Classified pursuant to coupling methods with CCTV, C mount lenses and CS mount lenses are differenciated by the flange back focal lengths as shown at right. The C mount lenses are currently the most commonly used, but the CS mount is becoming increasingly popular.
Rf(C mount lens) = 17.526mm Rf(CS mount lens) = 12.5 mm
Fixed focal lenses and variable focal lenses
Lenses with fixed lengths are commonly reffered to as fixed focal lenses, and those with variable focal lengths as variable focal (zoom) lenses. The zoom lenses change magnification of focused images by varying focal length. For example, when a zoom lens changes its focal length from 12.5mm to 75mm, its zooming ratio will be 6X(75/12.5=6). The focal length(f) has a direct relationship with the view field, which is calculated by the ratio between the focal length of a lens and the size of image pickup devices.
Focal length
The smaller the focal length is set, the wider the view field grows, while the larger the focal length is set, the narrower the view field becomes. Lenses with focal length between f12 and f16 mm are typically reffered to as standard lenses, while those with focal langths below these values as wide-angle lenses and those above these values as telescoping lenses
N(mm)
v(mm)
1 inch
12.8
9.6
2/3 inch
8.8
6.6
1/2 inch
6.4
4.8
1/3 inch
4.8
3.6
1/4 inch
3.6
2.7
View field
A view field is defined as the largest angle in a certain size of an image pickup device. The view field can be obtained by the following equation with a given focal length and size of an image pickup device: Particular attention should be paid when selecting lenses and CCTV, since the view field is determined not only, by the focal length of a lens but by the size of the image pickup device of camera.
Particular attention should be paid when selecting lenses and CCTV, since the view field is determined not only, by the focal length of a lens but by the size of the image pickup device of camera. The size of the image pickup device of the lens should be identical or larger than that of the camera. For example, a camera of 1/4" is available for every lens independant of the size fo the image pickup devices, and a camera of 1/2" is net available for lenses of 1/3" or 1/4".
