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. CCTV Cameras with an integral lens are less expensive but do not offer lens choices and are therefore limited in their usefulness. 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 Millimeters (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.
If, for instance, you wanted to be able to positively identify people, 25' away from a camera, a short focal length lens (e.g. 3.6mm) would give such a large field of view (37'x 26') that recognition would be uncertain. An 8mm or even 12mm lens would be far better.
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.
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 ). Our vari-focal, manual and auto-iris lenses have an "F Stop" value of 1.0, giving a far better aperture than most and consequently a better image, particularly in lower light conditions.
We supply a full range of C/CS mount lenses for CCTV camera bodies, including fixed focal length, vari-focal, fixed iris, manual iris and auto iris lenses. The most widely used lenses appear in the table below but others are available. Please contact us if the lens you need is not shown here:
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All lenses include precision glass optics and better than average F Stop values, thus providing optimal image quality and brightness.