Security cameras are the most important part of your security
system. After all, they are what actually does the "seeing" and if
they do it poorly, the most expensive viewing and recording hardware and software
will not be able to compensate for poor security camera system design.
Security cameras are complex, precision instruments with many different settings
and features. They can appear intimidating. A knowledgeable security solutions
provider can go a long way toward helping you design an appropriate CCTV system
for your situation. But how do you know that the video surveillance provider
that you’ve chosen is working in your best interest? By being as informed
as possible.
The intent of this white paper isn’t to make you an expert on security
cameras or CCTV system installation. What it will do, is help you to become
an informed consumer. Understanding the basics of a CCTV video surveillance
system will allow you to ask the right questions to insure you are getting a
video security system that will work for you.
In this white paper, we’ll look at the camera body and the
lens separately. We will describe the various aspects of each and discuss how
the attributes of the one work with and/or limit the functionality of the other.
II - Technical Features
The first thing you need to know is that when most people speak of a "security
camera" they are talking about the camera and the lens. These are two very
distinct, (usually) interchangeable parts of a "security camera" and
each bring different features to the table. Proper CCTV system design involves
selecting both the appropriate camera and the appropriate lens for each situation.
A. The Security Camera Body
1. Camera Type
Camera type refers to the physical configuration of the security camera. Obviously
size and weight are properties that fall under this category, but those things
aside, security camera bodies come in two common types, standard and pan / tilt
/ zoom (or PTZ) styles, as well as in many specialty configurations.
Standard Format
Standard format cameras are what most people think of when they think of a security
camera. They are more or less shaped like a rectangular cube with a lens attached
to the front. In the vast majority of situations, standard format camera bodies
are suitable. Standard format cameras should be used in any situation where
the camera is intended to be seen (overt) and a fixed view is acceptable.
As a quick side note, you may see the term "bullet camera" from time
to time. Bullet style cameras are just a variation on a standard camera body
where the body is cylindrical rather than box-like. Other than that, there is
little difference.
PTZ
PTZ stands for "Pan / Tilt / Zoom". Panning is the motion of a camera
from side to side. Tilting is the motion of a camera up and down. Zooming is
the motion of a camera that allows it to view a smaller area of a target but
at much higher resolution. Three different types of cameras fall into the PTZ
family:
Pan / Tilt (PT)
Pan / Tilt / Zoom (PTZ)
Zoom
PT and PTZ cameras use motors and gimbals to allow the camera to pan 360 degrees
and to tilt 180 degrees. If you imagine yourself inside a sphere, PT and PTZ
cameras will allow you to see half of the sphere. As PT / PTZ cameras are typically
mounted on the ceiling inside a building, this allows you to see all of the
action inside the building.
Unlike the pan and tilt functionality, zoom functionality is typically built
into the lens and not the camera body. Zoom capability is rated in power of
magnification so for example, a lens rated 16X would be able to magnify a subject
16 times it’s apparent size.
PT and PTZ cameras are significantly more expensive than standard cameras, usually
2-4 times as much depending on features. In addition, PT/PTZ cameras require
an extra cable to be run (a "telemetry" or "data") cable
to allow the PTZ functions to be controlled. Because of the extra expense and
complexity, PTZ cameras are not suitable for most situations. In general, they
tend to be utilized in situations where the security cameras are actively monitored
and where it’s important to be able to capture additional detail that
standard cameras are not capable of - for example, at a secure facility where
a guard might need to zoom in on a license plate, or in a casino or prison.
Specialty
Specialty cameras are cameras that don’t fall into the above two categories.
It’s impossible to list all the types of specialty cameras here; there
are hundreds. However, specialty cameras fall into two broad categories that
are worth mentioning: covert and environmental.
Covert cameras are cameras that are not intended to be seen. In most cases,
it’s desirable for people to know that they are on camera as this deters
theft and other crime, but there are cases where a facility owner might not
want to advertise that the premises is under surveillance. Pin-hole and other
covert cameras are perfect for these situations. While covert cameras are not
really more expensive than a traditional security camera, they don’t usually
produce as high of quality picture as a full size camera.
Environmental cameras are cameras designed to operate under extremes of temperature,
pressure, moisture, radiation, etc. or are designed to be vandal resistant.
Examples of these types of cameras would be cameras in nuclear reactor cores,
underwater cameras, cameras in foundries, cameras in parking structures, etc.
Some environmental security camera systems use standard cameras in special environmental
enclosures. For example, an outdoor enclosure will usually have a heater and
a fan to prevent condensation buildup on the camera and lens.
2. Color / Black & White / Infrared
The next most important criteria is the range of light frequencies the camera
captures. There are essentially three types of camera available:
Black & White
Color
Infrared
Color and Black & White Security Cameras
Standard black and white cameras are suitable for most situations though color
cameras are not that much more expensive and are often installed instead as
they do give additional information (e.g. color of a subject’s clothing,
color of their hair, etc.)
Black and white cameras are typically higher resolution and are better in low
light situations and are generally specified where high detail, low lighting
conditions exist.
Some cameras are available that switch from color to black and white when light
levels fall below a certain point. Sony’s ExView® cameras are an example
of this type of camera.
Infrared (IR) Security Cameras
For very low light, and even complete darkness, infrared cameras should be used.
While there are stand-alone infrared cameras available, most of the time you
will specify a color or black and white camera with infrared capabilities such
as the Silent Witness NightHawk™. That way you will be able to capture
surveillance video across the broadest range of lighting conditions.
Infrared cameras come in two types, passive and active, the difference being
that passive IR cameras use only the infrared light that already exists in a
scene while active IR security cameras have infrared LEDs that illuminate the
scene with infrared light (which is invisible to the human eye). Active infrared
LEDs generally can illuminate a scene fairly well up to 20 feet away from the
camera lens. Additional infrared illuminators can be installed to increase the
range and level of detail an IR camera is capable of taking in.
3. CCD Format
Almost all cameras on the market these days are CCD cameras. CCD stands for
charge-coupled device and is the part of a camera (a chip, actually) that captures
the light and turns it into an electronic signal.
CCDs come in several different formats which refer to the size of the CCD measured
along the diagonal. CCDs range in size from 1/4" to 1" and larger.
The most common size for security cameras is 1/3" with many 1/4" CCDs
on the market as well. 1/3" CCDs are suitable for standard and high resolution
video and should be specified unless cost is a major issue. Larger format cameras
are available for specialty situations.
4. Resolution
Resolution refers to the level of detail present in an image. Both analog and
digital devices have "resolution". A CCD is an example of a digital
device defined by resolution while a CRT monitor is an example of an analog
device defined by resolution. Film also is an analog "device" that
is rated (in part) by it’s resolution.
To complicate things, resolution is described in several different ways in the
CCTV industry:
Pixels
TVL
CIF
Pixels
The most easily understood resolution rating is "resolution in pixels".
Pixel resolution is given in number of pixels wide by number of pixels high.
An example of this would be a resolution of 640x480 which is approximately equivalent
to the resolution of your television. This means that each image is composed
of a grid of pixels, 640 pixels wide by 480 pixels high or 307,200 pixels total.
For purposes of digital storage and display (as with a digital video recorder)
standard resolution is 320x240 and high resolution is 640x480. Note that high
resolution takes 4 times as much digital storage space as standard resolution
(i.e. 76,800 pixels for standard resolutions vs. 307,200 pixels for high resolutions);
something to keep in mind when selecting a digital video recorder as this translates
directly into the size of the hard drive(s) the DVR must use. For most applications,
standard 320x240 resolution is sufficient.
Television Lines (TVL)
Modern digital video evolved from analog television and many of the conventions
and descriptors which were used to define picture quality in an analog world
are still used today even though the sensors, storage mediums (and some of the
displays) are purely digital. The one that causes the most confusion is TVL
(or Television Lines). Relating TVL to pixel resolution is a very complicated
subject which, for some aspects, even the experts disagree. What follows is
a simplified description and should serve only as a rough guide.
An analog television works by scanning an electron beam horizontally across
the CRT (cathode ray tube or picture tube). This defines the vertical resolution
of a television and is expressed in Television Lines (or TVL). For a NTSC television,
this is 525 TVL of which approximately 480 lines comprise the picture (in other
words, are visible). Therefore, a standard television picture can be said to
have an equivalent digital vertical resolution of about 480 pixels.
The horizontal resolution in an analog TV is just that, analog, and doesn’t
have a precise resolution per se. However, each horizontal line displays about
440 dots at broadcast frequencies (the higher the frequency, the more dots)
which would correspond to about 440 pixels digital resolution.
Therefore, to display a digital picture with the approximate look and resolution
of a television picture you would need a pixel resolution of 440x480 minimum.
Using the aspect ratio of a standard TV, which is 4:3, and extrapolating based
on the visible TVL yields a horizontal resolution of 640 pixels (4/3 * 480).
Therefore, a resolution of 640x480 pixels is generally considered to be "TV
Resolution" even though a standard television cannot display 640 dots per
horizontal line (TVL).
Everything discussed up to this point assumes a black and white picture. The
human eye is less sensitive to fine detail in a color picture and so equivalent
(to the human eye) resolution for a color picture can be achieved with a lower
TVL. That’s why you’ll see lower TVL numbers for the same resolution
on color cameras. We’ll get into that later on.
Common Intermediate Format (CIF)
Common Intermediate Format (or CIF) is a video format defined in the ITU H.261
videoconferencing standard and originally used to spec out videoconferencing
systems. It is also sometimes used to describe CCTV system capacities. The CIF
format was designed to work with both NTSC and PAL systems. One CIF or "Full
CIF" corresponds to 258 TVL with 352 dots per line. When translated to
digital systems, a Full CIF is generally understood to be a resolution of 320x240
pixels. Further resolutions based on Full CIF are in common usage as shown below.
Resolution Abbreviation Digital Analog
(hor. pixels x vert. pixels) (dots x TVL)
Full motion (30 frames per second) NTSC video is 120 CIF (4CIF x 30 fps).
This is more or less TV quality video.
DVR capacities generally assume Full CIF (not 4CIF!) when stating playback and
recording statistics (e.g. recording capacity) so keep that in mind if you plan
on recording in high-resolution. Cameras are usually rated as standard (Full
CIF) resolution or high (4CIF) resolution.
CCD Construction
A CCD is composed of thousands of sensors arranged in a grid. Simplistically,
each sensor captures one pixel’s worth of information. The more sensors
in the CCD, therefore, the higher the resolution of the camera. In reality,
it may take more than one CCD sensing element to provide one pixel’s worth
of video data. For example, in a color CCD, it usually requires a red, green
and blue sensing device sitting right next to each other to capture enough information
to create one color pixel. There are CCDs than can capture all of this information
in a single sensing element but they are expensive and more suited for high-end
photography than security applications.
Relating All of This to the CCD
Since CCDs are digital it makes the most sense to define their resolution based
on the "pixel" resolution measuring method. Nevertheless, CCD security
cameras are still commonly specified using the old TVL resolution measuring
method. This can be quite confusing. Just keep in mind that a full screen television
image (480 TVL visible / 525 TVL total) approximately corresponds to a CCD with
a pixel resolution of 640x480 pixels which in turn is approximately 4CIF.
Standard & High-Resolution CCDs
CCDs generally come in two "resolutions", standard (Full CIF) and
high (4CIF). For a color camera, standard resolution for a 1/3" CCD equates
to ~330 TVL while a high resolution 1/3" color CCD will have ~460 lines
of resolution. Black and white cameras, because they don’t need to use
as many sensing elements to create a single pixel, generally have higher resolutions.
For a 1/3" black and white CCD, standard resolution is ~420 TVL and high
resolution is ~580 TVL. These numbers can vary so check the manufacturer’s
specs.
Other Factors Affecting Resolution
Just because your CCD can capture a certain resolution, doesn’t mean that
you will be able to view video at that resolution. The way the video information
is presented onto the CCD is just as critical and that means the quality and
resolution of the lens is just as important as the CCD.
(NOTE: Your digital video recorder (DVR) can also have an effect
on viewing and storage depending on many factors including compression rates,
processing techniques, etc.. To add to this, the video cable you run is also
a critical component to keeping out electromagnetic and other types of interference.)
5. Broadcast Standards
In general, all video components in a CCTV surveillance system must adhere to
the same broadcast standard. Broadcast standards vary by region. The three most
widely used worldwide broadcast standards are NTSC, PAL and SECAM as shown below.
Standard Region
NTSC North America / Japan
PAL United Kingdom / Europe
SECAM France
In the United States equipment will almost always be NTSC.
6. Lens Type Accommodation (Flange Back Length)
Two lens styles are available to attach to the body of a security camera. Called
"C mount" and "CS mount", they differ in that the flange
back length of a CS lens is ~5 mm shorter than a C type lens. (The flange back
length is 17.256mm for a C type lens and 12.5mm for a CS type lens.) It is important
to select cameras that can accommodate the type of lens you are planning to
use. See "Back Focus" below for more information.
7. Back Focus
Back focus refers to an adjustment in the camera which sets the distance from
the back of the lens to the sensor (CCD in most cases). Back focus is used primarily
to adjust a camera’s back focus length based on which type of lens (C
or CS) is used. Not all security cameras have a back focus adjustment, relying
instead on an ~5mm spacer ring to fit a C type lens to a CS mount camera. You
can’t use a CS lens on a camera designed only for C mount.
Most manufacturers specify the recommended back focus length in the spec sheets
that are included with the camera lenses though most camera bodies have back
focus adjustments and back focus is normally set manually and then fine tuned
at the time of installation.
8. Auto-Iris Support
Security cameras are designed to use either manual or auto irising lenses. The
iris (or aperture) controls how much light enters the camera. See "The
Camera Lens" section for more information on iris, aperture and depth-of-field.
For a manual iris lens, no special considerations are required when selecting
a security camera. When specifying an auto-iris lens, however, you must make
sure that your camera supports the auto-iris standard of the lens you’ve
chosen. The two standards are Video Drive (VD) and Direct Drive (DD). Most cameras
support both types but check the spec sheet to be sure.
For more information on VD and DD lenses, see "The
Camera Lens" section of this white paper.
NOTE: Some cameras now come with an "electronic iris" which attempts
to compensate for changing light levels through circuitry. While these do work
for small variations in light level, the aperture of the lens is still fixed
and usually must be set wide open to accommodate the lowest light levels expected
in a scene. A wide-open aperture will result in a short depth of field, and
for this reason some objects in the scene may be out of focus.
9. Light Sensitivity
Illumination is expressed in "lux". One lux is defined as the illumination
equal to one lumen per square meter. Some common illumination ratings for various
indoor and outdoor applications are shown below.
Common Illumination Ratings Indoor Illumination (Lux)
Office Areas - Detail Work 500 to 1000
Office Areas - Normal 300 to 450
Retail - Display 500 to 1000
Retail - Normal 300 to 450
Factory Floors 75 to 300
Corridors & Stairs - Normal 75 to 200
Corridors & Stairs - Emergency Lighting 30 to 75
Warehouses & Distributors 20 to 150
Parking Garages 20 to 50
Oudoor Illumination (Lux)
Daytime - Full Sunlight 10,000 to 1,000,000
Daytime - Overcast 100 to 10,000
Dusk/Dawn 10 to 100
Twilight 1 to 10
Full Moon - Clear 0.1 to 1
Quarter Moon 0.01 to 0.1
Moonless - Clear 0.001 to 0.01
Moonless - Overcast 0.0001 to 0.001
Minimum Light Sensitivity
Security cameras are rated as to the minimum light sensitivity they will operate
at. Put another way, light sensitivity or "lux rating" describes how
dark a scene can be and still have an acceptable picture. The lower the lux
rating, the more sensitive the CCD in the security camera is to light levels.
Common, medium sensitivity camera lux ratings are 0.5 to 0.7 lux while highly
sensitive cameras may have CCDs with lux ratings of .003 or less (for black
& white cameras) or .05 or less (for color cameras).
Factors Affecting Lux Ratings
Many different factors come into play when measuring light sensitivity including
aperture setting (F-Stop), video output level (IRE), color temperature (expressed
in degrees Kelvin) and reflection ratio of the target object compared to the
background of the scene (expressed as a percentage of light reflected).
Because there are no standards that govern how light sensitivity is measured
for a camera, security cameras with the same lux rating can vary in their actual
sensitivity by several orders of magnitude from one manufacturer to another.
To truly compare one security camera to another, lux rating should be compared
with all of the above factors specified (e.g. 0.5 lux @ F1.4, 5600K, 30 IRE,
80% Reflectivity). However, it is often difficult to find light sensitivity
specified in this way, so having an experienced video integrator help you select
security cameras from reputable manufacturers is the best way to get quality
equipment.
10. Power Requirements
While some security cameras come with built in transformers and standard outlet
plugs (referred to as "main line" cameras), most professionally installed
cameras use either 24 Volt AC (VAC) or 12 Volt DC (VDC) power sources. For these
systems, line voltage (either 220/240 VAC or 110/120 VAC) is converted to 24
VAC or 12 VDC in a security power panel made specifically for this purpose.
See i-ViD’s white paper entitled CCTV Cable & Connectors for more
information on security system power requirements.
B. The Camera Lens
The camera lens is the second piece of the security camera puzzle. Some cameras
come with integrated lenses, but for many the lens is interchangeable. Selecting
the right lens for your application and the camera body that you have chosen
is vitally important if good results from your video surveillance system are
to be achieved. This section will cover the basics of lenses and lens selection.
1. Overall Quality
Lenses range wildly in quality and price and in general, you get what you pay
for. This is not to say you need to put a $1,000 Zeiss lens on a security camera.
This would be overkill and a waste of money. It does mean, however, that if
you put a cheap lens on a quality security camera you’ve wasted your money
on the camera and vice versa. Matching camera to lens is an important part of
good CCTV security system design.
Primary Quality Factors
The primary factors that distinguish one lens from another are how closely the
lens is ground to the ideal profile and how finely polished the lens is. Lenses
that don’t match the profile will create blurry areas of the image, especially
towards the sides. Lenses that aren’t finely polished will create grainy
pictures. Remember the Hubble Telescope? It had a lens that was very, very highly
polished but was ground to the wrong profile! The Hubble had a very high resolution
image of a blurry, out-of focus picture. You have to have both qualities to
have a good lens.
Other Quality Factors
Other quality factors include rigidity of the optical mounts, smoothness of
operation, type of materials, overall fit & finish, etc. The bottom line
is that the best way to insure you are getting a quality lens is to purchase
from established, reputable dealers and manufacturers.
2. Image Format
Just like CCDs, lenses come in different formats described by the area they
project onto the CCD (or other image capture technology such as film). A 1/3"
format lens will project an image sufficient for a 1/3" CCD. You could
use a lens with a larger format, for instance a 1" format lens, with no
problem as it will still cover the 1/3" CCD. In fact, using a larger format
lens than the equivalent CCD format will usually result in a higher quality
image as only the center part of the lens (where it will conform most closely
to it’s ideal profile) is used to project the image onto the CCD.
Lenses come in image formats of 1/4", 1/3", 1/2", 2/3" and
1".
3. Light Transmission
Light transmission is the amount of light that a lens will transmit. It depends
primarily on two things: the number of optical elements used in the lens (lenses,
mirrors, etc.) and the quality of those optical elements. The transparency ratio
is the technical measurement used to describe the light transmission and is
expressed as the percentage of light transmitted at a given light frequency.
The transparency ratio is a difficult specification to obtain. It is not commonly
listed on the lens specification sheet and is generally only available by contacting
the manufacturer, if at all. The good news is that, for CCTV security camera
applications, any lens made by a reputable manufacturer will have an acceptable
transparency ratio.
4. Focal Length
Focal length describes the magnification that a lens provides. Lenses that appear
to make things smaller are called "wide angle" lenses because the
field of view (the area of a scene that the lens projects on the CCD) is larger.
Another way to say this is that the angle of view is wider, hence the term "wide
angle". Lenses that appear to make things bigger are called "telephoto"
lenses. They increase the apparent size of an object but at the same time reduce
the field of view. Telephoto lenses are also known as "narrow angle"
lenses.
Common Security Camera Focal Lengths
For a 35mm format sensor (the format of traditional photography equipment)
a "normal" lens is 50mm. For a 1/3" format sensor (the most common
CCTV security camera format) a "normal" lens is around 12mm. A 2.8
mm lens is generally the shortest lens available and is considered a "wide
angle" lens. The longest CCTV lens available is usually a 50mm lens and
would be considered a "telephoto" lens. Wide angle lenses (4mm to
8mm) are the most common lenses used in video surveillance systems as they allow
the largest possible viewing area and the lowest light levels.
5. Field of View & Angle of View
Field of View
Field of view is usually described by the size of the area of a scene that is
in focus at a given distance from the lens. Because this measurement depends
on the distance from the lens, it is difficult to compare the field of view
from one lens to another if the field of view is specified at a different distance
from the lens.
Angle of View
Angle of view expresses the same thing as field of view but in a way that is
consistent from one lens to another*. Angle of view is defined as "the
angle formed by the outermost visible rays of light crossing at the secondary
focal point". Simply put, it’s twice the angle from the center line
of the lens (the line directly out of the front of the lens) to the outermost
objects that can be seen by the camera.
*The angle of view is a function of the focal length and the size of the CCD
(i.e. the CCD format) used with the lens. The larger the format size of the
CCD, the greater the angle of view will be for a given focal length.
Calculating Angle of View
The angle of view of a lens is approximated using the following formula:
Angle of View = 2Tan-1( L/2f )
where:
"L" is the dimension of the rectangular frame for which you want the
measurement. This is relevant because, even though the lens projects a circular
image over the CCD, the CCD itself is rectangular so you must specify which
dimension is of importance; and
"f" is the focal length of the lens.
6. Auto-Iris Type
Lenses are classified as either manual or auto-iris lenses. The iris (or aperture)
controls how much light enters the camera. With manual iris lenses, the iris
must be adjusted at installation. Any future adjustments must be done at the
camera. Auto-iris lenses are used in varying light situations to automatically
correct for changing light conditions as discussed below.
Manual Iris
Manual iris lenses have a manual adjustment ring that is set at installation
either by eye or with special diagnostic equipment. Lenses with manual irises
are suitable for situations where light levels will be changing very little
such as the inside of an office building. Auto-iris lenses are often less than
20% more expensive than manual iris lenses and are therefore worth specifying
unless you are absolutely sure the light levels in your scene will remain constant.
Automatic Iris
For situations where light levels change over the course of time, such as outdoor
installations, rooms with exterior windows or offices with different light levels
at night, auto-iris lenses should be used. Auto-iris lenses have a motor attached
to the lens that automatically adjusts the aperture (iris) of the lens to correct
for changing light levels.
Auto-iris lenses come in two different varieties: Video Drive
(VD) and Direct Drive (DD). With VD lenses, the lens itself contains the necessary
circuitry to control the iris motor and adjust the iris. With DD lenses, the
amplifier circuitry is not present so the camera must supply it. Because DD
lenses do not have the iris control circuitry, they are cheaper, and since most
security cameras now support both VD and DD lenses, DD lenses are the right
choice in most circumstances.
7. Aperture
Aperture, also commonly known as "F-Stop", describes how far open
the iris is. The iris is the mechanical opening in the camera that can be adjusted
to restrict the amount of light that enters a camera. It is analogous to the
pupil in your eye. In bright light, your pupil is very small (closed), while
in dim light, your pupil is much larger (open).
The iris in a lens is adjusted, either at installation in the case of a manual
iris lens, or continuously and automatically in the case of an auto-iris lens,
to let in the correct amount of light. Too little light and the image will be
dim. Too much light and the image will be washed out.
Understanding F-Stop
"F-Stop" is a term used to describe the state of the aperture. In
general, as the F-stop gets larger, the aperture gets smaller. This is easy
to remember if you think of the aperture setting as a fraction, thus f/2 is
larger than f/8 (i.e. ½ is larger than 1/8).
Each stop in the following series lets half as much light through to the lens.
This series is what you will commonly find on the aperture ring of a traditional
camera used in photography.
The trade off you get for setting a low aperture (e.g. f/1.4),
and thus letting more light into the lens, is that the depth of field is reduced.
(See the "Depth of Field" section below for more information on depth
of field.) The opposite is true for a high aperture setting. In general, a small
aperture (high F-stop) means greater depth of field but less light reaching
the CCD while a large aperture (low F-stop) means a shorter depth of field but
more light reaching the CCD.
For adjustable iris security camera lenses (and lenses in general) aperture
is usually specified as a range, as in "F1.7 to Close", which means
that the aperture is adjustable from a setting of 1.7, which is nearly all the
way open, to an aperture that is all the way closed.
Calculating the Aperture Ratio
The aperture ratio (F) is calculated using the following formula:
F = f/A
where:
"f" is the focal length of the lens; and
"A" is the area of the aperture.
Setting the Aperture of a Lens
Because of the way focal length, aperture ratio and depth of field interact
(see below), the proper way to set the aperture is with the lens focused and
the focal length (zoom) set to infinity. When the aperture is set correctly,
the video output level should be 1 Volt Peak-to-Peak.
8. Depth of Field
Depth of field refers to the range of distances from the camera lens within
which the image is in focus. In other words, with a very large depth of field,
all objects in a scene will be in focus from the very near to the very far.
With a small depth of field, only a small number of objects, or even just one
portion of a single object will be in focus. Photographers often use this effect
to blur the foreground and the background of a picture in order to highlight
the subject (e.g. a flower or an insect).
Focal Length vs. Depth of Field
Depth of field is inversely proportional to the focal length of the lens meaning
that wide angle lenses (those with shorter focal lengths) have greater depth
of field than telephoto lenses. With a short enough lens, the image will remain
focused from very near the camera lens (less than 0.1 inches) all the way out
to infinity. The shortest lenses do not need to be focused and therefore usually
don’t have a focus adjustment ring.
Aperture Ratio vs. Depth of Field
Depth of field is also inversely proportional to the F-stop (aperture ratio)
of the lens meaning that the smaller the iris is (the higher the F-stop) the
greater the depth of field of the image. Thus a high F-stop has the same effect
on depth of field as using a wide angle lens (i.e. the depth of field is large)
while a low F-stop has the same effect on depth of field as a telephoto lens
(i.e. the depth of field is small).
9. Exposure
Exposure (also known as shutter speed) is the amount of time the shutter of
a camera is open. The longer the shutter is open, the more light the sensing
device (CCD, film, etc.) will be able to capture. The downside to this is that
the longer the exposure, the more likely it is that fast moving objects in a
scene will be blurry.
For stop motion action, photographers typically use an exposure of 1/125th second
or faster. Because CCTV cameras are based on the NTSC standard, which limits
video to 30 frames per second, the exposure of a CCTV security camera is usually
fixed at 1/30th second which can lead to blurry video for fast action scenes
(like automobiles, fist fights, etc.).
For applications where crisp, fast action frames are required, special cameras
are available with high speed exposure times up to 1/10,000th of a second.
10. Light Levels vs. Depth of Field vs. Angle of View
- Pick Any Two!
The goal in designing video surveillance systems is often to maximize the light
entering the camera and the depth of field of the image. These two things are
at cross purposes as allowing more light into a camera by opening the aperture
results in a shorter depth of field and vice versa.
Shorter focal lengths allow you to have both a large depth of field and lots
of light impacting on the CCD. Of course the trade off in this case is the level
of detail in the image will be low as things appear small in a wide angle lens.
The most difficult thing to achieve in video (and photography) are low light,
highly magnified images with large depths of field (and fast action).
The easiest thing to achieve in video (and photography) are high light, wide
angle images with small depths of field (and no action).
NOTE: Exposure (i.e. shutter speed) also plays into these calculations but since
for CCTV video, shutter speed is usually fixed at 1/30th of a second, exposure
can usually be ignored.
11. Varifocal Lenses
Varifocal lenses are simply lenses that have a manual zoom adjustment. They
are specified by their minimum and maximum focal lengths (e.g. 2.8mm to 8mm).
They can be manually adjusted to any focal length within the range specified.
Varifocal vs. Fixed Focal Lengths
When specifying a fixed focal length lens (as opposed to a varifocal lens) for
a particular application, an integrator will generally ask the client what they
want to see, then they will measure the size of the target object (e.g. a door
and the area around it) and the minimum and maximum distances from the camera
for which the scene must be in focus (i.e. depth of field). The installer will
then calculate the correct focal length for the lens to meet the client’s
specifications.
The problem comes when the fixed focal length lens is installed. It is often
difficult for the client to describe exactly what they want to see and after
installation is complete, they may decide that they need more detail (a longer
focal length) or a wider view (a shorter focal length) than they have with the
installed lens. It’s not the client’s fault, nor the fault of the
integrator. It occurs simply because video doesn’t work like the human
eye and it’s difficult for the client to describe exactly what they see
in their mind’s eye. They may not even realize what they want until they
get the system up and running.
With fixed focal length lenses, a change of focal length is time
consuming and expensive as a new lens must be ordered and shipped (if not in
inventory) and then installed, configured and tested.
Varifocal Lenses Save Money
The solution to this is the varifocal lens which allows the integrator, with
the client present, to set the level of detail and field of view exactly as
the client desires. And if the situation changes down the road, varifocal lenses
are very easy to adjust to accommodate the new situation.
While fixed focal length lenses are definitely cheaper, the added flexibility
that a varifocal lens gives to the integrator and the end-user, and the time
and frustration saved makes them well worth the extra cost. A varifocal lens
may cost 20-40% more than a fixed lens but when you consider an overall project
cost in the thousands of dollars, the extra $40 or so premium you’ll pay
for a varifocal lens is inconsequential, especially when the benefits are factored
in.
III - About i-ViD Ltd
For more information regarding any information contained in this document, please
feel free to contact us.
We’d be happy to answer any questions you might have, provide you with
further information or help you design a security system that meets your exact
needs.
IV - Notices
DISCLAIMER
IMPORTANT INFORMATION - READ THIS!
This information is provided as-is and is intended solely to assist our customers
to make informed buying decisions. It may contain errors and/or omissions of
important facts and is not intended to be used to install, operate or maintain
any equipment or for any other purpose. Under no circumstance will iViD Ltd
be held liable for the information presented herein.
