When rays of light pass through a convex lens (the shape of lens shown in figure above) they tend to converge at a single point at some distance ahead of the lens. This distance between the lens and the point where they focus is known as the focal length as specified by the symbol f in the figure. In case of a CCTV camera, the lens focuses the rays at a point where the sensor is placed.
The combination of the focal length of the lens and the size of the sensor determine the field of view of the CCTV camera. The shorter the focal length, the wider the field of view, and vice versa.
Focal Length Formula
Lens Focal Length = (Distance) * 3.2 mm / HorizontalWidth
where 3.2 mm is the horizontal size of 1/4″ CCTV sensor (4.8 mm for 1/3″ sensor).
In the earlier days, rotating lens focal length calculators made out of paper were good and simple tools for quick calculation of the field of view. As well as the formula above.
But you should not use this formula for modern wide angle cameras!
Focal Length Calculator
Traditional calculators cannot calculate focal length for modern wide video surveillance cameras.
Modern security cameras with short lenses (2.1 – 2.8mm) have non-standard wide angles. For example 115 degrees.
The standard formula above doesn’t work for such lenses. You need to have a database of camera models with horizontal and vertical angles provided by camera manufacturers.
We created the unique FREE lens focal length calculator that has a database of camera models and shows correct angles and DORI zones with 3D graphics:
We also offer much more sophisticated CAD software for CCTV design, like our IP Video System Design Tool, that has a built-in database of more than 9000 camera models.
Picture 1. Advanced Camera Focal Length Calculator in IP Video System Design Tool.
Apart from the lens focal length calculator function this software has 3D engine for automatic design and visualization of all elements of video surveillance systems.
See also: FOV Calculator
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