Comparing Filters For Digital IR Photography
by Bjørn Rørslett

89B colour f/11, 1/4 sec	89B b/w f/11, 1 sec
U-360 b/w f/11, 8 sec	U-360 colour f/11, 3 sec
FF colour f/11, 1/2 sec	FF + Hot Mirror f/11, 1/2 sec
87 colour f/11, 2 sec	87 b/w f/11, 2 sec

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Most IR images obtained with digital cameras are presented in b/w. This yields stark contrast, but leaves one speculating as to how the images would look if shown with the colours recorded by the camera. It goes without saying that most colours should be in the reddish range, simply because the deep filters used for IR photography strongly filter out the shorter blue, green, and yellowish spectral bands.

Above, I have compiled test images shot with my D1 using a UV-Nikkor 105 mm f/4.5 lens. I selected this lens simply because I wanted to learn how D1 coped with UV and IR recording simultaneously (the UV bandpass filters do pass some IR as well). I put the white balance to "sunny" and alternated between acquiring NEF raw files in colour, and TIF files entirely in b/w. The shooting took place on an idyllic lake outside Oslo, under slightly hazy sun interspersed with some clouding. Thus, it is difficult to compare these images at extreme detail, because the pattern of shading on the ground may be different. However, the general features of UV and IR recording emerge clearly, and show that D1 indeed can capture pure UV and IR images. The exposure is given with filtration for each image.

Using the Hoya U-360 filter, the D1 captures soft images remniscent of vintage orthochromatic pictures. "FF" denotes Nikon's UV bandpass filter that is distributed together with the UV-Nikkor lens. This filter has a slightly wider UV transparency than either Wratten 18A or Hoya U-360, and passes some near-IR as well. The IR side-lobe can partially be blocked by combining the FF with a Tiffen "Hot Mirror" filter, exemplified above. Thus, we can obtain moody and unusual images that combine reddish rendition (from the remaining deep-red and near-IR ranges) with the soft bluish cast typical for pure UV pictures.

The UV images clearly show that such images confer a soft, hazy mood. This is due to the scattered and diffuse nature of the UV light field. By contrast, the IR photographs indicate their excellent haze-penetration ability. An 89B filter renders deciduous foliage and grass slightly darker than the 87 filter, the latter however imparts a nearly black colour to any water surface.