I have the Tamron AF70-300 Di LD f/4-5.6 macro. It's a decent lens. Here are a couple of simple shots taken recently with this lens + the Tamron 1.4x teleconverter, on my K10D:
Picasa Web Albums - William Porter - 20080112 lake
These shots were focused manually. Effective focal length in 35mm terms = ( 300 + 120 ) * 1.5 = 630mm. Shutter speed = 1/320th sec. Either shake reduction works really well, or I have a very steady hand, or a bit of both.
Be sure to look at a slideshow or zoom in on the first photo so you can see the detail in the pelican's feathers. I think the detail is quite satisfactory.
Zoom in on the second shot, too. Note that the in-focus duck's head and bill are quite sharp. I think the out of focus areas here (bokeh) are appealing, too. You will notice some funkiness in the color--say, along the neck of the second duck--that is, I guess, chromatic aberration. Sort of green fringing. I am pretty sure this was exacerbated by the way in which I post-processed this shot in LightZone: I did some selective editing with regions and may not have done a great job feathering the line between the selection and the non-selected part of the photo. But this is the kind of shot that generates chromatic aberration--strong backlighting. Anyway, while connoisseurs might look at this and decide to burn the lens, I still rather like the photo, and this problem (if it is a problem) doesn't occur often, in fact, this is the only time I remember noticing it.
Now, the deal here is that this lens + the converter cost me $300 or less. The superior alternatives are much more expensive. I continue to eye the Sigma 135-400 which looks very good and is the only long K-mount lens that is within my reach, but I doubt I'll get it any time soon. What I'd really like is a 300mm or 400mm f/4 prime. But I have not seen one for sale anywhere.
Note that both Sigma and Tamron make a couple 70-something-to-300 lenses. Both companies make a cheaper (older) version in this range and a slightly more expensive, newer and presumably better counterpart. The
Tamron 70-300 Di II macro is the "better" one (it's "Di" and the other one isn't). I have heard that Sigma's "better" version--the
70-300mm F4-5.6 APO DG MACRO--has very similar specs, but more blades/groups (not sure what that matters). It's also slightly more expensive. I suspect that the Sigma might be slightly better, but I would not expect a major difference.
If I had $600 to spend, I'd get the Sigma 135-400. If you want something longer or better, well, keep saving.
Will