Why is there a black border on the movie?

Cover movies are also called "false width movies". It is a non-deformable widescreen system, shooting with a standard 35 mm film camera and optical lens. When shooting, add a blocking frame in front of the camera window to block the upper and lower sides of the original standard frame, so that the aspect ratio of the picture is from 1? 33: 1, occlusion is 1? 66: 1 to 1? When the frame ratio is 85∶ 1, the aspect ratio of the picture increases obviously because the upper and lower sides of the frame are blocked. Using a short focal length lens to directly project the screen, the screen effect is the same as that of a wide screen. This method is simple and economical, and is widely used in the world.