It's normal, it does a little earthquake for you and then it's ready :)
Mine does the same, they all do. Don't worry.

What you described is a natural phenomenon with optical image stabilizers that need to work with a heavy de-centering lens which constitutes part of the image stabilization system.

You'll get the initial viewfinder image shake with Canon zoom lenses too, even the lightweight EF-S 55-250mm zoom. As said, there's nothing to worry about.

However, I would be concerned if my Tamron 17-50mm f.2.8 VC doesn't shake the initial image when I half press the shutter. It might mean I have a problem with the vibration compensation mechanism.

Thanks for your detail inputs. Now, I can enjoy my new Tamron lens.

Hi, i have got some VC related problems too...it seems that when the VC works the lens has a heavy front focus, but when i refocus it is ok.
This is not really a big problem, but it can be...it takes time to refocus after all :).
Any one noticed something like that?

Hi, not that I noticed with mine, but I sometimes subconsciously refocus my composition. Short of Canon's legendary L-series IS lenses or Nikon's ED VR line, with Tamron VC lenses it's usually a mixed bag. Some copies are great right off the assembly line, while others have back or front focus issues.

Because neither camera manufacturer licenses their AF routines to third party makers like Tamron (Sigma and Tokina included), each lens maker have to come up with their reverse engineered AF algorithms.

The de-centering stabilizer lens is not supposed to move front or back as that would change the focus rather than steadying the image circle, but if it's not firmly stationary that could have caused the front focus you've described. Just a guess. Then again, it could be the AF motor not up to Tamron's manufacturing tolerances.

Still, the SP AF 17-50mm f/2.8 VC is one of Tamron's high end models that are made in Japan. Tamron has another factory in China that makes their cheaper lenses. I'm not too sure about Tamron's degree of manufacturing QC compared to Canon or Nikon but thanks to Tamron, we can have affordable image stabilized, constant wide aperture standard zoom lenses. :-)

If your lens is still under the warranty period, maybe you can send it back to where you bought it from for a warranty claim.

Thanks Stratman, the bad thing is that my lens has no warranty, since it was bougth from England...long stoyry.
But anyways, i will go to the regional service for some assistance :).

You're welcome. And oh, since you'll be taking your Tamron to the regional service center, have them check your lens for any front or back-focusing problems and request them to recalibrate if necessary. :-)