Would you like to ask the relationship between camera focal length and field of view? How much area can an 8mm lens shoot at a distance of 50cm? Is there a corresponding conversion formula?

In the engineering practice of video surveillance, we often encounter such a problem: I want to see the people at 100 meters clearly, what kind of zoom lens should I choose? How much zoom do you choose? What is the appropriate focal length?

According to the lens transmission principle: f/d = h/h.

Where: f- lens focal length mm

D- the distance m between the lens and the object to be photographed

H-CCD lens height mm

H—— the height of the monitoring point (the actual height of the photos taken by the camera) m

Let's take an intelligent high-speed ball machine (TC-D26 18BNX) in Ye Wei as an example: the ball machine is 1/4CCD(w*h=3.6*2.7mm), 18 x optical zoom, F4.1mm ~ 77. ..

1. Suppose Party A wants to see a person's big head clearly, what is the distance?

First of all, we must make clear a concept (which is easy for people who have taken photos with a camera to understand): if we want to see the big head, the face accounts for about 1/2 of the picture, and the face is about 0.25m, then the height of the monitoring site is d = 0.25 * 2 = 0.5m ..

Substitution formula: d1= 4.1* 0.5/2.7 = 0.76m.

D2 = 73.8 * 0.5/2.7 =13.67m.

So this camera wants to see clearly the big head of Chu Chu, so 0.76 m ≤ d ≤13.67 m.

2. If Party A's requirements are not so high, for example, until the whole human body can be seen, and the human body accounts for 1/2 of the picture according to the average height 1.65, then the height of the monitoring site is d = 2 *1.65 = 3.3m. ..

Substitution formula: d1= 4.1* 3.3/2.7 = 5.01m.

D2 = 73.8 * 3.3/2.7 = 90.2

At this time, 5.01m ≤ d ≤ 90.2m. ..

3. Suppose there is no camera, but Party A says that we should shoot a clear human body at a distance of150m, and we can also calculate (assuming 1/4CCD is still used):

F =150 * 2.7/3.3 = 122.73mm, that is, to achieve this effect, the longest focal length of the selected camera should reach122.73 mm, and if the shortest focal length is 4mm, it is 3 1 multiple zoom.

From the above case analysis, in order to make the lens see farther and clearer, the lens should be large, such as 1/2 CCD, and the focal length should be long.

However, this is not infinite. If you use a zoom of more than 50 times to distinguish distant objects, it is actually of little significance, mainly due to environmental factors such as wind and water vapor.