25/03/2017
One of the difficult problems when setting up a CCTV system, is choosing the right lens for your cameras as different lenses give different fields of view, and different levels of zoom. Some of the higher-end lenses also offer features such as Auto-Iris, and manual zoom/focus adjustment. Choosing the wrong lens can lead to a camera monitoring too small an area, or conversely not having enough zoom to capture events with clarity.
Adjustable lenses are marked with their maximum and minimum sizes. Examples are 3.5 - 8mm lenses (of which the 6mm sample above would be roughly in the middle of this range), and telephoto lenses that range from around 5 to 50mm.
You need to understand the following when choosing a cctv lens:
*Focal Length*
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
*Depth of Field*
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 Depth of View is the distance within which objects in a picture are in focus. CCTV Cameras come in different chip sizes – 2/3″, 1/2″, 1/3″ and 1/4″.
*F Stop*
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.
*C or CS Mount*
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
*Auto or Manual Iris*
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.
*Video or Direct Drive*
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.
*CCTV Lens Calculator*
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 10m wide from a camera installed 30m away would be:
30 x 4.8 / 10 = 14.4
So you would need a lens with a focal length of 14.4mm
For further enquiries, feel free to drop your comments or send us a private message, we can assure you of our prompt response.
