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I took this picture of a tree on campus yesterday as I was walking around testing my new lens. As you can see, there are some technical flaws regarding the DOF. This post is mostly asking for some help...

I recently bought a used Canon 28-105mm f/3.5-4.0 USM (I) to replace my kit 18-55. I've been using it the past few days to test it out and I noticed that I'm having serious depth-of-field (or focus) issues that I didn't have with with the kit lens (or my Canon 35mm f/2 prime). Notice how in this picture, the focus point (in focus point) is about the middle of the picture or about 1/3 of the DOF. At 28mm and f/18, shouldn't most of the picture be in focus?

This is an extreme vantage point, but I noticed that even simple landscape photos taken with the same lens are having DOF issues also. I can't get more than the focal point +/- 25% in focus.

Help!? Is there something wrong with my technique, or is this used lens a really bad copy? Or is the Canon 28-105mm a bad lens? It shouldn't be because I've read good reviews about it. I can still return this used copy under the "try-out" period for used lenses.