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  HOW TO BACK FOCUS A VARIFOCAL

Q: I'm using more "varifocal" lenses vs. fixed lenses in my everyday camera installations but I am confused on how to properly back focus the varifocal lens.
A: Back focusing a lens to a camera that it's attached to is critical in obtaining optimum clarity. With a fixed lens it's quite basic since the focal length is "pre-set." The varifocal lens, having numerous focal length settings, poses the question: Where do I begin?

The very first thing you must know to obtain proper back focus is the minimum object distance (MOD). Let's use the 1/3-inch varifocal lens for this example. Most 1/3-inch varifocal lens manufacturers show an MOD of 0.3 meters or approximately one foot for this lens. Now, attach the varifocal to the camera and set the three barrels as follows:

Focus Barrel. Move completely to the "near" setting, opposite of "infinity" or "far." You also should notice the front lens element moves forward, away from the front of the camera.

Focal Length Barrel. Move completely to the wide "W" setting, the opposite end of the telephoto or "T" setting.

Iris Barrel. With the camera on and viewing the monitor, achieve the optimum lighting condition without washing out the picture by moving the barrel from close towards open.

Now put a "test pattern" or any white paper with either print or an image on it at the one-foot point from the lens. Fill the entire screen of the monitor with this "test pattern" and begin adjustment. This "adjustment" varies with camera manufacturers. Some allow you to move the CCD of the camera by way of a screwdriver placed inside a hole of the camera body. Others use a collar with the lens screwed into this collar further in or out of the camera to set the lens in relation to the CCD of the camera.

In either case, adjust the position to achieve optimum clarity on the monitor. Once achieved, lock this position of the lens to the CCD distance. Now remove the "test pattern." and change the focal length barrel of the lens to any position, then adjust the focus barrel to get optimum clarity. Remember, a varifocal lens means that anytime you adjust focal length, you also must adjust focus.

Do you have a question? E-mail your questions to info@tekstaroptical.com




Tekstar Optical, Inc.  |  75 Lackawaca Hill, Napanoch, NY 12458  | 
Phone/Fax: 845-647-5367 (LENS)  |  Cell: 516-582-3458
Email: info@tekstaroptical.com