What cameras can be used for local monitoring?

8mm and 12mm are the focal lengths of the lens. The MM lens we often talk about refers to the focal length of the lens, generally 2.8MM, 4MM, 6MM, 8MM, 12MM, 16MM, 25MM, etc. Our common cameras are generally 2.8MM and 4MM, shaking heads and card cameras. Most of these two sizes of lenses are used, and usually their viewing range angle is within 120°. Therefore, there is a blind spot in the monitoring area, and only a local range can be monitored (within the angle). Although the head-shaking machine claims to be 360°, it can only achieve directional rotation and does not involve the improvement of the lens angle.

Generally, 2.8MM/4MM and below are wide-angle lenses, which means the illumination range is relatively wide, but the effective distance is extended, and the people and objects in the picture will become smaller and farther away, so it is suitable for It is suitable for use in storefronts or home spaces, that is, when the distance to the illuminated object is 3-5 meters.

For example:

Someone is 10 meters away from the camera.

To see faces clearly, use a lens of about 20mm.

To see the outline of the human body clearly, use a lens of about 10mm.

To monitor people’s activities, use a lens of about 5mm.

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The focal length is formed by the optical lens (lens) of the camera lens. The metal barrel of a camera or projector houses a set of lenses that are curved (convex or concave) on either side or one side to form a composite lens. The light emitted from different parts of the object, after passing through the lens, is focused on a point on the film, giving the image a clear outline and real texture. This point is called focus. The so-called focal length is the distance from the middle point of the lens to the point where light can be clearly focused.

When a photographic lens is adjusted to infinity, it is actually a focal length in name only. In terms of design, the distance between the main plane of the lens and the film or imaging sensor is adjusted to the length of the focal length. Then, the image far away from the lens can form a clear image on the film or sensor. When the lens wants to capture a closer object, the actual focal length of the lens is changed.

Focal length is usually marked in millimeters (mm), but you may still see some old lenses marked in centimeters (cm) or inches. The size of the field of view depends on the ratio of the focal length of the lens to the size of the film. Since the most popular format is 35mm, the field of view of a lens is often indicated based on this format. The fields of view are different for the standard lens (50mm), wide-angle lens (24mm), and telephoto lens (500mm). The same is true for digital cameras. Their photoreceptors are smaller than traditional 35mm film, so the same image can be obtained with a shorter focal length.