( 1 ) F stop
Iris, like the human eyes, can
control the amount of light that passes through a lens in order to
provide clear image picture. The ability of a lens to gather light,
which is symbolized as F, also know as F stop, is equal to f/D where f
is the focal length and D is the aperture of the len . The smaller the
F stop number, the larger the aperture and thus the greater the amount
of light passing through the lens tothe camera’s imaging device, thus increasing the sensitivity of the camera.
Fnumbers conform to an
international aperture scale in discrete values such as F1, F1.4, F2,
F2.8, F4, F5.6, F8, F11, and F16. The light passed is 1/FxF. For
example, a light sensitive F/1.4 lens will pass 1/1.4 2= 0.5 = 50% of
available light.
Different Fstop lens
( 2 ) Focal Length
Once the viewing object is defined, the focal length of the lens is dependent on the following factors:
1) The object size
2) The distance from the object to the camera and
3) The CCD size
The focal length of the len is calculated according to the following formula:
h/H= f/L or w/W=f/L
W Width of object
H Height of object
w Width of format (1/2 format: 6.4mm; 1/3 format: 4.8mm; 1/4 format: 3.6mm )
h Height of format (1/2 format: 4.8mm; 1/3 format:3.6mm 1/4 format: 2.7mm )
f Focal length
L Object distance
Ex: Full image of 4.5m high object :
1/3 format, Object distance: 10m
3.6/4500=f/10000
f=8mm
( 3 ) 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 isdependent on lens focal length and camera format size.
The FOV width and height can be calculated using the following formulas:
FOV Width= Format (horizontal in mm) x Distance in feet from camera )
Focal Length
FOV Height: 0.75 x FOV width
When the focal length is
smaller, the field of view is wider, which is suitable for picture
taking within short distance. On the other hand, when the focal length
is greater, the field of view is narrower, which is suitable for
picture taking in the distance or a closeup shot.
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 resolutionlimited FOV (image to the
right). The resolutionlimited FOV width can be determined by using
camera resolution in horizontal lines per foot and the number of lines
of resolution per foot requiredto identify an intruder.
The following formula is used to calculate the resolutionlimited
FOV width:
Resolution limited FOV width = Camera resolution
Number of lines of resolution
A resolution of 16 lines per
foot is considered acceptable for identifying most people. If a camera
with 350 horizontal lines of resolutionis utilized, the resolutionlimited FOV width for a resolution of 16 lines per foot can be calculated as follows:
Resolution limited FOV width = 350/16=22'
The following table presents the horizontal camera format sizes of the image for various size imagers:
For example, the maximum
distance from a 350line, horizontal resolution, 1/2inch format camera
with a 75mm lens to the resolutionlimited FOV width at 16 lines per foot resolution will be as follows:
Maximum Distance= 22 x 75/6.4 =257.8 feet
( 3 ) Depth of Field
Depth of field is the area in
focus ahead of and behind the main object. When an object is focused,
there is certain area behind theobject and in front of the object that will still be in focus, although not as sharp.
Depth of field increases or
decreases based on the length of the lens, the lens’ aperture,
and distance from the camera to the subject.
Depth of Field is calculated by the following formula.
H= f 2
C X F
T1= B(H+f)
H+B
T2= B (Hf )
HB
F: F number
H Hyperfocal distance ( the
point of focus where everything from half that distance to infinity
falls within the depth of field )
f focal length
H object distance from the image sensor
T1: Far Limit
T2: Near Limit
C Circle of least confusion ( 1/2 format: 0.015mm; 1/3 format: 0.01mm; 1/4 format: 0.008mm )
Based on the above formula, the conclusion can be as follows:
• Lens length
Shorter lens (ex. wide angle) = longer depth of field
Longer lens (ex. telephoto) = shorter depth of field
• Aperture
Wide aperture (low Fstop) = shorter depth of field
Narrow aperture (high Fstop) = longer depth of field
3 Distance to object
Short distance = shorter depth of field
Long distance = longer depth of field
F3.2
F5.0
F9.0
<>
Short depth of
field Long
depth of field
