What is the appropriate focal length for monitoring?

With the continuous development of security systems, the models and functions of surveillance cameras are becoming more and more abundant.

This has also caused many friends to be very entangled when choosing a surveillance camera. One of the more concentrated problems is how to choose the focal length of the surveillance camera. Here we will talk about the focal length of the surveillance camera in detail.

1. What is the focal length?
Focal length, in an optical system, refers to the distance from the optical center of the lens to the focal point where the light gathers when parallel light is incident. It also refers to the distance from the optical center of the lens lens to the CCD or CMOS photosensitive element of the camera in the camera.

For surveillance cameras, the focal length of the lens determines the size of the captured image, the size of the field of view and the size of the depth of field. Angle of view: With the lens as the vertex, the angle formed by the two edges of the maximum range where the object image of the observation target can pass through the lens. Depth of field: refers to the range of distances before and after the object being photographed when the camera lens can obtain a clear image. In short, the depth of field is the distance range that we can see clearly.
PS: The depth of field is also related to the focal length of the camera, lens aperture and object distance, and the focal length is only one of the factors.

2. What is the effect of different focal lengths? Now the mainstream surveillance cameras are all network surveillance cameras, the focal lengths are generally 2.8mm, 3.6mm/4mm, 6mm, 8mm, 12mm, 16mm, etc. Some of them also have a zoom function, so what is the difference between different focal lengths?
simply put:
✅ The larger the focal length, the farther you can see and the smaller the viewing angle range;
✅ The smaller the focal length, the closer you can see, and the larger the viewing angle range.

can be seen:
✅ At the same distance (such as the first column), the larger the focal length, the smaller the viewing angle range, but the clearer the relative details.
✅Under the same focal length (such as the first row), the farther the distance is, the wider the range of viewing angles can be seen, but the lower the recognition.
✅ If you want to see the face or human figure clearly, the farther the distance is, the greater the focal length you need.

3. How to choose the focal length of the surveillance camera?
I have seen the shooting effects of different focal lengths before, so how to choose the focal length of the surveillance camera in practice? I think it can be considered from the following dimensions:
1. Monitoring scope

The focal length of the monitoring camera determines the size of the monitoring screen and the effective viewing distance, that is, the monitoring range.

For situations where the monitoring range is small and the viewing angle is large, such as the living room of a family or a small shop, a monitoring camera with a smaller focal length can be selected. For example, with a camera with a focal length of 3.6mm or 4mm, the viewing angle range can reach 75°-90°, the effective monitoring distance can reach more than 5 meters, and it can cover a space of 40²-80².

For a larger monitoring range, such as a space above 10m*10m, it is necessary to choose a monitoring camera with a larger focal length. However, due to the large focal length, the corresponding monitoring angle will become smaller. In order to achieve complete monitoring, we need to consider using a cloud billiard ball machine, or install multiple monitoring cameras to ensure complete coverage.

For situations where the monitoring range is fixed but the distance is very far away, for example, it is necessary to monitor a parking space 30 meters away, it is necessary to choose a camera with a larger focal length, such as a surveillance camera above 12mm.

More data can refer to the figure below:

2. In many cases, when we choose a surveillance camera, we consider the needs of specific scenarios. Here are some recommended focal lengths of cameras in common scenarios:
✅ For narrow spaces such as garages and storage rooms, it is recommended to choose a focal length of 2.8mm, and the best monitoring distance is within 3m
✅ In environments such as family indoors or small shops, it is recommended to choose a focal length of 4mm, and the best monitoring distance is 3-5m
✅ For home courtyards, balconies, doorways and other scenes, it is recommended to choose a focal length of 6mm, and the best monitoring distance is 5-10m
✅ Outdoor roads, alleys and other scenes, it is recommended to choose 8mm focal length, the best monitoring distance is 10-20m
✅ For some long-distance fixed location monitoring, it is recommended to choose a 12mm focal length, and the closest monitoring distance is about 20-30 meters
✅ There are also some camera lenses that are zoomable, and the size of the lens can be adjusted according to the scope of the monitoring scene

3. Specific requirements. The specific selection methods of the focal length of many surveillance cameras have been introduced above, and they all have a common premise: in the case of being able to effectively see people or objects.
In reality, we may pay attention to some different points, for example: sometimes we may not care whether we can see people or objects clearly, we only need to monitor the range is large enough, then we can choose a camera with a focal length as small as 2.8mm, or With some fisheye cameras, the monitoring angle of view can even reach 180°, and the range in front of the lens can be seen at a glance.
Sometimes we may be more concerned about a small area at a long distance, such as a fixed parking space 50 meters away. At this time, we can choose a camera above 16mm to focus on a long-distance fixed location.
In addition, sometimes the range we need to monitor is a narrow and long area, such as a range of 5*30m. If the focal length is small, the distance cannot be seen clearly, and the focal length is large, so we can consider using binocular or even trinocular zoom. The functional camera can take into account both distant and near monitoring.