Monitoring basic knowledge: how to choose the number of millimeters of surveillance camera lens?

Lens mm selection

Under normal circumstances, the same model of the camera, there will be more than a few millimeters to choose from. However, many friends do not have a specific concept about the number of millimeters of the lens, and it is not appropriate to determine how many millimeters of lens should be selected for what kind of monitoring scene.

Xiao Bian then deliberately organized an article, hoping to help everyone have an intuitive understanding of the lens mm.

Lens focal length

The lens focal length, which is often referred to as the number of millimeters of the lens, refers to the distance from the optical center of the lens to the focus of the light when parallel light is incident.

The length of the lens focal length determines the size of the imaging, the angle of field of view, the depth of field, and the perspective of the screen.

As shown in the above figure, in the same scene, the focal length of the lens is different, and the captured images are also different.

Object distance

The object distance is the distance between the camera lens and the subject. For example, if you need to photograph a vehicle across the road, the distance between the camera and the car is the object distance.

Field of view angle

The angle of view is simply the maximum angle that the camera can capture with the camera as the vertex.

The size of the field of view determines the field of view of the lens. The larger the field of view, the greater the field of view.

In layman's terms, target objects beyond this angle will not be captured in the lens.

Relationship between the three

The larger the focal length, the farther the visual distance, the smaller the angle of view; the smaller the focal length, the closer the visual distance and the larger the angle of view.

Scene recommendation

Lens selection tips

Locations with small viewing distances and large viewing angles, such as elevators, rooms, etc., generally use 2.8-4mm lenses.

In places with a large depth of field and a wide range of viewing angles, such as outdoor environments, walls, etc., a 6-8mm lens is generally used.

Smaller distances and longer distances, such as channels, workshops, warehouses, etc., commonly used 6-12mm lens.

Estimation method

Xiao Bian then shared with everyone a lens focal length selection method. This method is suitable for 4mm or more lenses.

Through the above table, we take 2 million cameras as an example, assuming someone is 24 meters away from the camera.

If you want to see the face, use a lens with 24÷1.5=16mm.

If you want to see the outline of the human body, use a lens with 24÷3=8mm.

If you want to monitor the motion picture of the person, use a lens with 24÷6=4mm.

Below we list some commonly used places to choose how the camera selects the millimeter of the lens?

In indoor environments such as small shops and homes, 4mm lenses are recommended. The main consideration is that the indoor distance is not too far (3-6 meters), and the monitoring angle is large enough.

As the cell unit door looks after, we generally choose a 6mm lens so that we can see the entrance and exit of the unit door and unit door.

In the outdoor road, the equivalent of the scene, you can choose 6mm or 8mm camera (optimum monitoring distance of 10-20 meters), with corridor mode, to make the picture more perfect.