Originally posted by mikesbike I certainly agree regarding the virtues of the Pentax DA 18-135mm. It is very under-rated in tests. In practice it is an outstanding lens especially for the range it offers.
As to the field curvature difference between the Tamron and Sigma 17-50mm f/2.8 lenses, there was apparently a variant of the lens produced when they designed the VC for the Canon version and likewise for the Nikon mount. The test you quote by opticallimits is this newer variant from 2010. The former version is the one still available for Pentax. Just google Tamron 17-50mm XR Di II without the VC, OS, etc. to get the former version, a 2006 report, where they go into the field curvature issue. The distortion figures were different between them too, with the newer one this time being the worst.
The opticallimits report of the Sigma (for Canon I believe) comments regarding the very low field curvature they measured, and again more comments about that in their conclusion.
In researching, I also came across reports of there being a slight shortcoming of max aperture in the Tamron, so that it seems to be actually more like f/3.2 wide open. Along with that is my finding that the Sigma, while being good to very good wide open at f/2.8 does indeed improve very, very quickly upon the least stopping down. I find this is true. Very acceptable wide open at the edges, but even at f/3.2 (maybe equal to f/2.8 on the Tamron), even the edges are already very good indeed. The AF is quiet and fast.
There's a lot of good digging there. Although odd, that the VS version has more distortion than the older version. That's something you really have to be student of the lens to know.
---------- Post added 08-06-18 at 09:21 PM ----------
Originally posted by timb64 Except that in your opinion the 18-135 is “better” than them both at 24mm,that’s what you said in post #19.
And it may well be, the information is inconclusive. I certainly don't discount the possibility. But lets not forget our positions, I regularly recommend people look the 18-35, the 16-85, the two 17-50s. I don't guide people one way or the other, just suggest possibilities. You on the the other hand proudly proclaim your undying love of the 16-85 without reservation. And tell everyone how you didn't like the 18-135, as if that's relevant to anyone but you.
The simple fact is these lenses are different enough that each has it's strengths and weakesses and in the end it all be one's persoanal biases in the way they shoot that will decide. The question is not that the lenses will deliver, they all do in their own way. The question is do they deliver on the images you think is right.
The 18-135 is the best of the above mentioned lenses for this image.
If that image interests you, then you should consider the 18-135, not the 16-85, not one of the 17-50s, not a 17-70. Now if that is not enough to make up for whatever weaknesses the lenses have (and test charts and images can help) then it's not for you. If on the other hand if you can look at the image and say, that's an ability I'd like to own, and I can live with the rest of it, it's the best lens for you.
There is no "best lens." It's nonsense. They all have their strengths, and you can buy even a kit lens because you like it's small size and weight and IQ combination over anything else that other lenses offer. If you have to resort to saying "this lens is a better lens" without saying what its better for, because it's not better for everything. then better to say nothing.