All are decent lenses, but I suspect that the Pentax has the best rendering in the way of contrast, colour fidelity, flare and possibly bokeh, it also has an extra 1 or 2mm at the wide end, okay it has the greatest distortion wide open and some CA, but both of those are correctable with software in PP, the other qualities aren't, in addition it has SDM (if important to you) and is weather-sealed. But it is considerably more money.
Of the remaining two, I would predict that the Tamron would be the better, going on the reports from the C&N folk who have been able to evaluate them both.
We will have to wait for a couple of months before we can decide. Again the Tamron has an extra mm at the wide end, the Pentax has 2mm, which anyone, who has switched from the kit (or any other 18mm) lens, makes a surprising difference in the available FOV.
If you can afford the Pentax, then I'd buy it, otherwise I'd actually buy the old DA 16-45 for now or maybe the Tamron if it shines in Pentax mount, the extra 5mm at the long end and the extra stop isn't the be all and end all!
Personally, I'm waiting to evaluate the DA 17-70 f4.0 SDM (due in the summer) before I make decisions to change from my 16-45.
Quality of any lens is subject to sample variation and overall QC, the latest batches of the 16-50 seem to be within an acceptable level. Again both Sigma and Tamron have sample variations reported, my personal experience is that Tamron is the better as I've never had a faulty Tamron, but have had poor experience with Sigma. I've also had poor Pentax lenses, but they were replaced by good examples, which I wasn't able to do with the Sigmas, although I tried, they were fundamental design issues.
Last edited by Richard Day; 02-05-2008 at 02:12 AM.
Reason: added a paragraph.
Originally posted by hll 
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