[Chris H]

Ever since I got my Canon 40D my Tamron 18-250 seemed to be slightly out of focus.

Well it was proved yesterday to be rather " soft ".

What I'm not saying is that the lens is necessarily faulty but can it be re calibrated to the 40D.

I have no idea whats involved but certainly the camera was considerably sharper with a Canon 70-300 IS USM on it.

Just a tad disappointed because the lens seemed really sharp on my 350D.

[LJR69]

Is it perhaps that the higher resolution of the 40D is showing up the imperfections of the lens that may always have been there? Are you getting the softness throughout all aperture settings? I'm not surprised that the Canon 70-300 is a sharper lens than the Tamron one, but of course I don't know how much a difference we are talking about here.

If you are sure the lens really is faulty then perhaps try contacting Tamron and see what they recommend (and see if they'll perhaps do you a deal )

[JMitchell]

It sounds to me like the problem described above. Cameras with higher resolution sensors will show up the quality of your lenses much more clearly, and as you've made the jump from the 350D to the 40D, I am not really surprised.
The range of the focal length is so vast that it needs a lot of glass elements, inherently reducing image quality, but as mentioned, try shooting at slightly narrower f stops if possible, and find out which ones render images the most sharply at each (general) focal length.
As also said, the 70-300 IS USM will be a lot sharper, just because it is a far superior optic to the Tamron, and I am guessing has less glass overall, reducing the effects diffraction, and thereby increasing sharpness.

HTH, and I've found this going from a D50 to a D300