Infrared focus point
From The Glossary of Digital Photography
The point at which infrared light rays come to a focus with a lens. This point is different than the point where visible light comes to a focus and that most cameras are set up to use. This infrared focus point must be allowed for when doing digital infrared photography. Autofocus systems will not operate properly. Sometimes the depth of field will be enough to compensate. Older film camera lenses had a mark on the lens to indicate the amount of adjustment needed to correct for this effect. See infrared focus indicator see figure on page 187
