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LENS CALCULATOR
Belajar tentang lensa kamera cctv
Format
Lenses and cameras now come in four formats: 1/3-, 1/2-, 2/3-, and 1-in. Lens format is actually determined by the size of the opening in a camera where the lens attaches. When the diameter of this opening measures 1/3 in., the camera is said to be a 1/3-in. formated camera. On the other hand, when the diameter is 2/3 in., the camera is said to have a 2/3-in. format.
For proper viewing, experts say to use only a lens that's formatted the same or higher than the camera used. For example, if the opening in a camera (camera format) is 1/2 in., a 1/2- to 1- in. formatted lens should be used for optimum viewing. Or, if a camera with an opening of 1 in. is used, use a 1-in. formatted lens. To do otherwise will result in the projection of only part of the viewing area onto the target in the camera
 
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| The function of the lens is to collect light
reflected from the scene and focus it into an image onto the
CCTV camera sensor. Most CCTV applications use a
fixed-focal-length (FFL) lens, which, like your eye's lens,
covers a constant angular field of view (FOV).
That is, the FFL images a scene with constant
magnifiction. A large variety of CCTV camera
lenses are available with different focal lengths (FLs), which
will provide different FOVs. Wide-angle,
medium-angle, or narrow-angle lenses produce different
magnifications and FOVs.
This handy FOV calculator will help you in detemining which
lens is right for your application. Simply enter
the distance to the object, camera format, and focal length of
the lens. Click "Calculate" to see the field of
view you can expect with that lens. It also works
in reverse. If you know the field of view you
need, enter it in and the distance to the object along with
camera format to find the focal length you will need to get
the field of view desired.
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www.cctvcamera-alarm.com
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A 1/3 inch camera is viewing an entrance gate to a factory. The car coming through the gate is the critical view.
1/3 chip; width ( c ) = 4.8mm height (v) = 3.6mm
distance to gate (d) = 100 ft
width of gate (w) = 12 ft
car dimension (front ) = 5 ft X 5 ft
Focal length f = c * d / w = 4.8 * 100/ 12 = 40mm
scene height h = v * d / f = 3.6 * 100 /40 = 9 ft
Scene area = 12 ft X 9 ft = 108 sq. ft
Critical area = 5 ft X 5 ft = 25 sq. ft
% size of car in monitor = 25 * 100 / 108 = 23.1%
The car will cover about 23% of the monitor. This will allow the positive identification of the car coming through the gate.
TECHTIPS FROM SPECTRA " K Field-of-View Calculation"
for Spectra™ Dome
If your field-of-view calculator does not have a setting for 1/4"-format cameras,
such as Pelco’s
Spectra™ dome, set up the calculator as follows: Use the 1/2" setting and set
the distance to the object
at half the actual distance. Then read the field-of-view and focal length scales,
which will be in 1/4"
format
size
|
Field of View (in ft.) |
5 ft
away |
10 ft
away |
15 ft
away |
25 ft
away |
50 ft
away |
100 ft
away |
| mm |
w |
h |
w |
h |
w |
h |
w |
h |
w |
h |
w |
h |
| 3.6 |
7.5 |
5.2 |
15.1 |
10.4 |
22.6 |
15.6 |
37.7 |
26.1 |
75.4 |
52.1 |
151 |
104 |
| 6.0 |
3.8 |
2.9 |
7.7 |
5.7 |
11.5 |
8.6 |
19.2 |
14.4 |
38.4 |
28.7 |
77 |
57 |
| 8.0 |
2.9 |
2.1 |
5.7 |
4.3 |
8.6 |
6.4 |
14.4 |
10.7 |
28.7 |
21.3 |
57 |
43 |
| 12.0 |
1.9 |
1.5 |
3.9 |
3.0 |
5.8 |
4.5 |
9.7 |
7.5 |
19.4 |
14.9 |
39 |
30 |
|
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HOW DO YOU CHOOSE A LENS?
Choosing the correct lens for an application is one of the most important decisions while designing a CCTV system. Experience helps but it is important to work with the end user to understand what field of view is required to be seen on the monitor. The field of view is the width and height of the scene as viewed by the lens. It depends upon the focal length and distance of the object.
Any field of view has some critical area which is the target area. For example when the camera is viewing the gate, the space the car is coming through is the critical viewing area or if one is watching the door, the space occupied by a person walking through the door is a critical a viewing area. In the same way every scene has a critical viewing area. This critical viewing area is usually ignored while selecting a lens for an application. After the installation is complete it is not uncommon to hear comments that the end user wanted to positively identify the person, but is not able to do so with the lens installed.
Step 1 - Identify the scene area, which needs to be covered by the lens and estimate the width or vertical height of the scene.
Step 2 - Estimate the distance from the camera to the scene.
Step 3 - Calculate the focal length of the lens. The following methods can be used:
The focal length can be calculated using the either the scene width or height formulas
c = width of the CCD chip
w = width of field of view
h = height of field of view
2. Lens wheel calculator - Many lens manufacturers provide this lens calculator. It is quite simple to use and the focal length of the lens can easily be calculated depending upon the object distance and scene dimensions. The limitation is that it does not tell how large the critical viewing area will be on the monitor.
Step 4 - As mentioned previously, in any scene there are areas or moving objects which are critical. It is important to understand what is required, a detection or positive identification.
Detection view - The critical viewing area should cover 5% of the monitor
In any scene there are areas or moving objects which are critical. It is important to understand what is required, a detection or positive identification.
Detection view - The critical viewing area should cover 5% of the monitor
Action view - The critical viewing area should cover about 10% of the monitor
Identification view - The critical viewing area should cover about 25% of the monitor.
Estimate the horizontal and vertical dimensions of the critical viewing area.
Step 5 - Calculate the viewing area of the scene and also of the critical viewing area by multiplying the horizontal and vertical dimensions. Divide the critical viewing area with the total viewing area to get the size of the critical viewing area in the monitor.
1. If the proportion of the critical viewing area is as expected, use the calculated focal length.
2. If not, then change the
- focal length till the correct proportion is found or
- change the distance of the camera until the correct proportion is found
3.If you still do not find what you want, you may have to choose a lens which is the nearest to your requirement.
A 1/3 inch camera is viewing an entrance gate to a factory. The car coming through the gate is the critical view.
1/3 chip; width ( c ) = 4.8mm height (v) = 3.6mm
distance to gate (d) = 100 ft
width of gate (w) = 12 ft
car dimension (front ) = 5 ft X 5 ft
Focal length f = c * d / w = 4.8 * 100/ 12 = 40mm
scene height h = v * d / f = 3.6 * 100 /40 = 9 ft
Scene area = 12 ft X 9 ft = 108 sq. ft
Critical area = 5 ft X 5 ft = 25 sq. ft
% size of car in monitor = 25 * 100 / 108 = 23.1%
The car will cover about 23% of the monitor. This will allow the positive identification of the car coming through the gate
Calculate Field of View
(FOV)
|
Calculate Required Lens
Size
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