The basic parameters of the monitoring lens include focal length, aperture, field angle, mounting interface, depth of field, etc. Today, we will simply discuss the knowledge of camera lens focal length mm.
The actual camera lens is composed of many unit lenses. The most basic two types of lenses are convex lens and concave lens:
The convex lens is used to “focus imaging”, which can make the incident parallel beams converge after refraction, that is, form a focus F on the other side of the refraction surface, usually called the real focus.
The function of concave lens is “astigmatism does not image”, that is, the light refracted by the parallel beam passing through the concave lens is divergent, but its ray extension line intersects the side of the incident light, forming a focus on the main optical axis, which is called virtual focus.
According to the optical principle, the general formula of convex lens imaging can be obtained:
Where, S is the object distance, S’ is the image distance, and f is the focal length. For video camera, in practical application, the object distance is often far greater than the image distance (i.e. S>>S’), so it can be approximately considered that f ≈ S’, that is, the image plane of video camera is almost coincident with the focal plane. If the object height is h and the image height is h ‘, the linear magnification m of the lens is calculated as follows:
Therefore, the image height h ‘is calculated as follows:
It can be seen from the above formula that the size of the image of the scene on the imaging device is related to the object distance, focal length and the size of the scene. When the focal length f is fixed, the smaller the object distance, the larger the imaging size. When the relative position of the scene and the camera is fixed, that is, when h and S are unchanged, the image size can be changed by adjusting the focal length f. The size of the image is proportional to the focal length f, that is, the larger the f, the larger the image.
In the surveillance camera equipment, the focus determines our angle of view, that is, the range of view. It is equivalent to the lens of our eyeball, which plays the role of gathering light. Lens has a variety of focal lengths (mm). Different focal lengths can correspond to different monitoring distances and monitoring picture angles.
● The smaller the focal length is, the closer the viewing distance is, and the wider the angle picture is, but the distance is not clear.
● The larger the focal length, the farther you can see, and the narrower the angle image, but you can’t see it clearly at close range.
● Install cameras with different focal lengths at the same position to obtain different picture effects.
Common monitoring lens focal lengths are: 1.6mm, 2.8mm, 3.6mm, 4mm, 6mm, 8mm, 12mm, 16mm, 25mm, 60mm, and zoom lens.
So how to choose the right focal length?
Household installation scenarios can generally be divided into indoor and outdoor:
The indoor area is generally no more than 10 meters, and the focal length below 4 mm can be selected. For those with very narrow space, such as elevator, small room, and utility room, the focal length of 2.8 mm or 1.6 mm (panoramic) can be selected. The space is relatively narrow and long, and 6mm or 8mm focal length can be selected. It is generally not recommended to use a focal length of more than 8mm indoors.
The outdoor doorway monitoring is generally selected according to the indoor standard. Try to select short focal lengths, such as 4mm and 6mm. If the focal length is too long, because the view angle is narrow, the door can not be completely covered, leaving a dead corner for monitoring.
For outdoor monitoring, long distance and small angle of view, such as long and narrow roads, corridors, fences, etc., usually use an 8mm to 12mm lens. With a telephoto lens, the larger the focal length, the farther the monitoring distance, the clearer the distant image, the smaller the focal length, and the larger the coverage, but the picture can not be seen far. If the budget is sufficient, you can also choose a zoom lens according to the actual situation, which can achieve free switching from far to near.
Finally, I will give you a simple way. If you are uncertain about the choice of focal length, you can choose the smaller one rather than the larger one. For example, 2.8mm and 4mm can be selected; 4mm and 6mm, 4mm can be selected.
Shenzhen Jinshikang Technology Co., Ltd. specializes in the research, development and design of camera modules. It has invested a lot of development and research work in face recognition technology, camera monitoring and other fields, and formed a complete optical industry chain together with its sister company, Shenzhen Jinchengtai (optical lens) manufacturer. Welcome partners who need camera modules and optical lenses to consult, investigate and cooperate.