when I zoom with the VC on, the image jumps at the beginning and again two seconds after I use the shutter-release button. Is it normal for the image to jump? I'll answer my own question for others to find: Yes it is. Tamron: "The phenomenon of the display shaking when VC starts and ends is a normal part of VC's operation and is therefore not abnormal. The VC mechanism in lenses for Nikon and Canon cameras uses a shift method that moves part of the optical system parallel to the image plane. When the VC mechanism is switched off, the VC lens element that compensates for vibration is locked in the center of the optical system. When the VC switch is turned on and the camera's shutter button is pressed halfway down, the lock is released, allowing Vibration Compensation to start. When the shutter button is released and not pressed again for a set time period, the lens decides that shooting has ended and locks the VC lens to stop Vibration Compensation.This locking mechanism uses a lock/release method to control the rotations of the VC lens when the lock is turned on/off. The display in the viewfinder shakes when the lock is turned on/off, because the rotation axis of the drive mechanism (VC actuator) that turns the lock on/off is not necessarily the same as the optical axis of the VC lens."