A day with the Sigma
Carrying the 50mm macro was nice, a good match to the k-5. Focus locked in pretty quickly. Let me rephase: the lens was quick to lock down on a point of focus, but a few times the subject I intended was not in focus. I kept the camera in center AF mode.. so this is odd. When it's off it is always back focused, and refocusing did not change the lens' opinion about the point of focus (AF confirms with no lens movement). At other times it locked perfectly though, so microadjusting the lens to a new focus would not solve this. It was easy to see when it happened in the viewfinder so I could shift back an inch or less and get a sharp image - so for me tne workaround was not a major pain. It should not be necessary though!
Exposure and color were nice, and overall I liked working/playing with the Sigma. I did not put on the hood so shading on extreme closeups was less of an issue.. plus it allowed me to use the lens cap (yes, still annoyed by that).
A day with the Tamron
A similar walk with the 90mm came the next day. Right off the bat (a bit before 7am.. both lenses were used at sunrise and noonish) I would say the Tamron hunts more but nails focus more consistently. If this persists it will make a more interesting choice between these two copies. Which would I prefer: to rock back for the occasional closeup shot at 50mm, or hum along with the occasional AF whine at 90? More study needed!
I find the Tamron a bit bulky, but I know that I'm more sensitive than most about size & weight. In fact the Rikenon 70-150 weighs 40g more but is quite skinny, which suggests that bulk is as big a factor to me as weight. This is an interesting discovery (to me, probably less so to you!).
My lunchtime stroll showed more of the same: great images and a few focus hunts along the way. I did not use the focus limiter with either lens so time to focus was as bad it it gets. Still, it is broad daylight though overcast, so it's too bad an f/2.8 lens is working so hard. A patient squirrel posed for a few shots; the extra reach of the Tamron helped here but I still have a lot of cropping space.