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By: Jo Chew
Why do you need to?
In many cases, you will not be required to adjust the back focus when you change a lens. You will need to, if the lens does not hold focus at both ends of the zoom range.
By adjusting the back focus, you are changing the distance between the rear of the lens and the CCD chip of the camera. Not all lenses have a back focus adjustment however.
Adjusting the back focus
Before starting, put the camera on a tripod and adjust your camera's viewfinder so it is in sharp focus. Ideally, you'd want a test pattern chart (looks like a dart board) to be at least 75 feet from the camera. Otherwise, as far as possible. If you don't have a test chart, Use a page from a magazine.
1. Set the iris to manual.
2. Set the zoom to manual.
3. Open the iris to 1.4 or its widest aperature. If the illumination on the test chart is too bright for the open iris, reduce the light or move the chart to a darker area.
4. Turn the zoom barrel to extreme 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.
10. When it is, tighten the back focus ring retaining knob to secure the ring.
Note: Most lenses are at their sharpest focus at about a middle iris position like F5.6.