Chapter 4 - Lenses and Optics

Abstract

This chapter reveals that the task of choosing the right lens for a security application is an important aspect in designing a video security system. The large variety of focal lengths (FLs) and lens types make the proper lens choice a challenging one. The lens tables and lens finder kit provide convenient tools for choosing the optimum FL lens that is to be used and the resulting angular field of view (FOV) obtained with the chosen sensor size. The common fixed-focal-length (FFL) lenses used in most video systems have focal length in the range of 2.8mm–75mm. Super wide-angle applications may use a 2.1mm FL. Super telephoto applications may use FLs from 100mm to 300mm. Most FLs in the range of 2.8mm–75mm are available with a manual or automatic iris. Vari-focal lenses are often chosen when the exact FL desired cannot be obtained with the FFL. The vari-focal lens can “fine tune” the focal length exactly to obtain the angular FOV required. Vari-focal lenses are available in manual and auto-iris configurations. Vari-focal lenses must be re-focused when their FL is changed. Zoom lenses are used when the camera and lens must be scanned over a large scene area and the magnification of the scene must be changed. This is accomplished by mounting the camera-lens on a pan-tilt platform that is capable of remotely panning and tilting the camera-lens assembly and zooming the zoom lens. Zoom lenses are available with FLs from 8mm to 200mm with zoom ratios from 6:50:1 and are available with a manual or automatic iris.