Infrared Photography

Most apochromatic ('APO') lenses do not have an Infrared index mark and do not need to be refocused for the infrared spectrum because they are already optically corrected into the near-infrared spectrum. Catadioptric lenses do not require this adjustment because mirrors do not suffer from chromatic aberration.
Zoom lenses may scatter more light through their more complicated optical systems than prime lenses, that is, lenses of fixed focal length; for example, an infrared photo taken with a 50 mm prime lens may look more contrasty than the same image taken at 50 mm with a 28–80 zoom.
Some lens manufacturers such as Leica never put IR index marks on their lenses. The reason for this is because any index mark is only valid for one particular IR filter and film combination, and may lead to user error. Even when using lenses with index marks, focus testing is advisable as there may be a large difference between the index mark and the subject plane.