Information on Common Lenses for Surveillance Cameras

1. Types of Lenses (Classified by Application)

Wide-angle lens: Field of view exceeds 90 degrees; offers a wide viewing range but causes distortion in images of nearby objects. 

Standard lens: Field of view is approximately 30 degrees; suitable for a wide range of applications. 

Telephoto lens: Field of view is less than 20 degrees; focal length ranges from tens to over a hundred millimeters. 

Zoom lens: Focal length is continuously variable, shifting from wide-angle to telephoto; a longer focal length results in a larger image. The focal length can be adjusted as needed to magnify or reduce the image of the subject. Common zoom lenses offer 6x or 10x zoom capabilities. 

Pinhole lens: Used for covert surveillance; typically installed in locations such as ceilings or walls.

2. Lens Aperture

An aperture is located at the rear of the lens to control the amount of light passing through it. Assuming the effective aperture diameter is *d*, the actual effective aperture is larger than *d* due to light refraction. The ratio of *d* to the focal length *f* is defined as the relative aperture *a* (i.e., *a* = *d*/*f*). The relative aperture determines the illuminance of the image; the size of the aperture is expressed by the reciprocal of the relative aperture value (the f-number). A smaller f-number indicates a larger aperture, allowing more light flux to reach the CCD chip. Therefore, for a given focal length *f*, a smaller f-number indicates a superior lens. 

Adjustable aperture: The aperture size can be modified. It should be adjusted according to changes in ambient lighting. Adjustment can be manual or automatic; lenses requiring manual adjustment are called "manual iris" lenses, while those using a built-in micro-motor for automatic adjustment are called "auto-iris" lenses. 

Fixed aperture: The amount of light admitted remains constant. These are primarily used in environments with constant lighting or with cameras equipped with electronic shutters.