CCTV Focal Length Lens Calculator:

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 = 19.2mm
so you would need a lens with a focal length of 19.2mm