The Canon TCR-101F Speed Servo Controller makes selective focus in studio or field settings an absolute joy. This controller gives you full manual override control over focus settings on compatible Canon lenses. It also gives you manual control over the speed of the zoom via a variable potentiometer, increasing the accuracy of your zooms.
This baseline model of Canon's TCR Series of remote lens controls is perfect for the slow and deliberate zoom, or for rack-focusing between near and far focus points in a demanding scene.