Originally posted by craig1024 Thanks walter!
I did a little informal test tonight and found that the lens is much sharper and in focus at f/4.5 than at f/2.8. So maybe it isn't the focus, but the sharpness wide open that is the problem. But what is the point of having a 2.8 lens if you can't use it at 2.8?
If you take a picture of a angled surface (like the floor at 45 degrees), it's easier to find out if the camera is front/back focusing. Keep in mind that the center autofocus point is a lot larger than the small red square so some shift is normal.
I think the Tamron is pretty sharp at f/2.8, but I would expect it to be softer (when pixel peeping) on your K20D than on you K10D because of the additional megapixels.