Back Focus adjustment for zoom lenses

How to adjust the back focus on a television camera zoom lens.

Most zoom lenses have a back focus adjustment ring which adjusts the distance between the rear element of the lens and the focal plane of the camera. As this distance is very slightly different between any given lens and camera, you will need to check and adjust the back focus whenever you mount a lens to a camera for the first time. Back focus can also drift with time so it's a good idea to check this periodically, especially if you are making frequent lens changes.

To adjust back focus

1. Put the camera on a tripod
2. Make sure your eyepiece or viewfinder is in focus for your eye (do this using colour bars)
3. Place a focus chart a good distance from the camera (at least 20 feet) Download Focus Chart pdf [32kB]
4. Adjust the iris to wide open and control the exposure using the ND filter wheels if necessary
5. If your lens has a 2X extender switch it to 1X
6. Zoom in to the end of the telephoto range and focus on the chart using the focus barrel
7. Do not alter focus
8. Zoom out to the wide end of the lens
9. If the focus chart has gone out of focus, loosen the back focus ring and use this bring the chart into focus
10. Zoom in again and refocus using the focus barrel
11. Zoom out again and refocus the back focus ring
12. Repeat steps 9 & 10 until the focus chart is sharp at both ends of the zoom range
13. Lock the back focus ring
14. Check focus again to make sure the back focus hasn't moved as you locked it
15. Back focus is complete