Part three intended to swap back to the Sigma right after the Tamron, so I could verify what I thought was different about the two lenses. Alas, my health failed me so I only took a few images with the 50mm. So much for plans and some degree of rigor.. but the points that I have hit so far were not refuted. Focus hunted less with the Sigma and missed a time or two. A squirrel photo-op with the 50mm came along just like with the 90, but I was at f/3.2 and the focus missed. To be fair to the Sigma, I believe I have learned that the center focus point on my K-5 is almost precisely at the lower-right corner of the square that displays in the viewfinder. The Sigma's hit rate shows more success with that in mind (I found this out after the squirrel scene, sadly). I need to verify this with the Tamron and other lenses, since that is something that should not drift from lens to lens!
After further review - yes the Tamron also prefers the lower-right area of the center-focus square. And a new item pops up that I had never tested when using the aperture ring on an A-enabled lens. I had presumed that if I were in Av mode and using the aperture ring in non-A mode, the camera would still know what aperture was set. It does not - on playback all shots show the max aperture of f/2.8. Not a big deal, just leave A-enabled lenses on the A setting - but a good thing to know! I probably knew this back in 1995 with my Program Plus.. but my favorite lens back then was an SMC-M 24-50 so maybe not.
