The focal length of the lens is different, the viewing angle and range that can be monitored will also be different. The following are the visible distances and ranges of some common focal lengths. You can refer to them when choosing a camera.
2.8mm, viewing angle 90 degrees, viewing distance 5-6 meters
3.6mm, viewing angle 82 degrees, viewing distance 6-8 meters
6mm, viewing angle 60 degrees, viewing distance 10-15 meters
8mm, viewing angle 45 degrees, viewing distance 15-20 meters
12mm, viewing angle 30 degrees, viewing distance 25-30 meters
16mm, viewing angle 22 degrees, viewing distance 30-35 meters
The smaller the focal length of the lens, the larger the viewing angle of the monitoring screen and the closer the viewing distance. The viewing distance is generally determined by multiplying the focal length of the lens by 2, and then floating it up a little, for example, 2.8*2=5.6, so the appropriate viewing distance for a 2.8mm lens is about 5-6 meters.
Of course, the above are only approximate reference values, which are very helpful for the selection of the focal length of the surveillance camera lens in ordinary household or small surveillance projects. More accurate lens focal length, viewing angle, and viewing distance are also affected by many factors, such as the target surface of the lens, the measurement standard of the viewing distance, and so on.