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By: Jo Chew
Updated 17/3/09 by Hugh Sherborne
2.8mm, 4mm, 25mm, 50mm... What do these numbers mean? CCTV lenses are available with varying focal lengths. So how do you know which lens you need? I've constructed a table showing different focal lengths and the field of view in degrees that will be achieved.
There are alot of variables that determine the field of view that you will achieve. Such as the size of the cameras imaging sensor (usually 1/3 or 1/4 of an inch).
These figures are approximate, but should give you a good indication of what lens will suit your needs. The figures are approximate because monitor underscanning and other variables will affect the end result. Degrees are of the horizontal view of the camera.
|Focal Length of Lens||1/3" Imaging Sensor||1/4" Imaging Sensor|
Lenses see a view which is like a TV screen - i.e. 4 : 3 proportions - see below
The angles shown above relate to the 4 dimension, or width, looking from above the lens. To help you to estimate the area a lens can see, we have produced a diagram below showing an estimation of the distances you could cover at specific angles of view. The coverage vertically would be approximately 3/4 of the dimensions shown.
For a better idea of the actual view of a lens, below are some camera views of a car approximately 40 metres away, at different focal lengths
View at 50 mm focal length
View at 25 mm focal length
View at 5 mm focal length
A 65 degree camera angle will give the following view in a square room