Infrared index
In photography , the infrared index is a marking on lenses that is used to ensure correct focus when taking infrared images .
Light of different color and wavelength is of lenses broken differently, resulting in blurring and color errors. Modern camera lenses are usually corrected achromatically or apochromatically and depict the range of visible light with only minor errors. However, normal lenses are not corrected for infrared light; the focal length for this light, which is invisible to the eye, can vary considerably. Without corrective action, this can lead to significant blurring when photographing on infrared film when focusing after the visible light.
Many lenses for system cameras , and more rarely cameras with built-in lenses, are therefore given an infrared index that marks the deviation of the focal point for a specific wavelength of infrared light, usually around 790 nm . In practice, the subject is normally focused according to visible light, the distance setting is corrected using the marking and then the picture is taken with the infrared filter attached .
particularities
- Mirror lens lenses usually do not have an infrared index, as their design means that they have only minimal chromatic aberration .
- So-called false color film is normally focused according to visible light despite an infrared-sensitive layer. In this case it is advisable not to use the lens with an open aperture in order to achieve optimum sharpness .
- Zoom lenses can have very large deviations depending on the set focal length.
- Different lenses of the same focal length can also have very different properties in the infrared range due to their design.