What do 8mm and 12mm on the camera mean?

8mm and 12mm are the focal length of the lens. How many MM lens we often say refers to the focal length of the lens, generally 2.8MM, 4MM, 6MM, 8MM, 12MM, 16MM, 25MM and so on. Our common cameras are generally 2.8MM and 4MM, and most of them use these two sizes of lenses. Usually, their visual range angle is 65438+. Therefore, there is a dead angle in the monitoring area, and only a local range (within the angle) can be monitored. Although the shaking head machine is called 360 degrees, it can only realize the rotation in the direction, and does not involve the improvement of the lens angle.

Generally, a lens with a width below 2.8MM/4MM is a wide-angle lens, that is, the irradiation range is relatively wide, but the effective distance is pulled away, and the people and things in the picture will also become smaller with the distance, which is suitable for shops or family spaces, that is, when the photographed object is 3-5 meters away.

For example:

Someone is 0/0 meters away from the camera/kloc-.

To see the face clearly, use a lens of about 20 mm

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

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

The focal length is formed by the optical lens of the camera lens. A group of lenses with curvature (convex or concave) on both sides or one side are installed in the metal tube of a camera or projector to form a comprehensive lens. The light emitted from different parts of the object, after passing through the lens, is focused on a point on the negative, so that the image has a clear outline and real texture. This point is called the focus. The focal length is the distance from the point in the lens of the lens to the point where the light can be clearly focused.

When the camera lens is adjusted to infinity, it is actually a nominal focal length. In design, the distance between the main plane of the lens and the negative or imaging sensor is adjusted to the length of the focal length, and then the image far away from the lens can form a clear image on the negative or sensor. When the lens wants to shoot a close object, the actual focal length of the lens changes.

The focal length is usually marked in millimeters (mm), but you can still see that some old lenses are marked in centimeters (cm) or inches. The size of the field of view depends on the ratio of lens focal length to film size. Because the most popular specification now is 35mm, the field of view of the lens is often marked according to this specification. The fields of view of standard lens (50mm), wide-angle lens (24mm) and telescope head (500mm) are different. The same is true for digital cameras, whose photoreceptors are smaller than the traditional 35mm film, so they can get the same image with a shorter focal length.

References:

Baidu encyclopedia entry focus