Lens for CCTV home security systems

Lens is made up of curved, ground and polished piece of glass or other transparent material used for the refraction of light. Their main function is to collect reflected light from objects and focus a clear, sharp image on the camera's imager.  Typically, the more light that passes through a lens, the better the quality of the image. The size and clarity of the image obtained are affected by factors such as distance from the scene, focal length, desired field of view, lighting and format used.
 

Field of View
The field of view (FOV) is the area covered by the lens' angle of view. Consider using a wide angle lens to get more coverage of an area.
Camera lenses can be divided into two basic types: fixed focal and varifocal (or zoom).  A fixed focal lens has a constant focal length, while a varifocal lens can change its focal length.  Focal length is simply the distance from the optical center of the lens to a focal point near the back of the lens.  This distance is written on the lens (in millimeters) and indicates the field of view produced by the lens.

Depth of Field
Depth of field is defined as the range of object distances within which objects are imaged with acceptable sharpness. This means that when you focus precisely on a subject, a certain distance in front of and behind the subject also will be in focus, although not as sharp. Depth of field increases or decreases based on the length of the lens, the lens aperture and distance from the camera to the subject.

Each of the three depth of field factors will yield the following:

Lens length
Short lens (i.e. wide angle) = longer depth of field
Long lens (i.e. telephoto) = shorter depth of field

Aperture
Wide aperture = shorter depth of field
Narrow aperture = longer depth of field

Distance to subject
Short distance = shorter depth of field
Long distance = longer depth of field

• A variable focus 1.8mm ~ 3.6mm lens is ideal for monitoring hallways, doorways, and average size rooms.
   

• A 5.0mm ~ 40mm lens has the ability to be used for monitoring hallways and doorways and can be easily adjusted to monitor long driveways or areas several hundred feet away.

Manual iris lenses provide an effective solution for applications where the scene and lighting requirements are relatively stable, especially when used with security cameras that are equipped with electronic shutters.
Auto focus lenses are slightly more expensive than manual lenses but it offers flexibility when used outdoor or indoor with varying light.
 

Other lens resources:
2MCCTV range of lenses
Lens Calculator