The core advantage of Fresnel screen lies in its "Fresnel lens structure"-simply speaking, there are countless tiny stepped prisms on the surface of the screen, which can concentrate the light from the projector into the eyes of the audience, so that the brightness of the picture is higher (that is, the "gain" is large), and the ability to resist ambient light is strong, and it is not necessary to draw curtains during the day. However, this structure is not designed casually, and it has to be perfectly matched with the "projection distance" of the projector, which is what we often call "focal length". If the lens of the projector is compared to "eyes", the screen is like a "target", and the way of "aiming" is different with different focal lengths, which is the fundamental difference between telephoto and short-focus screens. Let's talk about the "telephoto screen" first. This kind of screen is suitable for long-distance projection, and the picture will be more regular. As a result, it is found as soon as it is used-the micro-prisms on the telephoto screen are more "sparse", and the inclination angle of each prism is calculated according to "long-distance projection". For example, the angle of the light projected by the projector 3 meters away is relatively flat, and the prism of the telephoto screen can "break" these lights into the eyes of the audience; What if the camera is too close to the screen? For example, if it is only 1 meter, the prism structure of the telephoto screen can't keep up, and the light may be "refracted in the wrong direction", either the picture is blurred or the edge is deformed, just like boiling water with a magnifying glass. If the angle is wrong, it will not focus. Later, I went to my friend's house to see his telephoto screen effect. There was a wall in his living room that was meters long. He put the projector in the corner, about 4 meters away from the screen, and the projected picture was big and clear, even the small words on the subtitles could be seen clearly. And if he moves the projector to a place 2 meters away from the screen (at this time, it is a "short-distance projection"), the picture will be weak at the edge and even a little "scattered". My friend said that he didn't understand it at first, but he bought a telephoto screen and threw it near. He regretted it for a long time. Later, he realized that the telephoto lens was "long", which means "suitable for long-distance projection", and told us that "telephoto lens is suitable for long-distance shooting", which means "long-distance projection is more efficient". Let's talk about the "short-focus screen", which is even more interesting. I thoroughly understood it from a friend who runs an audio-visual studio. The audio-visual room in his studio is very small, only about 8 square meters, and the distance from the projector to the screen is at most meters. He said that in this case, a short-focus screen must be used, otherwise a clear big picture cannot be projected at all. The core design of short-focus screen is "high-density micro-prisms"-the prisms on the screen surface are arranged in a particularly dense way, and the inclination angle of each prism is optimized according to "close-range projection". Just like you scoop water with a small spoon, scoop it quickly and at the right angle to send the water to a nearby place. The prism of the short-focus screen is to "quickly drag the light of the projector to the right direction".