When using infra-red film, first focus the reflex image normally, then note the distance indicated by the red setting mark (either in feet or meters, or the infinity symbol), and turn the focusing ring to set this distance against the red dot (infra-red setting mark) which is either to the right or the left of the normal setting mark. By doing this the image produced by the invisible infra-red rays, which lies at greater distance from the lens than the image formed by visible light, will be brought into sharp focus in the film plane of the camera and consequently will appear sharp in the negative.