Focal Lens CalculatorThe 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 1 / (Object Distance) + 1 / (image distance) = 1/ (Focal Length)Try it here http://www.911securitycameras.com/lenscalculator.html