Back Focus Procedure

Back Focus Procedure

1. Aim the camera at a subject at least 12 feet away, in the worst case, beyond the minimum focus distance for the lens. The subject should provide you with sharp, easily recognizable features so that you can easily see when the subject is in focus.

2. Make sure the iris is fully open. This reduces the depth of field so when lighting conditions change, the lens will still perform correctly.

2.1. If you have manual iris or auto iris override, open the iris fully.

2.2. If you have an auto iris lens with no override, place a neutral density filter in front of the lens or perform this procedure in low light level conditions. The objective here is to get the iris in the lens to fully open so that the depth of field is reduced to a minimum.

2.3. Note: A neutral density filter is a filter that blocks all frequencies of visible light equally. There are three common measures of the filter density.

ND F Stop % Of light blocked
3
1
30
6
2
60
9
3
90
 

 
Two F stops is usually enough to open the iris. However, what you will need will depend on your, application (lens speed, and lighting conditions).

The amount of light you must filter to fully open the iris will depend on the sensitivity of the camera and the speed of the lens. Multiple filters can be stacked to reduce transmitted light to a level that forces the iris open.

3. Neutral density filters can be obtained at most camera shops and theatrical lighting suppliers. Camera shops will carry glass filters and theatrical suppliers will carry the same thing in Mylar (a thin plastic material). A 55mm diameter glass filter (made for a 35mm camera) will cost somewhere around $20-$30. A Mylar sheet 20″ by 24″ will cost around $5.00 and can be cut with a pair of scissors.

4. Adjust the focus to full far.

5. Adjust the zoom to extreme wide angle.

6. If the camera is equipped with a pick-up device or camera lens mount set screw, loosen that screw.

7. Adjust the pick-up device position (Back Focus) to obtain a clear picture.

telephoto

8. Without touching the focus, zoom into full telephoto.

9. Using the lens controller, adjust the lens for best focus. Do not adjust the camera (Back Focus).

10. Zoom out to full wide angle.

11. Repeat the above steps until the focus remains clear throughout the zoom range by focusing out to a very distant object at least 75 feet away. Check that the focus remains clear. If not, repeat above back-focus procedure (4 to 7).

12. Remember to make sure everything is set for normal operation when you’re finished.

– Remove any filter used to aid in this procedure.
– Restore auto iris lenses to full automatic operation.
– Make sure the back focus setting is secured (tighten screws if needed).

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