I got this Sigma DC 18-50mm f/2.8 EX MACRO HSM lens few weeks ago, but I noticed considerable sharpness problems with wide apertures. Until f3.5 (or even f4.5) this lens can't really be used.
I would like to know whether anybody else experienced the same issue? I've heared that Sigma lenses can be very inconsistent. You may bump into a very good lens or a lesser quality one (being the same lens type).
I would also like to know whether Tamron's equivalent 17-50mm f/2.8 lens is any better?
Here're examples of the same image taken few seconds apart. You can see the strange halo effect on the 2.8. Top part of both images is 25% original size (and cropped to fit size here), bottom part is 100% crop of strict lens centre.
50mm f2.8 - 25% and 100% lens centre crop
50mm f4.5 - 25% and 100% lens centre crop
One can of course see the difference in depth of field as well as focus issues I'm having with this lens. I wouldn't mind if it was soft on edges, but both of these images are lens centre crops.
Is this normal or do you think that my particular lens has issues of its own (which are either general for particular Sigma lens model or are just issues with my lens)?