces with caps
Pentax: 11.25 ounces with caps
-Pentax is slightly sharper in the center.
-Pentax is much sharper across the field, edges and corners much better than the Tamron, at f4 and f5.6.
-My Tamron is noticeably softer on the right side. I hadn’t noticed this before, so a better copy of the 17-50 may provide different results.
-Pentax has more CA wide open (more at 20mm than 40mm), decreases at f5.6.
-Pentax has noticeably more distortion at 20mm.
-Focusing is obviously quieter for the Pentax, but the speed improvement didn't seem too noticeable... but I wasn't testing for that. I'll play with focusing later, as I was just doing a quick optical test today.
I’ll try to post some test pics when I’m done playing. I noticed the exposure was longer for the Pentax test shot, so I may reshoot. Could have been the sun peeking through the clouds for a few seconds. Test shots were taken using a tripod. I'll also try to test flare-resistance later.
-Dan K
Last edited by wildboar; 12-11-2013 at 03:24 PM.