Some DV cameras have removable lenses, e.g., the Canon XL1. With these instruments the issue of back focus arises. Sometimes the lens does not hold focus at both ends of the zoom range. Then, it's time to adjust the back focus. The back focus is the distance from the rearmost element of the lens to the focal point on the CCDs.

These are the steps in making the adjustment.

1. Put the camera on a tripod and adjust the viewfinder for sharp focus. You can get a test pattern from this web site - http://www.rondexter.com. Set the test pattern up as far away as possible, but close enough so you can tell if it is in focus at full wide angle.
2. Set the iris and zoom to manual.
3. Open the iris to its widest aperature.
4. Turn the zoom barrel to maximum telephoto.
5. Focus on the chart.
6. Set the zoom to wide angle.
7. Loosen the back focus ring retaining knob.
8. Adjust the back focus ring for the sharpest focus.
9. Repeat steps 4 through 8 until focus is consistently sharp. Finally, tighten the back focus retaining knob.