A landscape that looks ordinary to the eye can be transformed by film that responds to wavelengths the eye cannot see. Foliage glows white, blue sky collapses to near-black, and skin and water take on an unfamiliar smoothness. This is the so-called Wood effect, and it depends on two things working together: an emulsion sensitised beyond the red end of the visible spectrum, and a filter that blocks the visible light the emulsion would otherwise record.

What the Wood Effect Is and Where It Comes From

The effect is named for the American physicist Robert W. Wood (1868–1955), who published some of the first photographs made on near-infrared-sensitive plates. His article “A New Departure in Photography” appeared in the February 1910 issue of The Century Illustrated Monthly Magazine and included an infrared exposure of his summer home, widely cited as the first published infrared photograph. The striking white foliage in those images gave the phenomenon its name.

The reason foliage records as white is structural, not chemical. Chlorophyll absorbs strongly in the visible red, so leaves look dark in a conventional red-filtered black-and-white photograph. But just past the visible spectrum, around 700 nm and beyond, leaf pigments stop absorbing and the internal cellular structure of the leaf — particularly the air-filled spongy mesophyll — scatters near-infrared very efficiently. Healthy vegetation therefore reflects a large fraction of incident near-infrared, far more than it reflects visible green or red, and an infrared-sensitive emulsion records that as a pale, almost luminous tone.

The dark sky comes from the same shift in wavelength. A clear blue sky is bright largely because of Rayleigh scattering, which falls off sharply as wavelength increases. By the near-infrared the sky scatters very little, so it records as a deep tone once the filter has removed the visible blue the film would otherwise capture.

Film and Filter Working Together

Two materials must be matched: the emulsion’s spectral range and the filter’s cut-off. Ilford SFX 200 is an ISO 200 black-and-white film with extended red sensitivity reaching to roughly 740 nm; Ilford’s own literature describes it as suited to a deep red filter, with skies rendered almost black and most green vegetation almost white. Because its sensitivity only just crosses into the near-infrared, SFX 200 gives a moderate version of the effect even with a red 25 filter, and a stronger one with a deeper red or 715 nm filter.

A more extreme result required film with deeper sensitivity. Kodak High Speed Infrared (HIE), discontinued at the end of 2007, was sensitive out to approximately 900 nm — the longest reach of any common pictorial film — and produced the most pronounced Wood effect.

The filter is what makes the effect possible. A red 25 filter passes visible red and the near-infrared while blocking blue and most green; it darkens skies and lightens foliage but still records a good deal of visible light. To isolate the infrared response, an opaque filter such as a Wratten 87, 87C, or 89B is used: these appear black to the eye, blocking essentially all visible radiation and passing only the near-infrared. The deeper the cut-off, the more the image is built purely from infrared reflectance, and the stronger the white-foliage, black-sky rendering becomes.

The Focus-Shift Caveat

A lens does not bring infrared and visible light to focus on the same plane. Because optical glass refracts longer wavelengths less, near-infrared rays converge slightly farther behind the lens than visible light, so the lens must be racked very slightly forward — focused a touch closer — for the infrared image to be sharp. A common rule of thumb places the correction at about 1/400 of the focal length.

For this reason many older manual-focus lenses carry a small red infrared index mark on the distance scale, offset from the main focusing index. The procedure is to focus normally, read the distance opposite the standard index, then rotate the lens so that distance sits opposite the red mark before fitting an opaque filter, which would otherwise make focusing through the lens impossible. The correction matters most at wide apertures; stopping down extends depth of field enough to absorb much of the shift, but at full aperture an uncorrected infrared exposure will appear soft.