For a quick 1/3" CCD Camera you can work out the lens required using this simple method:
Distance from object multiplied by 4.8, divided by horizontal or vertical area equals the lens required.
d x 4.8 / A = L
e.g.
To focus on an area 5m wide from a camera installed 20m away would be:
20 x 4.8 / 5 = 19.2
So you would need a lens with a focal length of 19.2mm
The Focal Length is measured in mm. As a guide,
a short focal length (e.g. 2.5mm) = a wide angle of view
a long focal length (e.g. 16mm) = a narrow angle of view
the smaller the CCD Chip - the narrower the angle of view
The Depth of View is the distance within which objects in a picture
are in focus.
A large Depth of Field means almost all objects in the Field of View
are in focus.
A small Depth of Field means only a small section of the Field of View
are in Focus.
The following factors can lead to a greater Depth of Field:
Using a Wide Angle Lens
High F-Stop
Using an Auto Iris
The size of the camera's CCD image device will affect the angle of view. CCTV Cameras come in different chip sizes - 2/3", 1/2", 1/3" and 1/4".
Approximate Horizontal angles of |
||||
| Focal Length | 2/3" | 1/2" | 1/3" | 1/4" |
| 2.0mm | - | - | - | 82 |
| 2.8mm | - | - | 86 | 57 |
| 4.0mm | - | 77 | 67 | 47 |
| 4.8mm | 83 | 67 | 57 | 40 |
| 6.0mm | 70 | 56 | 48 | 32 |
| 8.0mm | 56 | 44 | 36 | 25 |
| 12mm | 39 | 30 | 25 | 17 |
| 16mm | 30 | 23 | 17 | 13 |
| 25mm | 18 | 15 | 12 | 8 |
| 50mm | 10 | 7 | 6 | 4 |
F Stop or Aperture has 2 measurements. When the lens is fully open (Maximum
Aperture / Minimum F-Stop), just before the lens completely closes (Minimum
Aperture / Maximum F-Stop).
F Stop - is the lenses ability to gather light depending on the aperture
and focal length.
The 2 standard cctv camera lens mounts. The difference between the two is simply the distance between the lens and the ccd image sensor. C Mount - 17.5mm CS Mount - 12.5mm
For changing light conditions (e.g. cameras positioned outdoors), Auto
Iris lenses are needed.
For indoor use manual iris lenses are sufficient as light levels will
not vary too much.
The majority of CCTV Cameras will be able to use both types. Video Drive Lenses contain circuitry which converts video signal from the camera into iris control. Direct Drive Lenses require the cctv camera to contain the circuitry.