Lenses (Calculating)

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Online Lens Calculator

We are pleased to offer free access to the Online lens calculator developed by Constant Consulting.


Using a 35mm Camera to Calculate CCTV Lenses

There are many methods of calculating the field of view needed for a particular application and the appropriate lens selection. Previous articles in CCTV Today have covered this topic in depth. Probably the most useful method is to make a video recording using a camera and zoom lens from a hydraulic hoist at the actual camera positions being proposed. Many companies now offer a service using a custom designed vehicle with several types of camera mounted on hydraulic masts. Inside the vehicle are various switching and recording devices. However, they can only cover a limited number of surveys compared to the total number of opportunities. This article looks at other ways of determining fields of view and lens specifications.

There are several types of CCTV viewfinders which are like a small telescope with a zoom lens. You look through the viewfinder and adjust the lens until the required field of view is in the frame. The lens focal length for various camera formats is then shown on scales on the barrel. One problem is that some of these useful devices only adjust to comparatively wide angles and are mainly suitable for internal systems. Also they do not provide a permanent record of the field of view ,which could lead to disputes in the future.

If you have a 35mm camera with a zoom lens, you can accomplish two benefits at one go. First, find the correct lens angle for any camera by reading the zoom setting when the required field of view is composed through the camera viewfinder. Second, make a permanent record of this by taking a shot, keeping a note of the shot and the zoom setting.

The accompanying table lists the lens focal lengths and angles for various zoom settings on a 35mm camera and the equivalent focal lengths for camera formats.

Table4.gif

This chapter is supplied by Mike Constant and was originally published in CCTV Today. Mike is the author of 'The Principles & Practice of CCTV' which is generally accepted as the benchmark for CCTV installation in the UK.