What does the focal length in the surveillance camera mean?

Focal length is a measure of light convergence or divergence in an optical system, which refers to the distance from the optical center of the lens to the focal point of light convergence when parallel light is incident. An optical system with a short focal length has a better ability to collect light than an optical system with a long focal length. Simply put, the focal length is the distance from the focal point to the center of the mirror. Focal length f

For a thin lens in air, the focal length is the distance from the center of the lens to the main focus. For a converging lens (such as a convex lens), the focal length is positive, and a beam of parallel light will converge at one point. For divergent lenses (such as concave lenses), the focal length is negative, and parallel beams will spread out after passing through the lens.

Extended data:

Lens focal length classification:

Common ones are: 8mm, 15mm, 24mm, 28mm, 35mm, 50mm, 85mm, 105mm, 135mm, 200mm, 400mm, 600mm, 1200mm, etc. , the length is 2500 mm.

According to the length of its focal length, that is, the viewing angle when shooting, the lens can be divided into standard lens, wide-angle lens and long focal length lens.

The viewing angle of a standard lens is about 50 degrees, which is the viewing angle that a person can see with one eye without turning his head and eyes. The feeling observed from the standard lens is basically the same as the scenery we usually see.

The standard lens focal length of a 35mm camera is mostly 40mm, 50mm or 55mm. The focal length of the standard lens of 120 camera is generally 80mm or 75 mm. The larger the camera frame, the greater the focal length of the standard lens.

However, due to the size of imaging medium (CCD or CMOS) of digital camera, the focal length of standard lens is also inconsistent. For convenience and intuition, when talking about DC lens, the so-called equivalent focal length of a 35mm camera is often used, and this equivalence refers to the equivalence in viewing angle. Only the lens of 35mm camera can be compared with other cameras and digital cameras.

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