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.
Apart from the focal length calculator this software has 3D engine for automatic design and visualization of all elements of video surveillance systems.