Tamron 90 shot with Sigma 50, and vice versa
So both lenses are now in hand and been given a few simple tests.
What have I learned in 40 minutes of play?
Well, nothing magical or decisive (not a surprise to me). Both lenses show very nice color and contrast, decent blur to either side of focus, and similar enough focus speed and noise for me not to comment after changing lenses. Weight is almost exactly 100g different on our kitchen scale, 300g and maybe 405g with front caps on.
Having used the Tamron for a few months it has the upper hand here, as surprises with that lens were revealed in my distant past. The Sigma surprised me with its screw-on hood: with the hood screwed on, the lens cap cannot attach! Some would find that a thing worth doing, but Sigma chose not to make it so. Bummer. I suppose the hood protects the lens nearly as much as a cap, but still..
I was using auto WB for these tests so color variation may appear with more detailed testing - but I liked the colors captured by both of them. We had two Easter baskets on the table so it was easy to capture color! We will wait a while for specifics on bokeh as well, but both easily fall into my "more than good enough" rating.
A few test shots outside with the Sigma were good, but not all. I shot a twilight-silhouette tree at f/4 to check for fringing; it was evident (green) on a strongly front-focused image, but when the tree was in focus only a smidge of red-cyan was evident. My torture-test image of power lines showed no big issues. On a few plant shots I seemed to see some evidence of back focusing, so that will need to be explored and adjusted (thank you K-5!) before honest answers can be had. Later indoor shots did not show the back focusing, so I will need to pick my adjustments carefully. A tripod certainly will be helpful too!