Originally posted by calicojack Both of my Vivitar Series 1's are manual focus, and like the Sigma, are only 2.8 at the wide end of the zoom.
Brian
So, it isn't really meaningful to compare them to the 50-135. All evidence from what I see here is that that whatever problem you might have with backfocus in the AF system, it probably affects all your AF lenses that have a sufficiently shallow DOF to expose the problem. The focus chart you display is too small to say for sure, but yeah, it looks like it might be focusing a little bit behind where it should. This is more apparent in the poorly lit example than in the well-lit one, though, so I wouldn't be leaping to any coclusion. Multiple tests in good light (not flash, not good *constant* light for the AF system) would help. There is often a little bit of variation from shot to shot in where the focus system decided to focus. If it turns out to *consistently* produce results where the *entire* zone of acceptable sharpness is behind the focus target, I'd send it in for service. It it's just, as appear to be here, where the *center* of the zone is *sometimes* behind the target but the target is still in focus, I personally wouldn't worry about it - it shouldn't be enough of an issue in practice to be worth being without the camera for weeks.
The other examples you posted are pretty much useless, BTW. *You* may have wanted to focus on the tin can, but that's no guarantee the camera chose the can. it might just have well chosen the leaf in front of the dark shadow behind the can, which is the area of highest contrast, and *well* within the center focus point's zone. Ditto with the others. That isn't proof of BF, it's easily explained by the size of the focus sensor and the inherent difficulty in forcing a camera to focus on what you think it should focus on.