May 10, 2023
What is focal length
Focal length: refers to distance from optical center of lens to focal point where light is collected when light is incident parallel. The focal length of a lens determines size of captured image, size of field of view, size of depth of field, and perspective of image.
Types of video recordings from surveillance cameras:
The lens focal length of security cameras is typically 2.8mm, 4mm, 6mm, 8mm, and 12mm. The surveillance camera lens can be selected according to surveillance object, generally, smaller focal length of lens, larger range of viewing angles, but smaller surveillance distance.
Therefore, the 2.8mm lens is suitable for narrow spaces such as corridors and elevators, 4mm for rooms, 6mm for corridors or entrance doors, 8mm for workshops or corridors, and 12mm for narrow roads.
(Image taken from Internet)
How to choose monitoring focal length
Strongly recommended distance between monitors for wide-angle lenses with different focal lengths:
▶ The 2.8mm telephoto lens can be used in geographic surveillance environment designed for small indoor areas such as warehouses, and best surveillance interval is within 3m.
▶ The 4mm wide-angle lens can be used in a small shop or in a temperate room at home, and optimal distance between monitors is 3-5m.
▶ The 6mm wide angle lens can be used in small shops or at gate, and optimal distance between monitors is 5-10m.
▶ The 8mm wide-angle lens can be used outdoors, such as on roads and alleys, and optimal distance between monitors is 10-20m.
▶ The 12mm wide angle lens can mainly be used in special parts of monitor, such as some special inputs, etc., and optimal distance between monitors is 20-30m.
Different devices have different field of view angles, so actual usable distance will vary greatly, which should be based on specific device settings.
Location and angle of use of surveillance camera lens:
1. Wide Angle Lens: The viewing angle is greater than 90 degrees, and it is commonly used in elevator cabins, halls and other places with a short viewing distance and a large viewing angle; lens is usually chosen as 2.8mm or 2.5mm;
2. The viewing angle is greater than 60 degrees, and is used in places with a size of about 5 * 5 meters; lens is usually chosen in size 3.6–4 mm;
3. The viewing angle exceeds 50 degrees and is used in places at a distance of 8-10 m; lens is usually chosen 6 mm;
4. The viewing angle is over 40 degrees. It is used at a distance of about 10 - 18 meters, and lens is usually chosen 8mm;
5. The viewing angle is above 30 degrees. It is used at a distance of about 20-30 meters, and lens is usually selected from 12mm to 16mm.
In general, when size of target surface of a CCD camera is fixed (lens specifications are usually divided into 1/3 inch, 1/2 inch and 2/3 inch, etc.), ratio between focal length of lens length and field of view angle Lenses: The longer focal length of a lens, smaller field of view of lens. In addition, with a fixed lens focal length, relationship between lens specification and lens field of view is as follows: larger lens specification, larger lens field of view.