I just purchased a Sigma 10-20mm lens (Canon mount) and based on the reviews I was aware that many people receive copies of the lens that are soft or don't focus properly. I purchased from B&H, so I'm not worried about returning it other than the hassle. What are some tests that I can do to determine if my copy of the lens should be returned?
So far I've locked the camera on the tripod and taken comparison shots of a bar-code with both the Sigma lens and my 18-55mm kit lens. Below are crops (zoomed into 100% and cropped, only showing the center part of the image):
The kit lens seems sharper and the depth-of-field seems to be more centered around the focal point whereas the Sigma lens seems to have the focus point placed toward the very back of the DOF area.
Is it possible to tell if the copy of the lens is bad from these shots or are there other tests I should do?
Here are links to two (full-size) focus-chart images I took based on one of the resources in Eruditass' answer:
From these it seems that at 20mm, the focus is pretty spot-on, while at 10mm it might be shifted to the front by 5-10mm or so but with the target still in the acceptable DOF. Since most of my usage at 10mm will be landscapes stopped down for max DOF and the affect isn't at all focal lengths, I'm considering that the autofocus calibration of this lens is fine for my purposes.