First of all, we should understand the relationship between focal length and actual shooting and imaging. According to the imaging principle, the focal length of the lens is inversely proportional to the horizontal viewing angle, that is to say, seeing "far" and viewing "wide" is equivalent to having both fish and bear's paw. At this time, you need to choose the most suitable lens focal length according to your specific monitoring environment.

For example, a general civil surveillance camera will provide lens indexes of different specifications such as 3.6mm, 4mm, 6mm, 8mm, 12mm under the same product. A lens with a focal length of 3.6mm is a wide-angle lens with a wide viewing angle, but because of its short focal length, the viewing angle is not far, and it is not clear if it is a little far away. Therefore, this kind of camera with wide viewing angle but not far from shooting distance is very suitable for narrow and closed space, such as elevator room. A lens like 12mm is a telephoto lens with a large focal length, which can capture a clear portrait even at a long distance, but correspondingly, the angle range of the image can be smaller, which is more suitable for long aisles and corridors.

Therefore, there is no best, only the most suitable. Choosing a lens with a large focal length depends on the spatial characteristics and shooting distance you want to monitor. The lens with each focal length can only achieve the best monitoring effect within a certain range.

On the other hand, if you want to monitor a long distance and a wide range, the best way is to increase the number of cameras, or use the "electric zoom lens" to cooperate with the tripod head installation, which will be more expensive for ordinary people.