I am still focused (so to speak) on infrared, and I bought a 6-in-1 adjustable Fotga filter (range 530-750nm) to further explore the Rollei Infrared 400 film. What I have learned is that the film's sensitivity drops off significantly beyond 710nm. The photo above was shot with the filter set to 530nm, and shows the typical IR "woods effect," with dark (sometimes black) sky and glowing, white leaves. The 530nm setting was never problematic in these tests: the filter factor and the results appear to be about the same as with my Hoya 25A (deep red) filter.
The photo above is a more moderate editing, and was also shot with the filter set to 530nm.
This photo was shot with the filter set to 620nm. The IR effect is a bit more pronounced. The only problem is that it is slightly underexposed. My method in these tests was to set a slow shutter speed and let the camera set the aperture. The first photo was shot at 1/30 sec and f16: this one was shot at 1/30 and f5.6. But I was using a lens with a maximum aperture of f1.4. I judge the negative to be underexposed by one stop. Why didn't the camera select a wider aperture? What is happening here is that the film's sensitivity to infrared light is lagging behind that of the camera's internal CdS metering system. The correct exposure would be 1/30 at f4 for EV7 outdoor sunlight. That's an easy adjustment to make, and the time is still practical, so 620nm is quite usable. I like the effect, and I intend to use it a lot.
The photo above was shot with the filter set to 710nm, It is more underexposed than the preceding one, which accounts for the loss of shadow detail. It was shot at 1/30 at f4, which should have been 1/30 at f2, a two-stop difference. If this is kept in mind, the 710nm setting is usable.
The photo above was shot with the filter set to 720nm, a setting that I regard as marginal for this film. It was shot at 1/8 sec at f2 for EV5 artificial light (mostly quartz-halogen), and I consider the negative to be underexposed by three stops. In other words, the exposure really should have been 1/2 sec at f1.4, which is not really practical
Rollei's technical information and marketing site, www.maco-photo.de, recommends a filter in the range 715-750nm, which I think is gilding the lily. The drop-off in IR sensitivity beyond 710nm is a definite problem for practical purposes, though it would be less severe if one were to use this film only in sunlight. I made a rough, non-isometric graph to illustrate this:
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My estimated projection of the graph for 750nm (here cut off) was 8 sec at f 1.4! It's only a rough approximation, but it gives an idea of the problem.
Now that I have some approximate exposure benchmarks, I feel a need to test them. I only have one roll of IR film in my current stock, and it happens to be 120, so my next tests will be done on the Mamiya C33. That will also give me an opportunity to test my new (to me) 65mm lens. Meanwhile, shoot film, if you can!
(to be continued)
Text and images Copyright © 2022 by Donald C. Traxler aka Donald Jacobson Traxler.
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