These tests of the 17-35mm EX DG HSM Sigma lens I have listed on Ebay were taken by request of the buyer to demonstrate focus accuracy. All shots were taken indoors with no flash and ISO800 using my Canon 40D body.
High ISO (even if a little grainy when copped in) is independent of front/back focus accuracy results and is considered acceptable for the tests.
The first two images of batteries lined up are a simple recommended method for determining effects of front or back focus
Per the procedure, I used the widest appeture on the Sigma (f/2.8 at 17m) and positioned the camera at approximately 45 degrees from the line of batteries using the closest focus distance at which the lens can focus.
The first battery shot was done with manual focus. (I had to wear my glasses to ensure I was getting proper focus.)
The second battery shot was done with auto focus.
Results show the center battery is 'most focused', the batteries left and right of the center are similarly de-focused a bit, and the outer most batteries are de-focused more.
The amount of de-focus of the 2nd and 4th batteries appears similar and within spec.
Please excuse the fact that my shadow got in the way of the far left hand battery thus dulling it's appearance. It has nothing to do with the lens...
The next shot of a bottle sitting on top of an ornate decorated box was taken from about 5-6 feet away. I'm showing the full image + two crop-in's to illustrate the clarity achieved.
I took this in auto focus mode.
Last I clicked a portrait of our granddaughter which I think captures some of the color & contrast of the lens and some bokeh down the hallway. I used autofocus and aimed for the eye on the left side of the image. I was about 4-5 feet away for this shot.
Conclusions: The battery pictures illustrate the most definitive method to test for front/back focus issues. It's my judgement that this lens is well within spec and functions quite nicely.
I always liked the lens. It's a GREAT value for the price at which I offered it.
It's not as crisp as my much more expensive Canon L lenses, and there's a tiny bit of histerisis you can feel when focusing back-n-forth in tight, but this is typical for this level of lens. Bottom line is it's smooth, operating to spec and you're not having to spend another $1000 for L glass. That's why I bought it originally.
I hope all of this information helps you buy with confidence.
The final judgement is left to you.