Specifications

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KNOW-HOW: Lenses
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S55
Lenses promote the efficiency of a video surveil-lance system to a large
extent.
Lenses are an indispensable part of a surveillance system, no mat-
ter if the lens is already connected to the camera, as it is the case with
almost all compact cameras and camera modules, or attached to the
camera as an interchangeable lens.
To make it clear from the start:
A CAMERA WITHOUT A LENS IS USELESS
The right selection of the lens defines the visible picture detail to a large
extent. Although the necessary attention is not always paid, the selec-
tion of the suitable lens is at least as important as the selection of the
suitable camera type.
Different types of lenses are available for versatile applications. It is
worth having a look at the specifications when choosing a lens in order
to select the right one.
Features of lenses
Focal length
The focal length is always stated in mm. The visible picture detail is de-
fined by the focal length.
The value does not indicate the quality of the lens. Small focal lengths
indicate a large visible picture detail. Therefore, lenses with a focal
length of 2.5mm are wide angle lenses which allow to view a large
area; details are shown very small. On the other hand there are lenses
with large focal lengths, like e.g. 50mm telelenses which only show
small picture details, but persons and number plates are clearly identi-
fied, even from a distance of 10m.
When changing the focal length, the range which is visible on the
monitor changes in proportion to the focal length of the lens. Sounds
complicated, but it is really quite simple. By doubling the focal length,
the monitoring range is halved, but everything is displayed twice as
large. Details can thus be better recognised.
Inversely, by halving the focal length, the visible range on the mon-
itor is doubled. Details are displayed smaller.
The respective picture detail can be determined and the matching
lens can be selected by means of the focal length table.
Focus
A point in front of the camera can be adjusted via the focus adjustment
at the lens, allowing a clear reproduction on the monitor.
At zoom lenses, i.e. lenses with adjustable focal length (e.g. the
VZLCS series), the focus adjustment has to be readjusted after every
change of the focal length.
Iris and F-stop
The quantity of light reaching the chip via the lens is controlled with the
iris, just like the iris of our eyes is able to adapt to great fluctuations of
brightness.
The F-stop describes the smallest possible iris number. The F-stop is
shown e.g. as 1:1.4. The smaller the second value is, the higher the lu-
minous intensity of the lens. Even small changes of the value already
have got a large influence on the quantity of light which reaches the
image sensor.
If a lens is used with an F-stop of 1:1.8 instead of 1:1.4, twice the
quantity of light is required.
Of course, a lens of lower luminous intensity is available at a more
favourable price than a lens of high luminous intensity.
Nevertheless, it makes more sense to attach a high-quality lens to a
basic camera, than to plan a high-sensitive camera with a lens of low
luminous intensity.
Concerning the iris adjustment, the lenses can be subdivided.
Lenses with fixed iris
These are the most basic lenses. No facility for iris adjustment is avail-
able. The lenses for the camera modules as well as the lenses of the
VSL series with C mount thread are included in this class. If these
lenses are used, the camera adjustments (ES, AGC, BLC) will control
the image sensor in such a way, that a perfect picture is provided.
Lenses with fixed iris should only be applied for basic surveillance
applications.
Lenses with manual iris
The iris is adjusted manually via a ring which is attached at these
lenses. When mounting, the iris has once to be adjusted at a place
where it is used. If the lighting conditions change greatly, the iris at the
camera must be readjusted.
These lenses are suitable for all areas with a relatively constant illumi-
nation. These include e.g. all closed rooms which are illuminated with
artificial light. The classic store surveillance is part of this application
profile.
Please note: the adjustment to great fluctuations of brightness is only
possible directly at the lens.
This type is generally marked with the letter "M" at the end of the item
number.
Lenses with automatically controlled iris
With these lenses, the iris is adjusted electromechanically. The control
of these lenses is done via a cable at the camera from a particular con-
nection of the camera.
Lenses with automatically controlled iris can also be used with great
fluctuations of the lighting conditions, therefore they are mainly used
for outside surveillance.
Two different kinds of controls are available on the market at present.
These are distinguished by the position of the control electronics.
Lenses with video signal controlled iris, also called autoiris or AI.
This is the older version. With these lenses the evaluating electron-
ics is installed in the lens. Therefore, adjustments are available in order
to set the control response of these lenses.
ALC = setting, whether peak light (P) or brightness areas (A) are to
be controlled
Level = brightness level
Lenses