Originally posted by civiletti I went back to the Photozone test data and checked my conclusion by calculating relative camera MTF factors. After doing this a few different ways, I found that the DA70 actually has a slight edge in MTF, but it is VERY slight.\
Note that the two lenses have been measured on two different cameras. The MTF results are therefore not comparable. Even if you normalised them according to sensor resolution, they wouldn't be comparable because of different AA filter strength, different sharpening by Klaus, etc.
One sign that the Sigma 70/2.8 EX is the better lens optically is that its centre sharpness peaks at f/4. That's an indication of excellent performance (low abberrations). The Pentax DA 70mm peaks at f/5.6 which is very good, but not as good.
Originally posted by civiletti The Pentax lens is more expensive, of course.
If that's true, that's a bit of a joke.
The Sigma contains a lot more glass, more expensive glass (3 SLD elements) and has a focus limiter.
You'd be paying more for less, if you went for the Pentax (which can be exactly what people want, if "less" refers to the size).