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 surveillance 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:
Focal
Length
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 security 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. Take a look at the table below:
|
Field of View (in feet) |
Lens
Focal
Length
(mm) |
Viewable
Angle (Degrees)
Horiz. Vert.
|
5
feet away
Horiz. Vert.
|
10
feet away
Horiz. Vert.
|
15
feet away
Horiz. Vert.
|
25
feet away
Horiz. Vert.
|
50
feet away
Horiz. Vert.
|
100
feet away
Horiz. Vert.
|
| 3.6 |
74 55 |
7.5 5.2 |
15.1 10.4 |
22.6 15.6 |
37.7
26.1 |
75.4
52.1 |
151
104 |
| 6.0 |
42
32 |
3.8 2.9 |
7.7 5.7 |
11.5
8.6 |
19.2
14.4 |
38.4 28.7 |
77
57 |
| 8.0 |
32 24 |
2.9 2.1 |
5.7
4.3 |
8.6 6.4 |
14.4 10.7 |
28.7
21.3 |
57 43 |
| 12.0 |
22 17 |
1.9 1.5 |
3.9
3.0 |
5.8
4.5 |
9.7 7.5 |
19.4 14.9 |
39
30 |
| 16.0 |
15 11 |
1.3 1.0 |
2.6
1.9 |
3.9 2.9 |
6.6 4.8 |
13.2
9.6 |
26 19 |
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
surveillance 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.
