The IR (infrared) focus setting is a small red marking on a camera lens focus scale, used to adjust focus when shooting with infrared film. Because infrared light focuses at a slightly different point than visible light, using the IR focus mark ensures that the subject remains sharp in infrared photography.
Understanding and using the IR focus setting is essential for photographers working with infrared film, as it corrects the slight shift in focal plane caused by infrared light. This helps produce crisp, accurately focused images, whether capturing landscapes, experimental photography, or creative infrared effects.