Lenght from 2nd principal point on optical axis to focal point. The angle of the Field of View is greater with a short focal lenght and will show a wider picture. The angle of the Field of View is smaller with a long focal length and will show a narrower picture.   Back Focal Length The distance between last lens element of lens-group and focal point. Flange Back Length The distance between flange surface and focal point. There are 5 sizes image format used in CCTV camera, namely 1", 2/3", 1/2", 1/3" and 1/4". Lenses are specified according to these format sizes. The figures on the right show the approx- imate size of the light sensitive area for each format. A lens designed for a particular image format can always be used with a smaller format imaging device, but cannot be used with a format larger than that for which it was designed, if this is att- empted the corners of the picture will be lost. i.e. 1" format lens will work on all sized, whereas a 1/4" format lens will only work on 1/4" format. The angle of view of a particular lens is determined by the focal length in mm and the size of the size of the image device. The larger the image format the wider the angle of view for a give focal length.   DAIWON CCTV Lenses are supplied with either a C-Mount or CS-Mount. The style of C-Mount and CS-Mount is the same but each has a different flange back length. Flange back length of C-Mount is 17.526mm and CS-Mount is 12.5mm.       The Angle Field of View is that formed by the 2 lines from the 2nd principal point to the image sceme. Therefore the scene covered by a lens is dependant on the focal length and the image format of the camera being used. The focal length is not changed by the camera image format but the Angle Field of View is. The Angle Field of view can be calculate with the following formula.