Image format size
There are presently four main image format sizes, which are commonly used in CCTV cameras: 1″, 2/3″, 1/2″, 1/3″ (these measures refers to the sensor diagonal).
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A lens designed for a specific image format can always be used with a smaller format imaging device, but cannot be used with a larger format imaging device.
For instance: A 1/2″ format lens can be used with a camera having a 1/2″ or a 1/3″ imager; however it cannot be used with 2/3″ or 1″ format cameras.
Focal lenght :The rays from an infinitely distant object are concentrated by the lens at a common point of the optical axis, which is called focal point.
A lens has two focal points: the primary and the secondary principal point.
The distance from the secondary point to the focal point on the optical axis (where the imager is placed) is the focal lenght of the lens.
Angle of viewThe angle of view of a lens depends both on the focal lenght and on the sensor format size. The larger is the format size, the larger is the angle of view for a given focal lenght.
Aperture ratioIt is the ratio between the effective diameter and the focal lenght of the lens and shows the actual brightness of the lens itself; it is specified by the F-No. The smaller is the F-No, the brighter is the lens.
For instance: A lens specified with f1.3 is brighter than another one having f1.8.
Formula: F=f/A F=F number (brightness)
Angle field of view
It is the angle defined by two rays of light crossing at the secondary focal point.
It depends from the focal lenght and from the imager format size. Formula: w=2 x tan-1 D/2f
f=focal lenghtBack focal lenght
It is the distance between the last element of a lens group and the focal point. Flange back lenght
It is the distance from the flange surface (contact point between camera and lens) and the focal lenght and is always fixed, depending on mount type.
C mount and CS mount
These are the two standard mounts for CCTV cameras. The only difference between them is the flange back lenght C=17,256mm CS=12,5mm.
C mount lenses can also be used on cameras having CS mount standard, by using C/CS ring adaptors
CS mount lenses can only be used with cameras having CS mount standard.
Close-up focusingWhen the object to be viewed is at a smaller distance than the minimum focusing distance of the lens, the rays of lights will be focusing on a point beyond the imager surface; therefore the image is not focused. To focus the image two methods can be used:
a) Extension tube set
These are rings which are inserted between lens and camera.
Lens is thus positioned at a larger distance from the camera, bringing the object nearer. In this way the point where the rays of light are focused is shifted towards the sensor surface.
However the depht of field is reduced, therefore only objects at a certain distance can be focused properly. By changing the object distance, the thickness of the extension tube is also to be changed.
Extension tube sets are not recommended in case of using zoom lenses.
b) Close-up lenses
These are additional lenses, applied on the front of lenses themselves, to correct the convergence of rays of light and thus to reduce the minimum focusing distance. They are fitted in front of the lens. The close-up lenses have a given strenght which is measured in diopter, defined as the reciprocal of the focusing distance (in meters).