Resolving the Problem of Focus Shift

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Focus Shift is the condition that occurs when images that are sharp and in focus under artificial lighting (such as external cameras illuminated with IR lighting at night) are out of focus (appear soft or blurred) in daylight conditions and vice versa. The problem is caused by the nature of light. Different types of light have different wavelengths which means that an image viewed in different light conditions will appear slightly differently. Unless the lens is adjusted for different light conditions, it is impossible for a standard lens to produce a sharply focused image in all types of light.

To overcome this lenses either have to be adjusted manually for day and night time performance (impractical) or, if the camera is fitted with a remote controlled motor zoom lens, the picture may easily be brought back into focus (not possible with static lenses, of course, or on an unmanned site). A third option is to use the more expensive IR corrected lenses.

Ernitec A/S is a leading manufacturer of IR corrected lenses. Niels-Christian Andreasen, Lens Product Manager, is understandably keen on IR corrected lenses but believes they should be adopted more widely than just for day/night colour/monochrome environments:

"IR-corrected lenses should always be used and not only when using IR-illumination at night time. Many light sources include a part of IR-light, small or light. In connection with monochrome- or day/night cameras IR-corrected lenses will provide a sharper picture because all the light is focused, resulting in a far crisper picture compared to ordinary lenses. Sunlight contains much IR-light, but also many artificial light sources, especially halogen. Also, ordinary incandescent light bulbs include a considerable amount of IR-light. When having illumination with a mixture of visible and infrared light, it is possible to obtain a considerable improvement of the picture reproduction without having to replace the camera - just by replacing the lens. So there are excellent reasons for using IR corrected lenses in all environments.

"It is not possible to modify standard lenses to make them IR-corrected, e.g. just by coating the lens elements - at least not with optimum results. From the early stages of the design phase IR-correction must be considered and included in the design; special glass must be used for the lens elements and special coating of the lens element surfaces is required. Previously, this caused IR-corrected to be fairly expensive, but new efficient production technologies have reduced costs dramatically."

IR illumination manufacturer Derwent is pleased to see the increased use and market awareness of IR corrected lenses. Shaun Cutler, Derwent's Marketing Director, highlighted that 'the recent increased growth in day and night cameras has contributed towards the demand for IR corrected lenses and naturally, IR illumination. The industry is accepting that colour cameras can not perform as effectively at night as monochrome camera supported by IR illumination. Lens IR sensisitivity and Focus Shift are a real issue and it is pleasing to see that many of the leading lens manufactures such as Computar, Pentax, Fujion and Ernitec are responding to this.'

The Manufacturer Extreme CCTV believes that whilst single ccd day and night cameras can provide a good 24hour solution, their performance can not be compared to a twin camera solution. Extreme argue that a single camera solution can only provide a compromise between a perfect daytime picture and a perfect night-time picture (this is more evident with fixed filter models).

Extreme's engineers claim that the perfect, no compromise picture day and night picture can only truly be achieved with two separate cameras and two separate lenses integrated with IR illumination - in other words, two cameras in a single package.

Mark Vernon of Extreme CCTV commented: "This is not just a focus shift issue; it's also about accurate colour rendition, achieving the best picture in day and night conditions. During the day cameras must accommodate extremes of bright light yet at night gather as much light as possible. As a result it will always be a compromise trying to get the best picture 24 hrs a day from a single camera. With the dual camera each lens is optimised to get the best day and night image with the result that there is no focus shift, no colour rendition and very high IR sensitivity thereby matching the IR illumination to the camera field of view. When light levels drop the unit shifts to a monochrome camera which is then maximised for low light sensitivity with a high f-stop lens to gather as much light as possible and is already pre-focused for when the IR illumination switches on."

IR Corrected Lens Manufacturers:
Ernitec
Pentax
Ultrak

IR Illumination Manufacturers:
Bewater
Extreme CCTV
Dennard

This article was supplied by Norbain and originally appeared in 'Security Matters' magazine. Norbain are manufacturers of the popular Vista range of CCTV Equipment.