Whenever a zoom lens is attached to a video camera for the first time it must be adjusted to ensure that the lens holds focus throughout a zoom.

This is achieved by adjusting the back focus of the lens. The attached Siemens star chart is ideal for this.

Place the chart about 2 or 3 metres from the camera.
If the lens has a macro adjustment, it is essential that you ensure that the macro is in the indent position, that is to say, macro adjustment OFF.

Zoom in to the centre of the chart, and adjust the front focus ring on the lens to obtain the best possible focus.

Zoom out wide and adjust the back focus ring at the back of the lens.
There is usually a locking screw which of course has to be loosened to allow adjustment.
Tighten the locking screw only lightly, just enough to stop any movement.
Zoom in again and, if necessary, re-adjust the front focus ring on the lens to obtain best focus.

Then zoom out wide and re-adjust the back focus ring.
This may take two or three goes to ensure optimum setting. Each time the adjustment will get less, until finally no further adjustment is necessary. At this point the back focus is set correctly and the locking screw can be re-tightened.