This is set by adjusting the position of the entire lens closer to or further away from the camera sensor along the axis of the lens. This sets the distance between the rear lens element and the sensor. This type of adjustment is required only to ensure that the lens focus ring matches what is actually in focus. Vicon makes this adjustment when initially setting the camera up.
As back focusing is a delicate operation, it must be performed by qualified and experienced persons only. For this reason, if you require a change in the back focus of your cameras, please contact Vicon Support.