AlanF said:
Don't take seriously one person's measurements on their one copy of the lens. These lenses have pretty large copy variations, and single test results depend on just how good the particular lenses tested were. Different testers have produced conflicting reports on the Sigma vs Tamron 150-600mm. The only tester who does things properly is Roger of lensrentals who tests 5-10 copies using the best equipment. He has the Sigma C marginally ahead of the Tamron 150-600mm at the one focal length tested, 400mm:
https://www.lensrentals.com/blog/2016/08/the-sort-of-great-400mm-shootout/
The only lens that matters is the copy you have. I have had one each of the Tamron and Sigma C, and for mine the Sigma is much better. It could be quite different for another pair of those lenses.
Yes, it's absolutely correct that no one review should be taken as representative of the whole production variation of a lens. If I'm seriously considering a lens I prefer to have read at least three thorough reviews.
Actually Brian has noted that sometimes he picks the best copy out of multiple examples of a lens, but I wouldn't be surprised if that only applies to Canon.
Out of all the TDP samples, the worst offender of single copy variation I can think of is the Sigma 18-35A.
Reviews amazingly well everywhere else on the Internet, looks fuzzy at TDP.
Sharlin said:
...There are not many people trying to choose between a crop+400mm and a FF+600mm.
I'm pretty sure that is one of the most common comparisons any crop shooter interested in a supertelephoto lens will make (it's still something I like to check for regularly), and we all know what the sales ratio is like between crop bodies and full frame.
The 1DsMkIII and 7D2 are almost ideal for direct comparisons because the resolution is so similar.
The results are even more spectacular given the inherent disadvantage of the crop sensor.
Using the Canon lens and body combo will give drastically superior AF, and the AF point spread on the 7D2 is another improvement over Full Frame bodies.
The main point is that for reach limited shooting the Tamron 150-600 was never the best option, and if a supertelephoto zoom lens underperforms on the long end then it's still kind of missing the point of having a zoom range. Some reviewers even concluded that there wasn't much point in zooming in past 500mm (to be fair that was probably shooting wide open and not at f11 where the Tamron is sharpest, but that just further demonstrates the weakness of the lens).