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.
