Technical notes
CCTV lenses
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In this page you can find the main terms used when specifying CCTV lenses,
useful to select the more suitable product for your application.
Our technical service will be glad to help you, whenever further clarifications are needed.
To select the most suitable lens for your application, please use our optical calculator page.
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). | |
Moreover, 1/4" and 1/5" format sizes have been introduced recently in the market.
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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. |
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Angle of view
The 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 ratio
It 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)
f=focal lenght
A=aperture=effective diameter
Angle field of view
It is the angle defined by two rays of light crossing at the secondary focal point.
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Formula: w=2 x tan-1 D/2f D=object dimension
f=focal lenght
Back focal lenght |
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It is the distance between the last element of a lens group and the focal point.
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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.
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Close-up focusing
When 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).
For Instance: A close-up lens with a strenght of +4 diopters, will always focus an object at a distance of 25 cm (1/4 m), regardless of the focal lengh of the lens.
Formula: D=1/d D=diopters
d=focusing distance (in meters)
Depht of field
Is the range of distance from an optical system where the image of an object is always focused.
The depht of field is:
a) inversely proportional to the focal lenght.
b) inversely proportional to the aperture ratio (f number).
Consequently:
a) wider angle lenses have larger depht of field ; eventually, the image remains always focused at the lower focal lenghts. This is the reason why in most wide angle lenses there is not the focusing ring, but the iris ring only.
b) by closing the iris the depht of field increases. For this reason, the iris setting should be made when the iris is completely open (lower light), with the higher aperture ratio (for instance at night). Object focused in this light condition, will be focused also with a more closed iris position (higher light intensity).
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