Conclusion
Testing Sigma and Tamron lenses can be frustrating: someone will always be there to say: you got a bad one. All I can tell you for sure is that this Tamron 28-75mm f2.8 hugely outperformed this Sigma 24-70mm f2.8 and that on this showing it offers a very credible alternative to the Zeiss 35mm prime.
If you have to carry a zoom, this is a good one - nowhere near as good as the massively undervalued Contax 35-70mm which is even sharper than the 35mm prime - but for offering credible sharpness on the 1Ds II and very well controlled distortion in such an appealingly lightweight package, it must be highly commended.
But how can one forget how beautiful the Distagon prime is? Surely it would be unthinkable to throw away such performance for the sake of a miserly pocketful of cash. This isn't a case of either/or, I'm afraid: you really need both.
The good news is that, unlike the Canon 24-70mm f2.8 L, these are not expensive lenses - none of them will cost you more than $300 / £160 a piece on the second hand market.
