Marcel, How do you know? What didn't you like about the Sigma? I have read a lot of people are happy with the Sigma, is there that much of a difference? I'm not interested in specs. More interested in side by side shots taken with the two. Most complaints I have read about Simga is they are a little soft on the long end. Just wondering.
Hi Rick, no, I don't know about the sigma 17-50 2.8. I only know about the Tamron and the Nikon (17-50 and 17-55). The image quality is close but the construction is better with the Nikon. The original poster was asking about sigma 17-50 vs Nikon 24-70. I was barely mentioning that the 24-70 is FX while the other is not. Now I've read great reviews about the Sigma 24-70 and the Tamron 28-75. Both cover FX and are apparently very sharp. I know the Nikons are more expensive, but if you ever want to trade, you should get more of your money back with the Nikon.
For comparaison, have a look at this link:
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You can select any lens in combination with any body and look at pictures produced with that particular combination (many at full size).
I have to say that I'm quite happy with the two sigmas I have (10-20 3,5-5,6 and 105 2.8 macro) and am seriously considering the 50 1.4 and the 85 1.4 from Sigma.
There is enough choices to please everybody, but personally, I know that I sometimes buy the one that seems to be the bargain and then decide later to get the more expensive one. So for me, getting the more expensive one to start with is less expensive. But that's my problem and some others wouldn't do what I do.