Exposure meters are not calibrated for infrared radiation, and the ratio of infrared to visible radiation varies with the light source. Make trial or bracketing exposures over a five-stop range to determine the proper exposure for your application. Because infrared radiation is longer in wavelength than visible radiation, the focus point is further from the camera lens. Therefore, the lens must be moved slightly farther from the film to focus an infrared image. This focus difference is most critical when using filters to block all visible radiation from the film.
Most camera lenses have an auxiliary infrared focusing mark.
Consult your camera manual for the appropriate use. For best definition, make all exposures at the smallest lens opening that conditions permit. If you must use large apertures and the lens has no auxiliary infrared focusing mark, establish a focus setting by trial and error. Try extending the lens by 0.25 percent of its focal length beyond the correct focus for visible light. Electronic Flash Use the appropriate guide number in the following table as a starting point for your equipment.