Important parameters

Image size

The CCTV lens camera has a variety of sensors with different sizes and specifications, and the screen aspect ratio is usually 4: 3 (horizontal width: vertical height). The sensor size has an effect on the field of view, and the field of view is narrower on smaller sensors with the same lens. The size of the lens has nothing to do with the angle of view. It only needs to cover the entire sensor image, that is, a camera of the same size or larger. This also means that 1/3 “cameras can use all lenses from 1/3”-1 / “, for example, the field of view of 1/3” 12mm lenses is the same as 2/3 “12mm lenses. The image pixels and imaging quality are improved because only the image in the center of the lens is obtained, and the image in this range is usually sharper.

focal length

The light from the infinite object distance converges at a common point on the optical axis inside the lens. The point where the sensor of the CCTV lens camera focuses is called the focal point. In the optical design, the lens has two main points, a major point and a minor point. The distance between the minor point and the focus determines the focal length of the lens.

Field of View

The field of view of CCTV lens is the angle between the lens and the edge of the image sensor. In theory, when the size of the camera’s image plane is not considered, the focal length of a lens is fixed and the angle of view is fixed. But in fact, the size of the field of view also changes with the size of the face. When the size of the phase plane is fixed, if the focal length becomes shorter, the field of view angle will increase accordingly. The focal length in the catalog is the nominal focal length, so the field of view calculated from the formula is an estimate.

aperture

It is an index to measure the amount of light passing through the lens. The aperture value is represented by the F value. The smaller the F value, the greater the amount of light passing through, and the brighter the image formed by the lens. The F-number is inversely proportional to the lens aperture and proportional to the focal length.
The ratio formula is as follows:
F value = f / D (f: focal length, D = lens aperture)

Auto and manual iris

The lens is roughly divided into three types: DC-driven aperture, video-driven aperture, and manual aperture. The DC drive diaphragm is controlled by the internal circuit of the lens. An amplifier is installed inside the video drive diaphragm, which is controlled by the video signal and the DC power provided by the camera. The manual aperture is adjusted manually by the lens’s external adjustment ring to achieve the aperture change.

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