Hi, have you guys encountered when trying the lens for Tamron 17-50mm F/2.8 VC, when you first press the half shutter, the image in the viewfinder shakes alot before auto focusing? i'm interested in getting this lens, went to Cathay photo and tried 3 copies of the lens and it all had the same issue. i asked the sales person what is the issue but no one can answer me.. anyone know if this is a case of defect lens?