There are a few possibilities. Use or lack of use of the high-precision f/2.8 center AF point is one of them, but probably the least impact.


Here's the thing - the thinner the depth of field, the more critical focus becomes and the more significant the impact of AF problems becomes. Even though f/4 is not really 'fast', aperture is not the only factor affecting DoF. The other two main factors are focal length (longer focal length for a given subject distance means shallower DoF) and distance to subject (closer subject for a give focal length means shallower DoF). So, with a relatively long (200mm) lens and a relatively close subject especially for that long a focal length, you'll get a very thin DoF. Using the DoFMaster calculator, a 1.6x crop body, 200mm f/4 and the subject 12 feet away, the DoF is around 2" (<5 cm).


So, the first question is, how large was your subject? If it was anything other than a small mammal or small bird, a 2" DoF means that the whole creature is not going to be in focus, even if the AF is spot-on. Perfectly correct AF would get you the eyes (which should be under your AF point) in focus, but maybe not the ears/beak/body/whatever. That means you may not have any AF issues at all!


However, if the exact spot you're aiming at is out of focus (OOF), you may have AF issues to sort out. If you open an image (RAW or JPG) in DPP, you can select AF Point on the View menu to check which point you used (unless you focused then recomposed - but if you did that in the situation above, with a very thin DoF to begin with, you're pretty much guaranteeing an OOF shot).


If you are having bona fide AF issues, it could be the lens, could be the body, or could be the combination of the two. Read "This lens is soft" and other myths on Roger's site for a complete discussion of the issue. The bottom line is that with your camera, your only option would be to send the lens into Canon, and if that tests out fine, you'd have to send both the lens and the body (and if they calibrate the body, you may find that the 'problem' is transferred to the Tamron lens!). AF Microadjustment is one reason why I moved from the T1i to the 7D.


In this case, with a very thin DoF, the f/2.8 center point might help. The f/5.6 points are accurate to within one DoF (whatever that is for the aperture/focal length/focus distance), whereas the f/2.8 high-precision AF point is accurate to within 1/3 of the DoF.


One way to test for AF issues is to use a printable focus test chart, like this one. You tape it to a door (so you can adjust it to a 45° angle to your camera), and mount your camera on a tripod at ~25x the focal length, the longest end of a zoom (so for your 70-200mm zoom, that's ~16 feet from the chart), and test at the widest aperture.


Hope that helps.


--John


Ps. Adorama might be selling a new one now for the same price you paid for a refurb, since there is currently a $45 rebate on that lens (the rebate runs 5/9 - 7/10).