Originally posted by mikesbike As to your Tamron 17-55mm, f/2.8 do you mean the current 17-50mm f/2.8 XR Di II LD?
Yes, exactly. Sorry for writing it wrongly. Actually I sit in a Canon system, where all my similar lenses end at 55mm, while the Pentax belongs to my wife. She got the SP AF 17-50mm F/2.8 XR Di II LD from me and also the Sigma 18-35 Art was purchase for her, after she has seen what such lens can do with my cam. I have a pretty good experience with buying second-hand lenses (already >13 of them in total) but there is really good choice in Canon system. For Pentax you have to hunt.
It's a pitty, as they make soooo good cams with so good price/quality and price/features ratio. And... yeah, it didn't work.
Originally posted by mikesbike Tests have revealed this Sigma lens as having exceptionally low field curvature, where the Tamron's was high. This can affect edge sharpness in certain circumstances.
Correct but testing the corner sharpness I enter LiveView, enable 10x magnification, scroll to the place where I want to set focus, do it manually and then take a shot. So the field curvature should play no role.
This is how it looks for the Tamron 17-50 f/2.8 @ 17mm, f/2.8. Shots made with K-3 (24Mpx).
Full frame, focused manually in the bottom left corner
Close to the center the sharpness is perfect
Close to the corner it is also pretty good
But the very corner is bad. It is a very small area of the whole shot but still disappointing.
The kit DA L 18-55 AL WR has much better corner performance. Same it true for Sigma 10-20 f/4.0-5.6, which my wife also got from me. I find it hard to find a full-size sample from Tamron 17-50 and Pentax, where there would be something detailed in the corner and within the focal plane, for a sake of comparison. This lens is still on warranty, I wonder if it's worth sending it for a service. I don't have best experience with their service: my 28-75 for Canon has awfully bad AF performance (it's tack sharp but AF misses reproducibly). Service tried to calibrate it, they mean it's within their spec but still doesn't work good on any of my cams (and there are 3 different models, including a mirrorless one).
Originally posted by mikesbike In any case, as an alternative I can definitely recommend the Sigma 17-50mm f/2.8 EX DC for Pentax. It is very well built, sharp, and has quiet, fast, and accurate AF performance. It also has a handy AF/MF switch which will go from and to either setting without having to also use the camera's switch.
Thanks for recommendation. I considered it before buying but finally decided to take a Tamron. Don't remember anymore why. It was a weighted decision based on reviews, taking sharpness, reflections, distortion, aberration, coma, behavior with light spots in viewing field into account. But maybe Sigma would be a better choice indeed.