Espysecurity
 | | | | Digital
Video Setup | Cable
Connection | Hidden
& Wireless Setup | Technical
Words & Concepts |
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| Important |
| The format size of a lens must be equal to or greater than
the format size of the camera it is being used on. If the lens of smaller format
size than the camera then the corners of the scene being viewed will be cut-off.
You can usa 1/2" lens with a 1/3" camera. But you can not use a 1/3"
lens with a 1/2" camera. |
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| Lens
Mounts |
| A CCTV lens will be specified as either
C or CS Mount. Both types of lens look very similar and there is nothing that
can be physically measured on a lens, e.g. thread diameter or pitch, to determine
whether it is C or CS. The difference between the two types is the position of
the focused image behind the lens. CS lenses focus 12.5mm behind the lens whereas
C lenses focus 17.5mm behind.Most new lenses tend to be CS mount because they
are similar and therefore cheaper to manufacture. Ideally the camera mount and
the lens mount should be the same but it is possible to use a C Mount lens on
a CS Mount Camera by using a 5mm spacing with most body cameras. |
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| Lens Type |
| There are two main types of lens used in CCTV, these are fixed
focal length type and zoom (varifocal) lens type. |
|
Focal length of a lens defines its effective viewing angels both horizontally
and vertically. Hence the focal length of a lens determines the size of a particular
image on the monitor screen or the area of the scene being covered by the camera. |
| 1/3" format lenses on a 1/3" format camera, the
following lens focal lengths are commonly available. |
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| Lens Measurement - Sizes |
| The
choice of focal length of lens fitted to a camera affects how big a particular
object, etc. a person, will appear on the monitor screen or a video recording
from the camera. Obviously, the bigger the image apperas on the screen, the better
the chances of recognizing or identifying a person being viewed or recorded. |
| Guide: Smaller the focal length number = wider
field of view |
| A. Warehouse or Office, recommendation
to go 2.8 - 4.3mm lens (gives from wide to distance view) |
| B.
Entrance or Lobby, recommendation to use 8mm lens |
| For
best result on view angle use varifocal lenses to get flexibility to adjust to
different angle/distance view. |
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| Following Chart indicates rough estimate, viewing depends on the
distance from camera to destination view. |
| Lens
Focal Length | Horizontal
Viewing | Vertical
Viewing |
| 2.5mm |
85 - 95 degree | 65
- 75 degree |
| 2.8mm |
75 - 85 degree | 59
- 65 degree |
| 3.6mm |
59 - 69 degree | 45
- 50 degree |
| 4.0mm |
56 - 60 degree | 43
- 48 degree |
| 4.3mm |
53 - 58 degree | 40
- 45 degree |
| 5.0mm |
40 - 50 degree | 30
- 35 degree |
| 6.0mm |
15 - 25 degree | 15
- 20 degree |
| 8.0mm |
10 - 20 degree | 10
- 15 degree |
| 12.0mm |
6 - 11 degree | 5
-10 degree |
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| Lens
Lighting |
| Most CCTV lenses are equipped with
an iris that consists of four or six opaque metal vanes which are arranged to
give a roughly circular hole or apertaure through which light can pass and then
fall on the camera sensor surface. The vanes can be driven together to alter the
size of the lens aperture to control the amount of light falling on the sensor
and hence alter the picture brightness. |
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| Aperture Ratio |
| the size of
the aperture is defined by an aperture ratio or f- number where: |
| F-number or Aperture Ratio = | Focal
length of Lens | |
| | Diameter
of Aperture | |
| Example: A 6mm
lens at f1.0 has an aperture of 6mm in diameter |
| The
same lens at f1.4 has an aperture of 4.25mm in diameter. |
| The
area has halved and so has the amount of light allowed to pass through the lens.
Most lenses are marked with f-numbers; with each mark equaling one 'stop etc.
halving of the area of the aperture through which light can pass. The standard
f-number series is f1.0, f1.4, f2, f2.8, f4, f5.6, f8, f11, f16, f22 and each
successive mark represents a halving of the aperture area from the one previous. |
| Note: The smaller the f-number, the larger the lens aperture
and so the more light the lens can collect. |
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| Depth of field |
| Depth
of field is range of distances from the camera/lens for which the image obtained
is in sharp focus. A large depth of field will have images of objects from as
little as a 1m from the front of the camera through to images of objects at infinity
all in sharp focus. The depth of field for a camera and lens is greatest at smallest
aperture of the lens and vice-versa. If a lens aperture is set open etc.. at f1.4
this will let most light into the camera but the depth of field will be at its
narrowest. It is important therefore, when focusing a camera and lens to ensure
that the iris setting is at large as, or larger than, it is ever going to be when
in normal use. If this is not done then the camera can be focused and give perfectly
good pictures during the day but when night falls, the iris will open and the
image to be viewed will go out of focus. Focusing of CCTV cameras should be done
either at night nuder worst case lighting conditions or the lens should be fooled
into thinking it is night by placing an optical filter (ND2 or 3) over it to open
the aperture fully and then focus the camera on the scene to be viewed. |
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| Iris Lens |
| Manual Iris Lenses
- Recommend for area where light changes is not consistent |
| The
simplest type of iris control is termed 'Manual Iris'. The lens is equiped with
a ring on the body that can be turned to alter the aperture directly. In CCTV
systems this can only be used generally in fixed lighting conditions or where
the camera is readily accessible and it is not inconvenient to have to continually
adjust the lens for correct picture brightness. |
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| Auto Iris Lenses - Recommend
for area where light changes consistently |
| A far
more common arrangement is to use auto-iris lenses where the iris vanes are driven
by a galvanometer or servo motor which is controlled by an 'iris-amplifier' circuit
within the lens, the control input of which is the video signal from the camer
itself. Hence an auto-iris lense is used where the lighting level is liable to
continuous changes etc. outdoors where lighting can change from full sunlight
down to near total darkness. |
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| As
a scene gets brighter the video level increases. This is sensed by the lens and
the iris is driven to give a samller aperture, letting less light onto the sensor
and so causing the video level to be reduced. Damping is applied to prevent the
lens from hunting, etc. causing the picture to continually change brightness. |
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| The relationship between
acutal video level and corresponding aperture size can be adjusted by the installer
using two control potentiometers on the lens. These pots are usually marked 'Level'
or 'Gain' and 'ALC' or "P-A', abrreviations for average Level Control and
Peak-to-Average respectively. |
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| Direct-Drive Lenses (FCS) |
| These are a development on auto-iris lenses where the motor
or galvanometer coil moving the iris vanes in the lens is driven directly by the
camera, 'Direct-Drive'. There is no active electronics in the lens and hence they
are simpler, smaller and cheaper than an equivalent auto-iris lens. They must
be used with a camera capable of driving 'DD' lenses and this is normally identified
by the presence of the characteristic 4-pin square 'Hi-Rose' (Panasonic) plug
on the side of the camera. |
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| Direct
Drive lenses have no adjustment controls other than focus (and angle of view in
the case of Vari-focal lenses) |
| Picture brightness
is now controlled by a level setting on the camera. DD lenses slightly less flexible
and optimisable for outdoor use. |
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| Setting auto-iris lens |
| The
iris controls of a lens should be adjusted when the lighting levels are at or
near the highest that the camera will be subject to. Never adjust an iris setting
of an outdoor camera when light levels are low. If you do then it will almost
certainly be necessary to re-adjust them in daylight. |
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| Set the P-A pot to fully average. Point the camera at a bright
scene or the brightest part of the scene to be viewed and adjust the level pot
until the bright part of the picture just over-exposes and then adjust the pot
down slightly until the bright areas are not overexposed. If it is required to
make the lens stop down when a highlight enters the picture etc. a door opening,
then adjust the P-A pot towards peak. If the average control is adjusted then
the level control selling should again be checked and adjusted if necessary. |
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| Other Lens Types |
| Vari-focal Lens |
| Often it is not possible to determine the focal length of a lens
required to meet a particular application or perhaps the angle of view required
is not provided by one of the standard lenses available. Narrow range, manually
zoomed lenses have been developed to meet these application needs. These are known
as range, manually zoomed lenses have bee developed to meet these application
nees. These are know as 'Van-focal' lenses. (Variable focal length) and are available
in a number of different configurations. The exact angle of view can be set on
installation but is is important to ensure that the angle required is within the
range available from the lens. |
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| Pin-Hole Lens |
| These
are primarily used for covert surveillance. They have very small front 'objective'
lenses which mean that they can be put behind ver small holes and be virturally
undetectable. Straight and right-angled models are available to facilitate the
mounting of the camera and to reduce the depth requried behind the concealing
surface. Due to teh samll objective lens, pin-hole lenses cannot gather as much
light as conventional lenses and so their use dictates that the scene illumination
is better than would otherwise be required. |
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| Typically the maximum apertures available on pin hole lenses
is F2.5 to F3.5 which is approximately 2 to 3 stops less than standard lenses.
Between 4 and 8 times the normally quoted minimum scene illumination is required
for the camera to product a useable picture. |
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| Zoom Lens |
| For
CCTV applications these are almost exclusively motorised types to enable zooming
and focusing of the lens from remote control positions. Zoom ratios etc. the ratio
of maximum to minimum focal lengths are most commonly either 6:1 or 10:1. The
viewing angles obtained are shown below: |
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| 1/3" Zoom Ratio | Focal
Length Range | Horizontal View Angle (wide) | Horizontal
View Angle |
| 6:1 |
5.7 - 34.2 mm | 46
degree | 8.1 degree |
| 10:1 | 6.0
- 60.0mm | 44 degree |
4.7 degree |
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| Understand Lens Concept |
| BLC
- (Back Light Compensation) The cameras are fitted with a backlight compensator,
this is used to compensate for very bright areas in the picture. In order to counter
this problem switch the BLC switch to ON and the camera will try and adjust to
compensate for this problem. |
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| AGC
- (Auto Gain Control) Can be switched off by selecting off on the AGC switch.
This controls an amplifier that is used to boost the video signal; this may also
amplify any noise and may result in a poor picture quality. |
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| AES - (Automatic Electronic
Shutter) is used when a manual or fixed iris lens is fitted this will allow the
camera to adjust to varying light levels. The AES should be switched off when
an Auto Iris Lens is fitted. |
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| Video
Drive Lens - If a Video Drive Lens is fitted connect the Iris lead
to the four-pin conncetor on the camera and set the auto iris selection switch
to DC Drive the level may be adjusted by turning the level adjust. |
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| Level
Adjust - The level adjust is used when a direct drive lens is fitted,
the level adjust is used to set the size of the aperture for normal conditions.
Turn the level adjust so that the picture appears very dark then turn the level
adjust the opposite way until the picture is just to bright, then turn the level
adjust back untile the desired picture brightness is achieved. |
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