As with the camera’s autofocus system, metering will not work when an infrared filter is attached to the front of the camera, as there is no visible light for the camera to meter. The same applies to handheld light meters – as they are measuring visible light they are useless for infrared film photography.
When taking infrared photographs, it is best to use manual exposure, and to take multiple exposures of the same scene, using the bracketing function of your camera. Since infrared film doesn’t have an ISO rating, there are various exposure ‘starting points’ for different scenes. For example, using Kodak film on a bright, sunny day with a Wratten #25 infrared filter, your starting exposure would be an aperture of f/11 and a shutter speed of 1/125. However, with Konica infrared film – which is slower – the starting aperture would be f/5.6. On a cloudy day, you should open the apertures up by at least two stops to compensate for the lower light levels.