Q: How important are lenses in CCTV systems?
A: Lenses are, arguably, the most important parts of a CCTV system. If a poor or incorrectly adjusted lens produces a blurred image on the camera sensor, the rest of the system can do nothing to restore the lost detail and quality. This issue will be highlighted by new standards for CCTV installations that will be introduced next year.
Q: So what do I have to take into account when choosing a lens?
A: The first thing is to get the right focal length. Short focal length lenses provide wide-angle coverage, but the details in the scene will be small. For the same target distance, a long focal length lens will provide bigger images and, therefore, more detail, but the angle of coverage will be smaller. Good CCTV suppliers will be able to provide details about the relationship between focal length, target distance and image size. It is no surprise that varifocal lenses are so popular, as they allow a greater degree of flexibility.
Q: What about zoom lenses?
A: These also have a variable focal length, but are motorised instead of manually adjusted. They do need to be set up properly, particularly when used for external applications as the point of focus shifts slightly with the application of infra-red light. Many cameras allow the position of the sensor (the back focus) to be adjusted during installation to compensate for this effect, but not all installers take advantage of this. It’s a simple adjustment which must be carried out with the lens iris wide open. The best way to ensure this is to do it at night when light levels are low, but if that’s not possible, the lens should be temporarily fitted with a neutral density (ND) filter to ensure that the iris is fully open.
Q: Don’t autofocus cameras eliminate this problem?
A: They should do, but remember that even autofocus cameras aren’t perfect. The focus can, for example, vary continuously (hunt) in low light, or if there are several targets at similar but not identical distances from the camera. The critical factor is the level of light available for the application.
Q: Are special lenses needed for cameras used with IR lighting?
A: Yes, the lenses must be IR corrected. This means that the lens is more sensitive to IR light and therefore will perform better at night.
Q: What about lens quality?
A: In general, the cost of the lens will reflect the amount of time spent on design and manufacture to eliminate distortion. Always check the f stop number (the lower the better generally). With megapixel cameras this issue is further exacerbated. Specially made lenses must be used.
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