One of the most important
parts of any CCTV system is the lens/lenses, consideration should
be given to each site dependent on changing light levels, the
capabilities of the camera you are using and the object you wish
to see. Once you have decided you should consider what type of
lens best suits your application.
The most important points to consider when choosing
a lens are the focal length and the iris type.
The focal length of a lens sets the angle of
view that the camera can see. A short focal length lens has a
wide angle of view.
Fixed focal-length lenses give a fixed angle
of view. A vari-focal lens can be adjusted (within limits) to
give exactly what is required.
The common focal lengths of fixed and vari-focal
lenses and the approximate angles of view they give using 1/3”
cameras are shown below:-
|
Focal Length (mm) |
Fixed/ Vari-focal |
Horizontal Angle of View
( ° ) |
|
2.6 |
Fixed |
95 |
|
3.6 |
Fixed |
72 |
|
4.0 |
Fixed |
60 |
|
8.0 |
Fixed |
30 |
|
3.5-8.0 |
Vari-focal |
30-75 |
|
10-25 |
Vari-focal |
11-27 |
|
5-40 |
Vari-focal |
6-48 |
Avoid using wide-angle lenses as they distort
the picture so that vertical lines bend outwards. The amount of
detail that can be seen from distant objects is low.
Manual iris lenses are best suited to indoor
applications where light levels are more constant.
For outdoor applications, auto-iris lenses,
either video-drive or direct-drive should be used.
back to top...
