After some more experimentation with my Tamron 28-300 Di VC, I think I finally understand what this lens is and what to do with it. It is a closeup lens, not a telephoto lens. That is, it takes really good pictures fairly close (within a few meters) and pretty crappy ones far away. For example look at this nice close-up of a juvenile American coot taken at 300mm:
Santee Lakes, San Diego County, 2009-08-23
1/800 s, f/6.3, ISO 400, handheld
It’s pretty good, especially around the body where you can easily make out the individual feathers. In fact, if anything, it’s over-sharp; though that’s probably just a combination of me over-sharpening it in Lightroom and the JPEG conversion. The original full-size photo is even better than this.
Now compare to this shot of a far-away coot taken with the same lens. You’d expect the subject to be smaller, but it’ worse than that: very, very fuzzy and unsharp, even with a much faster shutter speed:
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