Originally posted by Entropy Only issue I have with my Sigma 17-70 2.8=4.5 (and mine may just be a bad sample) is that it seems a bit soft when focused at infinity.
However, when focusing on anything even remotely nearby, it's an unbelievably sharp lens.
A little update: I did some research, and the AF sometimes has trouble with focusing on faraway objects when at wide angle settings, simply due to everything being in reasonable focus at those zooms. Since the lens focus adjustment is independent of zoom (i.e. you should be able to zoom w/o refocusing), I have found that if I focus on a distant object at 70mm, then zoom out, I get much better results.
This is not needed for anything that is nearby.
Also note that to allow for thermal expansion, this lens CAN focus well beyond infinity, and at wide angles the AF can often lock into this region. (related to my comments above.)
Again, my lens could just be a bad sample. I still love it because when shooting anything other than landscapes and vistas, it performs extremely well.