What is the effect of film against screen printing?

If it's a thick picture, it doesn't matter whether it's front or back, but if there are dots or thin lines, it will affect it if it's used in reverse. You can think about it carefully, the film also has a certain thickness, and the thickness of the film may be different in different places, and the blackness (opacity) may also be different. If it is used backwards, there will be gaps between the black graphics and text on the film and the screen, and through the transparent film (especially thick film), when printing, the places on the screen that should not be exposed may be seen because of the diffuse reflection of light, resulting in blurred dots and thin lines or even no exposure at all.