Infrared focus marks on lenses
Many manual focus SLR lenses have a red mark offset a little from the regular, white focus mark – normally a line, diamond or dot – to help the user achieve proper infrared focus. When an infrared filter is attached to a lens, all visible light is blocked making it impossible to compose and focus through the viewfinder. To compose and focus, the infrared passing filter must be removed. When desired focus is achieved, the infrared filter should be reattached. The focus on the lens then has to be turned by the difference between the current visible focus and the infrared focus mark. As mentioned before, this is because in many lenses infrared light has a different focal point than visible light. Most modern lenses do not include infrared focus marks. Many older zoom lenses had two IR focus marks on them – one for wide and one for the telephoto end. If the photographer were using a focal length between the widest and longest setting, then they would have to guess how far to shift the focus, normally around the middle of the two infrared marks.