Lighting Details
Formats/Resolution
Tips/Guides
Need more
information on
video
surveillance?
Can't decide
what camera will
best fit your
application?
Among other
information,
this section
contains
excerpts from a
research report
from the
National
Criminal Justice
Reference
Service that
may assist you
in your
decisions.
A
more technical
discussion of
lenses and field
of view
Lenses
. A camera
lens focuses
light reflected
from objects
within a scene
onto the imaging
device of the
camera. The
imaging device
converts light
to an electrical
signal. Lens
focal length and
aperture are two
important
parameters to
consider.
Lens focal
length describes
the relative
magnification of
the lens. The
camera field of
view (defined
below) will be
dependent on the
lens focal
length, along
with the camera
imager format
size. Similar to
the camera
imager format,
there is a
format size for
lenses. For most
cases, the lens
format size
should be
matched to the
camera imager
format size.
Mismatched
format sizes can
result in the
focused image
being too large
or too small for
the camera
imaging device.
Different camera
and lens formats
can be used
satisfactorily
in a few
instances.
Except for
the most
uncommon sizes,
there usually is
not a large
price difference
between various
lens sizes. The
most common
sizes are
3.6, 4.8mm,
5.6mm, 8mm,
12mm, 16mm,
25mm, and 35mm.
A 35mm lens has
the longest
range with the
narrowest field
of view. The
4.8mm lens can
see much shorter
distances, but
it will have a
much wider field
of view. Most
lens sizes can
be used in
exterior
applications,
depending on the
view desired.
Shorter focal
length lenses,
such as 4.8mm or
5.6mm, are
typical for
interior
applications,
due to the
shorter
distances
involved.
The important
thing to
consider is that
the camera field
of view depends
on the focal
length and
format size.
Camera field of
view is
expressed in
horizontal and
vertical angular
fields of view.
Most camera
manufacturers or
manufacturers'
representatives
who sell lenses
with their
cameras can
provide charts
that list the
angular fields
of view for
common lens
sizes. The image
to the right
shows the
difference
between two
different lens
focal lengths.
The left portion
of the image
uses a 6mm lens
while the right
portion of the
image uses a
60mm lens to
provide for a
manual zoom.
The lens
aperture, or
speed of a lens,
is a relative
measure of the
ability of the
lens to gather
light. Aperture
is expressed as
the F-number.
The F-number is
the ratio of
lens focal
length to its
clear aperture.
Clear aperture
is the diameter
of the inside of
the lens where
light passes
through when the
lens iris is
fully open. A
lens that is
designated as an
F/2 will have a
clear aperture
size that is
one-half its
focal length,
meaning that a
16mm focal
length lens will
have a clear
aperture of 8mm.
The lower the
F-number of a
lens, the more
light the lens
can gather. This
becomes
important when
operating a
camera at low
light levels,
such as at night
with artificial
lighting. Most
security camera
lenses today
have F-numbers
of 1.8 to 1.4.
These are
usually adequate
for night
applications
given that the
minimum light
levels for CCTV
are provided.
Not all
lenses are the
same, however.
Two different
lenses with the
same F-number
can have
different
light-gathering
capabilities.
This is
particularly
true when it
comes to fixed
focal length
lenses versus
variable focal
length (zoom)
lenses. Zoom
lenses have more
glass elements
than fixed focal
length lenses.
Because of the
additional glass
elements, an
F/1.8 zoom lens
will not be able
to pass as much
light as an
F/1.8 fixed lens
with fewer glass
elements. An
amount of light
transmission is
lost in each
glass element.
This is
important to
consider during
night operation
under artificial
lighting. A zoom
lens will
require higher
lighting levels
than a fixed
focal length
lens if an
equivalent
picture quality
is desired.
Most lenses
for security
cameras will
have an
adjustable iris
to control the
amount of light
that is received
at the camera
imager. The iris
is either
manually
adjustable or
electronically
controlled. The
electronic iris
(or auto-iris)
monitors the
camera video
signal output
and will open
the iris for
decreasing light
levels and close
it for
increasing light
levels. This
keeps the video
level
(brightness and
contrast) fairly
constant under
varying lighting
conditions. In
the case of a
manual iris
lens, the user
or installer
adjusts the iris
opening for the
proper video
signal level for
the expected
operational
lighting level.
If light levels
change, an
adjustment to
the iris will be
required in
order to
maintain a
proper video
signal level.
Manual iris
lenses are used
mostly in
interior
applications
where no outside
light comes in
and the light
levels remain
constant. For
all exterior and
many interior
applications, an
auto-iris lens
will be
necessary.
A relatively
new feature in
many cameras is
the electronic
shutter. The
electronic
shutter is part
of the imaging
device and can
perform close to
the same
function as an
electronic iris.
It controls the
amount of light
that the
light-sensitive
elements within
the camera
imager receives.
Electronic
shutters have
limitations,
however. They
may not have as
much range as
auto-iris
lenses. This is
an important
consideration
for exterior
applications. If
light control is
totally
dependent on a
shutter (a
manual iris lens
is used instead
of an auto-iris)
in an exterior
application, the
shutter may not
be able to
reduce light
enough on
bright, sunny
days, resulting
in portions of
the picture
washing out. If
the manual iris
lens is
partially closed
to compensate
for bright
sunshine,
low-light
conditions may
produce a dark,
noisy picture.
Many shuttered
cameras intended
for exterior use
will also come
with an
auto-iris lens.
Field of
view .
Field of view (FOV)
relates to the
size of the area
that a camera
will see at a
specific
distance from
the camera. The
field of view is
dependent on
lens focal
length and
camera format
size.
The FOV width
and height can
be calculated
using the
following
formulas:
Manipulating
the FOV formula
allows a
calculation of
the distance in
feet from the
camera for a
required FOV
width. The
formula becomes:
Before the
FOV for a camera
is selected, the
minimum desired
resolution for
an intruder or
object to be
viewed must be
determined
(i.e., whether
it is desired to
identify a
person or to
just determine
if a person is
within the
scene). This
will limit the
maximum FOV
width and is
referred to as
the
resolution-limited
FOV (image to
the right). The
resolution-limited
FOV width can be
determined by
using camera
resolution in
horizontal lines
per foot and the
number of lines
of resolution
per foot
required to
identify an
intruder. The
following
formula is used
to calculate the
resolution-limited
FOV width:
A resolution
of 16 lines per
foot is
considered
acceptable for
identifying most
people. If a
camera with 350
horizontal lines
of resolution is
utilized, the
resolution-limited
FOV width for a
resolution of 16
lines per foot
can be
calculated as
follows:
The following
table presents
the horizontal
camera format
sizes of the
imager for
various size
imagers:
Example:
Calculate the
maximum distance
from a 350-line,
horizontal
resolution,
1/2-inch format
camera with a
75mm lens to the
resolution-limited
FOV width at 16
lines per foot
resolution.
Viewing the
image above
illustrates that
there is camera
coverage beyond
the
resolution-limited
area but the
resolution will
decrease as the
distance from
the camera
increases.
People may be
seen but not
identified
beyond the
resolution-limited
FOV area. The
figure also
demonstrates
that, as people
walk toward the
camera and into
the blind area,
they disappear
from view
starting with
their feet.
Another
method of
calculating the
field of view is
to use a lens
selection wheel.
These are
mechanical
computing wheels
that are
available from
many lens
manufacturers
and CCTV
manufacturers.
They will give a
good
approximation of
FOV parameters.
A viewfinder
can also be used
to determine the
field of view of
a lens. This is
a specially
designed lens
through which
one can view the
scene of
interest. The
scene is masked
through the lens
in such a way as
to represent the
picture that
will be seen on
the monitor. The
scene desired
can be dialed up
on the
viewfinder and
the focal length
of the lens
required for the
particular
imager format
size of the
camera read from
the side of the
viewfinder. A
viewfinder only
determines a
lens focal
length value;
other parameters
must still be
calculated.
Some lens
manufacturers
have developed
tables for
determining the
field of view.
The format size
and focal length
of the camera is
cross-referenced
to the column of
the desired
distance, and
the width/height
of the field of
view is read
from that
column.
In summary,
whether a camera
scene is useful
depends on
whether objects
can be
distinguished in
the scene.
Camera
resolution,
camera format
size, lens focal
length, as well
as lighting,
shadowing,
camera aiming,
and camera
sensitivity all
play a role in
being able to
distinguish
objects.
Resolution and
performance of
other components
such as TV
monitors,
recorders, and
signal
transmission
equipment must
be considered
also. Cameras
are specified
with the number
of horizontal
lines of
resolution and
active picture
elements. Most
security cameras
available today
range from 300
to 700
horizontal lines
of resolution.
Black-and-white
security cameras
commonly have a
horizontal
resolution of
500 to 600
lines, while
color cameras
for security
applications
have 300 to 400
lines. In many
exterior
applications and
some interior
applications, a
greater number
of
low-resolution
(200-300 lines)
cameras may be
necessary in
order to
distinguish
objects than
would be
necessary using
higher
resolution
(500-600 lines)
cameras.
