Camera Guide.pmd
Guide to Cameras and Lenses
PRODUCT SAFETY
WARNING
• Installation and servicing of cameras is only to be carried out by suitably qualified and experienced
personnel.
• Mains cameras contain hazardous voltages
• Do not remove camera covers as there is a risk of injury or death by electric shock.
• Cameras connected to mains supplies must be earthed.
• Only power low voltage cameras from a class 2 isolated power supply.
Guide to Cameras and Lenses
Contents
Introduction .............................................................................................................................................................. 4
Colour Balance ......................................................................................................................................................... 5
Gamma .................................................................................................................................................................... 6
Automatic Gain Control (AGC) ................................................................................................................................. 6
Electronic Iris (EI) ..................................................................................................................................................... 6
Synchronising Cameras ........................................................................................................................................... 7
Linelock phase adjust (LL-PH) ................................................................................................................................. 8
Backlight Compensation (BLC) ................................................................................................................................ 8
Flickerless ................................................................................................................................................................ 9
Shutter Speeds ......................................................................................................................................................... 9
Sensitivity ............................................................................................................................................................... 10
Resolution ............................................................................................................................................................. 10
Peak White Inversion ............................................................................................................................................. 10
Common Lens terminology ..................................................................................................................................... 11
Choosing a lens ...................................................................................................................................................... 12
Focal Length .......................................................................................................................................................... 12
Fixed Focal Length ........................................................................................................................................... 12
Variable Focal Length ....................................................................................................................................... 13
Lens Formats ......................................................................................................................................................... 13
Lens Mounts ........................................................................................................................................................... 13
Lens Iris .................................................................................................................................................................. 14
Fixed Iris ........................................................................................................................................................... 14
Manual Iris ......................................................................................................................................................... 14
Automatic Iris .................................................................................................................................................... 14
Peak / Average Adjustment for Automatic Iris Lenses ................................................................................. 15
Direct Drive DD ................................................................................................................................................. 15
Fitting a Direct Drive Lens Connector .......................................................................................................... 15
DD Lens Level Adjustment for Fixed and Zoom Lenses .............................................................................. 16
Depth of Field ......................................................................................................................................................... 16
Adjusting the lens back focus ................................................................................................................................. 17
Camera Mounting ................................................................................................................................................... 18
Cables .................................................................................................................................................................... 18
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Guide to Cameras and Lenses
INTRODUCTION
This short guide is aimed at trying to assist installers and engineers to correctly set up and understand the many
advanced features of Baxall cameras.
Almost all of Baxall’s cameras have similar features and functions whether they’re a monochrome or colour version.
Usually, the features are controlled using dip switches that are located under the hinged side door of the camera. A
label on the camera shows the settings for each function.
Genlock
Connector
Video
Connector
Function
switches
Mounting bush
Back focus
adjustment
Line-Lock
Phase Adjust
GENLOCK
VIDEO OUT
LENS
+
V
LLPH
MADE IN EU
AI Lens
Connector
CD9252
230V NOM.
50/60Hz
A.C.
POWER
Power LED
DD Lens level
adjustment
DD lens
connector
Typical Camera Layout
Some cameras in the Baxall range (OSD Series), are set up remotely by means of an on-screen menu system. All the
functions of these cameras can be changed or set via the video coaxial connection.
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Guide to Cameras and Lenses
COLOUR BALANCE
As the name implies, this feature is common to colour cameras only. Cameras that have the colour balance feature
generally have four modes of use; Indoor, Outdoor, Fluorescent and Auto.
The white balance feature compensates for the temperature colour ‘casts’ that different light sources can cause.
Colour casts can make white appear with a slight hue under different light sources (e.g. tungsten and fluorescent). To
see the effects caused by different lighting conditions, point a camera set to auto colour mode out of a window. Allow
the camera ten seconds or so to balance to the outside lighting, then point the camera indoors at a room scene lit with
artificial lighting. Any white areas in the scene will show a definite colour tint. After a few seconds you will see the
camera compensate and the white areas will be rendered correctly.
The camera cannot do this unless the colour mode is set to auto colour balance. It will only correctly reproduce white
for the specific lighting type it is set for. Baxall set the three fixed colour modes on their cameras to compensate for
indoor (tungsten), outdoor (daylight) and fluorescent lighting types.
Because no single lighting type has a fixed colour temperature, accurate rendering of white cannot be guaranteed.
Colour compensation should only be used if the scene being viewed contains a number of different lighting types and
this causes the auto white balance circuit to ‘hunt’ as it tries to balance itself. For cameras fitted with this feature,
Baxall recommends that it is always set to Auto. Figure 1 below shows typical colour casts for everyday lighting
Fig. 1. Typical colour casts
Auto White Balance on
Outdoor (natural) lighting
Fluorescent lighting
Tungsten lighting
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Guide to Cameras and Lenses
spectra.
GAMMA
The reproduction characteristics of most cameras and video displays are not linear. For example, imagine a scene lit
with low ambient lighting, a small change in the light level falling on the CCD will produce a given output signal.
However, the same change in light level within a scene lit with strong ambient lighting will not produce the same
magnitude of output signal. This non-linearity is known as gamma and it usually results in poor discrimination of grey
areas in the scene at low light levels.
To compensate for this and improve linearity of the system, Baxall cameras have a built-in gamma correction circuit
which normally has two settings. Usually specified as Normal (0.45) and Linear (1.0) (but sometimes High and Low
respectively), you can choose the optimum setting for the scene being viewed.
The Normal setting has the effect of stretching the camera’s response to the black and mid-grey components of the
scene whilst compressing the white copmonents. This makes it easier to see differences in those shades of grey that
are close to each other and is therefore useful when more visibility is required in darker areas of the scene. However,
the white compression can make it more difficult to differentiate shades of white in lighter areas of the scene. Compare
the grey bars in the two images (figure 2) and notice how the Normal setting seems to ‘reveal’ more of them.
Fig. 2
Normal setting
Linear setting
AUTOMATIC GAIN CONTROL (AGC)
Baxall cameras contain a video amplifier which applies gain to the video signal as required (up to a maximum of
28dB). The circuit is designed to compensate for fluctuations in scene illumination which would cause the video
output level to be too low. If the video level is adequate, the circuit will not apply any gain to the signal. As the video
level drops (e.g. the scene illumination level falls), more and more gain is applied by the AGC circuit to the video
signal. The camera only applies as much gain as is necessary to bring the video signal up to a reasonable level
(typically 1V peak to peak).
It should be understood that the AGC circuit cannot work miracles and some light must be present within the scene.
Note that as a consequence of amplifying a poor signal, the noise present in the signal is also amplified. Therefore a
poorly lit scene with a lot of gain applied to it will appear noisy or grainy. This is usually accepted in deference to the
alternative of having no picture at all. Obviously, the ideal solution is to provide adequate illumination for the scene
wherever possible.
It is recommended that the AGC feature is left permanently switched on since it will have no effect as long as the
scene illumination is adequate. When setting lens levels, switch the AGC off. This way you can be sure that the
picture you are seeing is not due to the effects of the AGC circuit. After the lens level has been set up, switch the AGC
back on.
ELECTRONIC IRIS (EI)
In contrast to the Automatic Gain Control, the Electronic Iris (EI) feature is used to compensate for increases in the
video level. Consider a camera fitted with a manual iris lens that is being used for low light surveillance. The iris has
to be left fully open so the camera’s performance is not degraded. However, in bright daylight conditions, the video
output level will be excessive and the resulting video display will be overexposed (white-out). The electronic iris can
overcome this problem. The electronic iris feature continuously varies the shutter speed between 1/50s and
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Guide to Cameras and Lenses
1/100,000s (1/60 and 1/100,000s for NTSC cameras) according to the light level. The ‘shutter’ in a CCTV camera
actually regulates the amount of time that light is allowed to fall onto the imaging CCD element. Shorter duration
shutter speeds allow the CCD less time to gather light (or charge) thus preventing overexposure. The EI circuit is
designed to ensure the camera’s video output level is maintained at the optimum value of 1V peak to peak.
Once again, the electronic iris feature has its limits. If too much light falls onto the CCD, it can result in the phenomena
of ‘smearing’. Usually the result of bright point sources of light such as car headlights, smearing appears as a bright
band above and below the highlight. The onset of smearing has reduced as CCD technology has improved, but it can
still occur when the CCD is overexposed.
SYNCHRONISING CAMERAS
There are many surveillance applications where more than one camera is used—to cover a large site for example.
Normally, some means of switching between cameras is employed. If the cameras are not synchronised correctly, the
image on the monitor will roll as the video switcher is used to select different cameras. This is because the video
output from each camera is essentially free running and frames arrive at the video switcher at different times. Baxall
cameras provide several features to ensure synchronisation. The first of these is Line Locking and is selected by a
switch on the side of the camera. When the camera is line-locked, it is synchronised using a fixed point on the A.C.
supply’s cycle—usually the point that the A.C. cycle passes through zero (figure 3). Setting all of the cameras to line
Fig.
lock 3mode will ensure that they all output video frames at the same time relative to one another. Obviously, to utilise
this feature, cameras must be operating from an A.C. supply and be on the same mains supply phase.
NOTE: Cameras on different mains phases can still be synchronised (see LL-Phase Adjust below).
A camera set to Line Lock is synchronised
to the negative going zero volt crossing point
A.C. Cycle
The other way to achieve synchronisation is to use Genlocking. Genlocking requires an externally generated
synchronisation signal which is sent to each camera via a separate coaxial connection. Many Baxall cameras are
fitted with a Genlock BNC to allow the connection of such a device. The generator produces an accurately clocked set
of video timing signals.
Fig.
4
Although
genlocking is the best way to synchronise cameras, it has obvious disadvantages such as the need for an
extra cable for each camera which can add significantly to the installation costs. In addition, the video generator will
need as many outputs as there are cameras in the system. Figure 4 shows a typical genlock setup.
Sync In
Video Out
Sync Out
Sync In
Video Out
Synchronising Generator
Sync In
Video Out
Baxall cameras also have an internal oscillator which is selected using the Internal mode switch. This feature can be
especially useful where the camera is designed to run off a D.C. supply or if a camera designed to operate at 50Hz
(e.g. PAL standard), is installed in a country where the mains supply frequency is at 60Hz. Ordinarily the camera
would not be able to use the Line Lock mode. Setting it to synchronise to its internal oscillator will overcome this
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Guide to Cameras and Lenses
problem. A side effect of such a setup however may be that the lighting in the scene seems to flicker. This is caused by
the discrepancy in the frequency of the camera (50Hz) and the frequency at which the lights are running (60Hz)
resulting in a mild strobing effect.
LINELOCK PHASE ADJUST (LL-PH)
Cameras can often be supplied from different A.C. phases especially when they are physically remote from one
another or because of the way the site’s A.C. supply is distributed. However, as we have already mentioned, it is still
possible to line-lock these cameras together. A dip switch on the side of the camera allows you to select between
Fixed or Adjustable. Either setting provides line-locking, however the Adjustable setting permits the point of
synchronisation
on the A.C. supply cycle to be shifted. A potentiometer, (usually located on the rear panel of the
Fig. 5
camera), provides up to 120O of adjustment either side of the zero volt point. Since supply phases are 120O out of
phase with one another, this control allows enough adjustment to enable accurate camera synchronisation (figure 5).
Adjustment range
O
120
+120O
-120O
Trigger point
3-phase A.C. Cycles are 120O apart
BACKLIGHT COMPENSATION (BLC)
Backlight compensation is a feature that is often misunderstood or applied incorrectly. Situations frequently arise
where the brightest light in the scene is coming from behind the subject of interest. Imagine a camera monitoring a
doorway. In this example, the light outside the door is much brighter than the ambient light in the room where the
camera is located. The camera’s exposure system sets itself according to the average light level in the scene.
However as someone opens the door the exposure system reacts to the increased light level and as a result, anybody
entering the room is seen in silhouette. The backlight compensation feature can help to overcome this problem and
figures 6 and 7 serve to illustrate BLC control.
Fig. 6 Backlight Compensation off
Fig. 7 Backlight Compensation on
Normally, the exposure circuit within the camera takes an average reading from the illumination present in the entire
scene and uses this to adjust the electronic iris (or the lens iris in the case of a motorised lens). Ideally, the camera
would calculate the exposure based on the light level in the part of the scene that is of interest to the viewer. The
backlight compensation feature uses a ‘window’ to set the exposure. Everything outside the window is ignored by the
exposure system.
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Guide to Cameras and Lenses
Indicates area used to calculate the exposure
Fig. 8 Fixed BLC window
Fig. 9 User-selectable BLC windows
On most cameras, the exposure window is fixed to the central portion of the scene (figure 8). Some baxall cameras
allow you to choose several different windows (X series cameras); and some cameras allow you to completely control
the size/position of the window and the amount of exposure compensation applied to the selected area (OSD series
cameras). Figure 7 shows the range of selectable BLC windows available on Baxall X series cameras.
The window at the top right in figure 9 would be ideal where the surveillance target is a street scene. The bright sky
area would normally cause the camera to bias the exposure and the street would appear dark in comparison. Setting
a window that forces the camera to ignore the sky will ensure that detail in the street is retained. Note that because the
camera is calculating its exposure based on the lighting in a darker area of the scene, the lighter areas will overexpose
and, in some cases, ‘white out’ completely. This effect can be seen in the background of figure 7.
FLICKERLESS
In some lighting conditions, particularly fluorescent, the image can be seen to flicker. This is usually caused by the
interaction of the shutter with the A.C. frequency of the lighting. The Flickerless setting changes the shutter speed of
the camera to a value that will not cause flicker (1/120s for PAL system cameras; 1/100s for NTSC). The disadvantage
to fixing the shutter speed in this way is that the sensitivity of the camera will be reduced. This is because the
electronic iris feature has effectively been turned off and it will no longer control the optimum exposure setting for the
available light conditions.
SHUTTER SPEEDS
The electronic shutter available on Baxall cameras is analogous to the shutter in a conventional 35 mm camera. The
shutter speed is usually selected using a bank of dip switches located on the side of the camera. A faster (i.e. briefer)
shutter speed can arrest the motion of a fast moving object rendering it sharp. Fast shutter speeds allow less light to
fall on the CCD and can darken the image. If fast shutter speeds are required, ensure that there is adequate lighting.
Selecting a shutter speed manually will override features such as the electronic iris and flickerless settings.
Fig. 10
Slow shutter speed
Fast shutter speed
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Guide to Cameras and Lenses
SENSITIVITY
The sensitivity of a CCD camera, broadly speaking, is a measure of its performance in low light conditions. Baxall
quote the sensitivity levels of their cameras as the minimum scene illumination required at a given lens aperture to
provide a useable video output. A typical figure may be 0.1 lux @ f1.2. This means that at a lens aperture setting of
f1.2, the minimum scene illumination would need to be 0.1 lux (approximately equivalent to the full moon) in order to
provide a useable video output from the camera. Some manufacturers quote sensitivity figures that require expensive,
‘faster’ lenses (e.g. f1 or better) in order to achieve them. The table below shows some typical lux levels that can be
expected under common lighting conditions.
LIGHTING
Unobstructed sun
Sun with light cloud
Sun with heavy cloud
Home/office lighting
Sunrise/sunset
Street lighting
Twilight
Full moon
Quarter moon
Overcast moon
Clear night sky
Average starlight
Overcast night sky
LUX LEVEL
100,000
70,000
20,000
100 - 1000
500
1 - 10
4
0.2
0.02
0.007
0.001
0.0007
0.00005
RESOLUTION
Resolution is the ability of a camera to discriminate fine detail in a scene. The resolution of a camera is usually
expressed in terms of horizontal TV Lines (TVL). Cameras specifications quote resolution based on the number of
horizontal elements that can be captured by the camera and, confusingly, this relates directly to the number of
vertical lines that can be discerned for an equivalent picture height. Typical video display devices have the proportions
of 4:3, therefore equivalent picture height means that only ¾ of the horizontal line is used to quote resolution. This is
done to preserve the natural proportions of the image.
In effect, a camera with a resolution of 570 TVL being is able to display ¾ of a single horizontal line as 570 individual
segments. The higher the number of segments used to display the line, the more fine detail can be resolved in the
image. Obviously the number of horizontal lines displayed vertically (i.e. the vertical resolution) in a given system is
fixed according to the CCTV standard in use (approximately 400 lines for PAL/CCIR; 330 lines for NTSC; etc.).
Cameras are often described as medium and high resolution. This typically equates to medium resolution of 330 TVL
and a high resolution of 480 TVL for colour cameras; and a medium resolution of 380 TVL and a high resolution of 570
TVL for monochrome cameras. If it is important to be able to resolve fine detail in an application, choose a camera
with an appropriate resolution.
PEAK WHITE INVERSION
Some surveillance applications naturally contain many point sources of light — one such being highway monitoring
during night-time where oncoming vehicles have their headlights on. This can cause the camera’s electronic iris, or
the iris of an automatic lens, such as a Direct Drive or Video Drive type, to react and close thereby losing detail in the
darker areas of the scene.
The Peak White Inversion feature found on Baxall OSD and CDX series cameras, can resolve this problem. It allows
the user to render selected areas of the scene above a certain predefined threshold level as black. This stops the
camera or lens reacting to the ‘peak’ white areas which would normally cause it to incorrectly control the iris mechanism
thus preventing underexposure of the scene. Figure 11 shows a typical application of the Peak White Inversion feature.
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Guide to Cameras and Lenses
Fig. 11
Peak White Inversion off
Peak White Inversion on
COMMON LENS TERMINOLOGY
Aperture - The ‘opening’ of a lens indicating the measure of its light gathering capability or performance. Relative
Aperture is a ratio between its focal length and effective aperture and is measured in f numbers e.g. f1.4, f1.3, etc.
Generally, the lower the f number, the more light gathering capability the lens has.
Aspherical Lens - A lens designed with special curvature so that the image distortion inherent at the edges of a
conventional lens are lessened.
Auto Iris (Al) - An electronic circuit controlling the iris of a lens to help compensate for large changes in illumination
levels.
Back Focus - The mechanical aligning of the imaging device with the focal point of the lens. Correct back focus
setup is particularly important on zoom lenses to ensure the image stays in focus throughout the zoom range of the
lens.
Depth of Field - The zones in front of and behind the principal focus point that will remain in focus at a given setting.
Direct Drive (DD) - A lens that takes a reference DC voltage from the camera to open or close the iris aperture. The
video level control is part of the camera.
Electronic Iris (El) - This is a system that uses the camera’s electronic shutter to control how much of the light falling
on the CCD sensor is used to produce a picture. The system allows manual or fixed iris lenses to be used in a wider
range of applications.
Field or Angle of View - The part of the scene visible with a particular lens. Generally, shorter focal length lenses will
have a wider field of view than those with longer focal lengths.
Focal Length - The distance in millimetres, between the lens’s secondary principal point and its focal point. The
higher the number the greater the magnification and the narrower the field of view.
Focal Point - The point on the axis of a lens at which rays of light entering the lens will converge.
Iris (iris diaphragm) - Mechanically adjustable leaves or plates that regulate the amount of light passing through a
lens.
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Guide to Cameras and Lenses
CHOOSING A LENS
Few things can the impair the performance of a CCTV camera more than an incorrect or poorly chosen lens. Lens
choice depends upon several factors such as the physical position of the camera, the available scene illumination
and the type of view of the scene that is required.
The easiest way to select the correct lens for a given application is to use a viewfinder. This is an optical tool along the
lines of a simple telescope that will give the user an exact representation of the scene that can be expected with a given
lens/camera combination.
If a viewfinder is not available (or practical), a lens calculator may be used. This is often a circular device with rotating
scales that represent the field of view. Usage varies from manufacturer to manufacturer but usually the desired
horizontal and vertical scene dimensions are entered and the necessary focal length of the lens required to achieve
this can be read directly off the calculator.
If neither of these aids is available, the only alternative is to measure the distances and angles that are required and
use the specification sheets and tables provided by the lens manufacturers.
FOCAL LENGTH
The focal length of a lens determines its field of view at a given distance. A wide angle lens as its name suggests has
a wide field of view at a given distance. This means that it can ‘see’ a wide area of the scene in both the horizontal and
vertical planes. Because of this, objects in the scene will appear far away and show little detail. The opposite is true
of a telephoto lens. Most lens manufacturers provide tables or charts for their lenses providing the necessary data.
Broadly speaking, the focal length of a lens falls into two categories: fixed or variable.
Fig. 12
Field of view with an 8mm lens
Field of view with a 25mm lens
Fixed Focal Length
A lens having a fixed focal length is often the least expensive. Since the focal length is fixed, so is the field or angle
of view. This means that accurate calculations will have to be performed in order to correctly select a lens for a given
application. A change in the requirements of the application will often result in a change of lens.
Fig. 13
Typical field of view
with 8mm fixed lens
47.4O
12
Typical field of view
with 25mm fixed lens
15.7O
Guide to Cameras and Lenses
Variable Focal Length
Although more expensive, these lenses are easier to use, set up and change. It is much simpler to obtain the correct
view of a scene when it is possible to vary the focal length (and therefore the angle of view) of the lens. Variable focal
length lenses should not be confused with zoom lenses which have a much larger adjustment range.
Zoom Lenses
Zoom lenses are the next step up from variable focal length lenses and offer the greatest functionality. They can be
continuously adjusted throughout their range, usually remotely, to vary the focal length and field of view (figure 14).
Note that because the depth of field is also dependent upon the focal length of the lens, it will continuously vary
throughout the zoom range being at its greatest when the lens is zoomed fully out (wide angle). Remotely controlled
zoom lenses are often used by the operator to closely examine critical areas of the scene.
Fig. 14
Typical field of view with
a 6 to 36 mm zoom lens
33O
5.9O
LENS FORMATS
The format of a lens, often quoted as 1”, 1/2”, 1/3”, is derived from the ratio of the diameter of the lens to the image
size produced. The usual practice is to match the lens format to the CCD sensor size but it is possible to use larger
format lenses on cameras with smaller CCD sensors (e.g. a 1/2” format lens on a camera with a 1/3” CCD). The rule
when choosing which format to use is that the image size produced by the lens must always match or be larger than
the CCD sensor. In practice, this means that a 1/3” format lens would not be compatible with a camera fitted with a 1/
2” CCD. The image projected by such a lens would not entirely cover the surface of the CCD and the corners would
be truncated (vignetting).
Larger format lenses can offer advantages such as greater depth of field and the image produced by such a lens will
have less distortion at the edges than one with a smaller format.
LENS MOUNTS
Historically, the larger format cameras such as 1” and 2/3” have used the C-mount type lens system to
physically couple the lens to the camera. With the advent of smaller CCDs such as 1/2” and 1/3”, the CCTV industry
has adopted the CS-mount. The main dimensional difference in the two systems is the distance that the back (or
flange) of the lens protrudes (figure 15). The unique back-focussing mechanism on Baxall cameras allows both types
of lens mount to be used—see adjusting the lens back focus. This is because the CCD assembly can be physically
moved backwards and forwards in relation to the back of the lens. If this were not the case, the flange of a C-mount
lens would mechanically interfere with the CCD causing damage. CS-mount lenses are often less expensive and, in
general terms, for a given focal length, a CS-mount lens is physically smaller than an equivalent C-mount lens.
Fig. 15
12mm max.
7mm max.
C-mount lens
CS-mount lens
NOTE: A CS-mount lens will not work on a camera designed to be used only with C-mount lenses
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Guide to Cameras and Lenses
LENS IRIS
The amount of light that falls on the surface of the CCD sensor needs to be within certain limits for optimum performance.
Too much light and the image is overexposed or washed out. Too little, and the resulting image is dark losing detail in
the shadow areas of the scene. The lens iris is used to control the amount of light falling on the sensor. The iris
consists of a number of thin metal plates arranged in such a way that they produce a circular opening at their centre
(figure 16). This opening, called the iris or aperture, can be made smaller or larger usually in fixed increments called
f-stops. In addition to controlling the amount of light entering the lens, the iris has a secondary function in controlling
the depth of field—see Depth of Field below. Lenses can incorporate fixed, manually adjustable or automatic irises.
Lens Iris
Fig. 16
Iris open
Iris closed
Fixed Iris
Fixed iris lenses cannot be adjusted for different lighting conditions. These lenses are most suited to indoor conditions
where the lighting level will remain constant. However, the Electronic Iris and Automatic Gain Control features of
Baxall cameras can make this lens much more flexible in use.
Manual Iris
The iris on a manual iris lens is usually set up when the camera is installed to suit the prevailing lighting conditions.
These lenses cannot react to changes in scene illumination and are best suited to indoor applications where the
ambient light will remain constant. The Electronic Iris and Automatic Gain Control features of Baxall cameras can
allow this type of lens to be used in a wider range of application areas.
Automatic Iris
For external conditions, and where the scene illumination is constantly changing, a lens with some sort of automatically
adjustable iris is preferred. The iris aperture is controlled by the camera and is constantly changed to maintain the
optimum light level to the CCD. Automatic iris lenses usually conform to one of several types: AI (Automatic Iris), DD
(Direct Drive) and Galvanic drive.
AI
This type of automatic iris lens is usually self-contained with the image analysing circuitry built into the lens itself. A
voltage to operate the lens is usually taken from the camera and is supplied to the lens by means of a plug or other
type of connector. A video reference signal is sent to the lens from the camera and the lens attempts to maintain this
at a 1V pk-pk voltage level by opening or closing the iris as necessary (figure 17). For example, if the light level in the
scene is low, the camera’s video reference signal will also be low. The lens analyses this and a small motor opens the
iris sufficiently to re-establish the optimum 1V peak to peak video output signal.
Fig. 17
1V
ALC LEVEL
POWER
Iris ‘closed’
14
1V
ALC LEVEL
POWER
COMMON
COMMON
VIDEO
REF.
VIDEO
REF.
Iris opens
Guide to Cameras and Lenses
PEAK / AVERAGE Adjustment for Automatic Iris Lenses
AI type lenses are usually fitted with two potentiometers Level and ALC for adjusting the response and type of
operation. The Level control sets the iris opening based on the average scene illumination. The level control should
be adjusted to take into account both daytime and night-time operation i.e. if it is set incorrectly, the image may be
satisfactory during the day but too dark at night. The ALC potentiometer controls the degree to which the Auto Iris
responds to bright areas in the picture, e.g. windows, street lamps, car headlights, reflections, etc. The ALC potentiometer
usually has two settings Peak and Average located at the anticlockwise and clockwise ends of its travel respectively.
Fixed focal length lenses
If the ALC control is set at or towards Peak, the lens will respond more to any bright highlights. This will have the effect
of reducing the overall contrast in the scene but allowing more detail to be seen in the bright areas. If the ALC control
is set at or towards Average, the lens will respond less to highlights. This will produce over-brightness in the highlights
and may cause flare but will improve the contrast in the darker areas of the scene. For most CCTV applications the
ALC control should be fully set to Average before attempting to set the level control.
Often, the ALC control is set at Peak during installation and, since there are no peaks in the picture, the control seems
to have no effect. However when conditions change and peaks are introduced, the overall contrast is reduced and a
site visit has to be made. Therefore it is best to ensure that it is set fully to Average unless in one of the rare occasions
it is necessary to see detail in a bright area of the scene.
Zoom Lenses
On most zoom lenses the Peak/Average control functions as little more than a speed control for the iris motor and is
best left in the centre of its travel to ensure consistent operation.
DIRECT DRIVE DD
Direct drive type lenses (sometimes referred to as DC-drive) use circuitry within the camera to provide the necessary
iris control signals namely a drive signal and a damping signal. The lens contains no signal analysing electronics and
is therefore directly driven by the camera. The drive signal controls the iris and the damping signal is used to prevent
‘hunting’—a condition where the iris reacts too quickly to changes in the scene illumination. The camera has to be
capable of supplying these signals via the correct connector, and is a common feature of all Baxall cameras. A
potentiometer on the side of the camera is used to adjust/set the drive level for the lens.
Fitting a Direct Drive Lens Connector
A Direct Drive lens may or may not be fitted with the correct connector. If it is not, use the connector supplied with the
camera and connect it as follows:
1. If necessary, remove the old lens plug from the cable.
2. Remove the plug cover 1, add heat shrink tubing to the wires and solder the lens cable wires 2 to the pins A
through D on the supplied connector. Apply heat to the heat shrink tubing.
3. Replace the connector cover.
Fig. 18
Pin No.
1
Cover.
DD-Lens
2
Lens cable.
Damping (-)
3
Rib (if the cable is too thick
and the connector cover
does not seat properly, the rib
can be cut off).
4
Lens connector.
Drive (+)
Drive (+)
Damping (+)
15
Guide to Cameras and Lenses
DD Lens Level Adjustment for Fixed and Zoom Lenses
The only correct method of setting the lens level control is to monitor the camera output on an oscilloscope and adjust
the level control for a reading of 0.3 volt sync. and 0.7 volt video = 1 volt peak to peak total (figure 19).
Fig. 19
0.7V
VIDEO OUT
0.3V
When an oscilloscope is not available the following method can give some surprisingly accurate results with a little
experience. You should have an installation/test monitor, and use a camera known to be set correctly to 1V peak to
peak. Connect the camera to the monitor and adjust the contrast and brightness until YOU think the picture is correct.
Mark the controls so you can always set them to this position again. As you set up each camera, adjust the ‘level’
control until you see that the contrast of the picture is similar to that achieved with the test camera. You should then
find all the cameras set to equivalent contrast.
NOTE: Never adjust the monitor controls no matter how tempted.
DEPTH OF FIELD
The focus ring on a lens is usually adjusted so that the object of interest within the scene is sharp. Up to a certain
point, objects in front of this setting, and behind it, are also in focus. This zone of focus is referred to as the Depth of
Field. As objects get further outside of the depth of field (either further from the lens or closer to it), they will lose
focus. The depth of field can be controlled by the iris setting on the camera. As the iris aperture is decreased in size,
the depth of field will be greater—that is, more objects either side of the focus point will be in focus.
Figure 20 shows a camera looking along a line of people. The lens is focussed on a point mid way along the line
represented by the ceiling light. If the iris is wide open, only a few of the people on either side of the focus point will
be in focus. If the iris is closed towards its minimum, more people in the line will be in focus in other words, the depth
of field will be greater.
Point of focus
Point of focus
Fig. 20
Depth of Field
LENS
LENS
IRIS SETTING
IRIS SETTING
Depth of Field
One disadvantage of increasing the depth of field by closing the iris, is the amount of light admitted to the camera will
be reduced and the image will become darker. Depth of field is also dependent upon the focal length of the lens. Wide
angle lenses (i.e. those with a small focal length) will have a greater depth of field than telephoto types. The depth of
field is inversely proportional to the focal length of the lens so as focal length increases, the depth of field will
decrease.
Automatic iris lenses, because of their very nature, will cause the depth of field to vary. Consider an auto iris lens
fitted to a camera that is used in a day/night role. During the day when the ambient light is at its strongest, the iris will
16
Guide to Cameras and Lenses
be narrow. This will represent a good depth of field. As night approaches and the iris opens to compensate for the
reduced overall light level, the depth of field will become more shallow. This phenomena should be taken into
consideration where the depth of field is an important factor in the performance of the system.
When focussing the lens on a camera, ensure that the lens iris is fully open. If the iris is closed when the lens is
focussed, the increased depth of field may give a false impression that the lens is correctly focussed when in fact it
is not. This would be seen when the lens iris opened up and focus was lost.
ADJUSTING THE LENS BACK FOCUS
Baxall cameras contain a mechanism for adjusting the position of the CCD assembly in relation to the back of the lens
(figure 21). The back focus adjustment screws are located on the top and side of the case and should be adjusted
using an appropriate screwdriver. If possible, always use the top screw to adjust the back focus mechanism.
Turn the adjuster screw clockwise or anticlockwise to obtain focus. When the focus is sharp, turn the back focus
adjustment screw 2 or 3 turns anticlockwise. The picture will lose sharpness. Turn the back focus screw clockwise
until focus is once again obtained. If you have turned the back focus screw clockwise past the point of best focus,
repeat the procedure. The last turn of the back focus adjustment screw must always be in a clockwise direction.
This will ensure that any mechanical lash in the system is taken up. Do not use force or turn the back focus mechanism
adjustment further than it is intended to go.
Fig. 21
CCD Assembly
Back focus
Adjustment Range
NOTE: It is important that the lens iris is fully open before the back focus is set. This is because the depth of field will
be at its minimum and won’t be tending to ‘focus’ the image.
Fixed iris lenses
Set the lens focus to infinity and view an image greater than two metres away. Focus the image using the back focus
screw. Set the lens focus as required.
Manual iris lenses
Open the iris fully and set the lens focus to infinity. View an image greater than two metres away. Focus the image
using the back focus screw. Set the lens focus and iris as required.
Automatic Iris and Direct Drive Lenses
Fully open the iris by covering the lens with a suitable neutral density (ND) filter. Set the lens focus to infinity. View an
image greater than two metres away. Focus the image using the back focus screw. Remove the ND filter and set the
lens focus as required.
Zoom Lenses
Set the lens focus to infinity and fully open the iris by covering the lens with a suitable neutral density (ND) filter. Zoom
out to the widest field of vision and view a distant object. Adjust the back focus screw until the object is in focus. Next,
zoom fully in and adjust the lenses focus until the object is again focused. Repeat these steps until the full zoom range
may be viewed with the minimum loss of focus.
17
Guide to Cameras and Lenses
CAMERA MOUNTING
All Baxall cameras are fitted with mounting points on the top and bottom of the case (figure 22). They are designed to
accept standard photographic mounting bolts (1/4” BSW or 20 UNC). The mounting bracket and its fixing, must be
capable of supporting the weight of the camera and its lens. In cases where the lens is substantially heavier than the
camera, it is better to use the mounting bush on the lens itself.
NOTE: In some countries, notably the U.S., installation codes dictate that the mounting bracket and its fixing must be
capable of supporting four times the weight of the camera and lens.
Fig. 22
Top Mounting
Bottom Mounting
CABLES
Always use the highest quality coaxial cable possible from a reputable manufacturer such as Belden. Poor quality
cable can result in a noisy picture with interference and cross-talk. As a rule of thumb, typical coaxial cable runs of
250 metres for RG59, and 500 metres for RG11 will give good quality clear pictures. Acceptable pictures can be
obtained on cables runs of twice these distances but cannot be guaranteed.
If exceptionally long cable runs are required, a video amplifier will be required at one or both ends of the cable.
Electromagnetic interference (EMI) can be induced into coaxial cables running in close proximity to high voltage or
high current carrying cables. This will cause hum bars in the picture and degrade image quality. Because of this, the
installation of video cables next to high voltage cables is not recommended.
Cable Type
Max Recommended Length
Typical Nominal DC
Ω /1000ft)
Resistance (Ω
Feet
Metres
RG59
820
250
10.5
RG11
1,640
500
1.24
Table of Maximum Recommended Cable Lengths
Application
Distance
Indoor
Outdoor
250m
500m
Surface installation
Trunking, Conduit
RG59
RG11 CT125
PTZ Head
Catenary or flexible
URM70
—
—
Ducting
URM70
RG11, CT125
—
Directly buried
—
CT125RBS
Table of Recommended Cables for Common Applications
18
Guide to Cameras and Lenses
A CCTV system has to be capable of handling signals that can be at a nominal 5MHz and above in frequency. The
coaxial cables in common use exhibit resistive characteristics at such frequencies. The effect of this in real life is to
attenuate or reduce the video signal slightly. This attenuation increases with cable length and is commonly measured
in decibels (dB). As a rule of thumb, 2dB loss will leave approx. 80% of the signal unaffected.
Cable
Loss in dB/100m @ 5MHz
URM70
2.3dB
RG59
2.2dB
RG11
1.2dB
CT125RBS
1.1dB
Table Showing Typical Cable Losses
19
Baxall Limited, Stockport, England. Visit our Web site: http://www.baxall.com
Baxall Limited reserve the right to make changes to the product and
specification of the product without prior notice to the customer.
HB-CAMGUIDE-2
Issue 2 03/01
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