Focusing of infrared photography

In traditional infrared photography, because the film itself solves the problem of selecting the wavelength of light, no special filter is needed when shooting. However, due to the long wavelength of infrared, the refractive index in optical glass is smaller than that of visible light. When imaging through the same lens, the imaging position of infrared is behind the visible light, so it is necessary to correct the focusing system of the camera. In order to facilitate infrared photography, the previous manual focusing lens often has an "R" mark or a red dot mark to facilitate focal length correction.

If the subject is far away, the focusing error of infrared imaging and visible imaging is relatively small and can be basically ignored. If the shooting distance is close, it is necessary to correct the focal length. The correction method is to adjust the focal length according to the daily shooting method of visible light, and then turn the focusing line forward slightly to aim at the "R" mark or the red dot mark.