I just received a copy of the aspherical IF Tamron AF 20-40 mm F2.7-3.5 SP for my Sony a7 (using the Sony A / E adapter that is not AF) on eBay. I'm seeing what I find a surprising degree of softness at the long end of the lens. This is especially surprising to me, since the revisions (see the link) praise the clarity of this lens. However, I have not used a wide-angle lens before, much less this specific lens, so I'm not sure if this degree of smoothness is to be expected or indicates a problem with the lens. (It was listed as "excellent" condition without optical problems on eBay).
All of these shots are untrimmed, manually focused, approximately 25 feet (~ 7.5 meters) away from the fence, and hand held because I was too lazy to set up my tripod for three shots. In each case, the sharpest focus was timid of infinity. (The lens does not have focus marks between 10 feet / 3 m and infinity). I took these shots in Raw, I loaded them in On1 Photo Raw and exported them as 90% of JPEG files without any editing. Photo Raw does not seem to have a profile for this lens, so it does not seem to have applied any correction. (At least, I do not see any difference in activating and deactivating the lens correction panel.)
First, for reference, here is a photo at 20 mm, 1/200 sec, f / 10, ISO 160. With a 50% zoom I can see some small differences in the sharpness between the center and the edges, but that does not worries. I consider that this photo has a good sharpness for a lens of the mid 90's.
Then 40 mm, 1/200 sec, f / 10, ISO 125. The center has a good sharpness, but everything else is extremely smooth, even in the small preview version. The right side is especially bad. This is the problem that I am trying to understand.
Finally, only in the case that f / 10 can be considered "open" in this lens, or if there is something about the depth of field with wide lenses that I do not know, I tried the smallest aperture in this lens. 40mm, 1/200 sec, f / 32, ISO 1250. I think I can still see more sharpness decrease towards the edges, compared to the 20mm shot, but it's somewhere between tolerable and good.