Metering for Infrared Films

Metering for an infrared exposure using a light meter is difficult because the spectral sensitivity of the meter does not match the sensitivity of the film with a filter installed. In other words, the film will pick up red light and the meter is measuring blue light and there isn't a constant ratio between the red and blue levels, so you cannot trust most light meters at all with infrared.

Approach to Metering

The simplest approach is to ignore the meter and use a known exposure in known conditions, i.e. the Sunny 16 Rule with an ISO of 1 for IR820 or 2 for SFX200. In other words, go out in full sun on a summer day when there is absolutely no haze in the sky, use an R72 filter (720nm lowpass), set the camera to f/16 and 0.5 to 1 second and you will get a decent exposure.

As clouds cover the sun, the infrared light does not decrease at the same rate as the visible light, so exposures become a little more risky and some bracketing is called for. Much of the beauty of this film is lost without a clear blue sky and strong direct lighting though.

Because the film is operating largely with longer wavelengths (800nm being twice the wavelength of 400nm blue light), the image is twice as susceptible to diffraction, therefore larger apertures are called for. However most IR films are so coarse that the grain is likely to be the limiting factor until approximately f/16 anyway.

Focus Shift

An infrared focus correction (to account for the different focal plane at the longer wavelength) seems unnecessary for near-IR films such as IR820 and SFX200 even at f/4 with the (many) lenses that I've tried. There is some shift but it lies within the usual human focusing errors.

Focusing corrections are required primarily for wavelengths approaching 1um, which are not captured by the films currently available.

Appearance of IR Film

Taking good IR shots requires some pre-visualisation, taking into account the fact that tonal placement in infra-red is going to be quite different from what it is in the visual band.

Assuming you use an R72 filter, the following items are likely to be relatively dark:

And the following items are likely to be fairly bright:

Rocks are pot luck in their apparent brightness depending on their chemical composition.

Back to Analogue Photography and Film FAQ