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It is important to use the right lens for the light conditions and desired field of view, otherwise the images seen and recorded will not be satisfactory. The important factors that govern the choice of lens are:
Lenses either have a fixed or variable focal length. Manually variable focal length lenses are called Vari-Focal Lenses. Electrically powered variable focal length lenses are called Zoom Lenses and while they are often used in high-end video surveillance systems, they are generally too expensive for use in most systems. The focal length of a lens is usually given in Millimetres (mm). Focal lengths of most CCTV camera lenses vary from 3.6mm to 16mm for fixed focal length lenses, to well over 70mm for zoom lenses. So what does that mean?
In general short focal length lenses (e.g. 4mm) have wide fields of view. This is good for close-ups or for seeing a large area. Objects appear smaller, rapidly, as distance from the camera increases. As lens focal length increases, the field of view narrows and more distant objects are easier to define.
The Lens Iris:
CCTV Cameras that are used in good and constant light conditions (e.g. indoors with electric lighting) will perform satisfactorily with a “Fixed Iris Lens”. Fixed Iris lenses with fixed focal length are the least expensive and the most widely used.
In varying light conditions (e.g. outside or inside facing a window ) the iris on the camera will need to open and close to accommodate changes in light intensity, otherwise the camera images will be too bright or too dark. Lenses with electrically powered irises which automatically adjust as light conditions change are called “Auto Iris Lenses”. They are more expensive but do a much better job in varying light conditions.
In constant light conditions that are either dim or very bright, a “Manual Iris Lens” may be the answer. As it’s name implies, this type of lens can have the Iris aperture adjusted by hand to give the optimum camera image quality. Pricing for “Manual Iris” lenses is higher than Fixed Iris and lower than Auto Iris lenses.
There are 3 main types of Iris: Manual, Auto Iris and Direct Drive (DC Iris)
This type of lens only has an iris which is manually adjustable by hand and therefore can only be used in areas where the light level is fairly good and constant and there is little effect from changing light levels e.g. sunlight through windows. This allows the use of cameras which have electronic iris to be used.
This type of lens is needed when using cameras outdoors or in areas where the light level is changing or where affected by external sources e.g. sunlight. The lens contains an amplifier which converts the video signal from the camera into a voltage which drives the iris motors on the lens to open or close.
These lenses work in much the same way as auto iris lenses but they do not contain an amplifier. These lenses utilise amplifiers which are built into the camera on which it is to be used. This tends to make them slightly cheaper than auto iris lenses.
The “F Stop” value of the lens.
While “F Stop” is actually a ratio between focal length and lens aperture, in practice, when comparing lenses of the same focal length, it is an indication of the lens aperture. The smaller the “F Stop” the wider the lens aperture and the more light that lens will let into the camera. This is of particular importance in lenses with adjustable irises (both manual and auto).