Choosing the correct lens

August 2002 CCTV, Surveillance & Remote Monitoring

Choosing a lens

This depends on camera position and the view required. The wider the field of view, the less detail will be available and vice versa.

The easiest way to choose is to use a viewfinder which, when held to the eye, will indicate the view to expect on a monitor.

If a viewfinder is not available, a lens calculator, or lens calculator software, will make the task easier. The only other alternative is to resort to measuring angles and comparing them to lens specification sheets.

Back focus fixed focal length

This is a simple setting, if it is needed at all. With the camera viewing a subject the same distance as the final image distance on site, set the focus ring to midway and adjust back focus for optimum clarity.

Back focus zoom lenses

Before adjusting, note how the focus ring on the zoom lens appears to have little or no effect when the lens is set to wide angle and, conversely, it has a large effect when on telephoto. With the focus set on far (fully clockwise viewed from the front), aim the camera at an object over 30 m away, set to wide angle and adjust the back focus for optimum clarity. Now zoom into telephoto and focus on a nearby object or track, all the way out. If possible, it is better to perform this adjustment in low light conditions or with a filter covering the front optic as this will ensure the iris is completely open giving the smallest depth of field and, therefore, the best results.

Mixing C and CS cameras and lenses

The difference between C and CS mount equipment is the distance between the flange of the lens (the part of the case that butts up against the camera) and the focal plane of the lens (where the CCD sensor must be positioned). This is known as the flange back distance. On C mount lenses, this is 17,5 mm and on CS mount lenses, it is 12,5 mm.

Therefore if you have a CS mount camera and a C mount lens you can add a 5 mm spacer to obtain the correct focus. If, however, you have a C mount camera and a CS mount lens, correct focus cannot be achieved.

Some C mount cameras do allow you to swap the whole mount from C to CS or vice versa.

For more information: Reditron, 011 887 1546, jhb@reditron.co.za



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