John A. Lind |
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Roland,
This is a _tough_ question! Modern AF camera bodies, especially with zoom lenses, are not user friendly with IR film. The focus compensation often shifts with the focal length, and can be different depending on which IR film you are using.However, I believe there are some work-arounds that may work for you. I don't have any AF bodies, but here is what I've read in the past about IR films, AF bodies and zoom lenses. I don't know how familiar you are with the various IR films, so there's information included about each of the IR films I know about. 1. To reduce the effect of any focus error, use f/8 or f/11 apertures. This maximizes your depth of field. I recommend _not_ using f/16 or narrower apertures! The wavelength of IR is longer than visible light. Thus, IR will encounter diffraction limiting, which degrades resolution, sooner than visible light as you stop the lens down. The limit for IR with most 35mm lenses is about f/11 before you start to encounter diffraction. 2. Kodak's Ektachrome EIR:
This is a color slide film which renders "false colors" and is intended for special AR-5 processing. It can also be processed using E-6 as you would with any other Ektachrome provided it is handled properly to protect it from being fogged by IR. It is a bit different from other IR films as it renders in color without any correlation to the true visible colors, includes the visible region, plus extends down into the 700-900nm near IR region. Because of this it requires the least tweaking of focusing. Kodak recommends using a #12 yellow to eliminate all UV and blue, which all color layers in the film are sensitive to. Letting the AF focus through it seems to work with many users. Because the Ektachrome is a combination of visible plus near IR, it is usually a good compromise as only the longer visible wavelengths plus near IR will pass through the filter and be used by the AF system. A simple test is to focus without any filters, then add the #12 and see if it shifts focus slightly. If not (or not enough), then try a #25 red (more radical, see the next section) and see if the focus shifts enough. 3. Kodak B/W HIE (a.k.a. 4143)
This film is also sensitive to visible plus extends down to about 900nm in the near IR region. Kodak recommends a #25 red which is considered a color separation filter that blocks out visible green and blue. If you want only IR then try an 89B, 87 or 87C which will shift the IR admitted downward in that order. You cannot see through these three filters though so you have to compose first, then put the filter on. Some AF systems that use IR will focus through them _if_ there is enough IR. "Your mileage may vary" with your F50 AF and one of the pure IR filters (89B, 87 or 87C). If it won't focus lock, drop back to a #25. 4. Konica Infrared 750:
Another B/W _with_ an anti-halation layer, but only extends down into the near IR to about 800nm. Konica recommends an orange #15, red #25 or red #29, each of which will shift what it lets through downward in that order. As with the other films, a #15 may not work as well as using a #25 or #29. 5. You are going to have to experiment some with whatever film you decide to use. Keep in mind the only IR film I'm aware of with an anti-halation layer is Konica's. This extra layer on the film (which is removed during processing) keeps light . . . or in this case IR also . . . from reflecting back into the film emlusion from the pressure plate that holds the film flat against the rails. Some pressure plates are more reflective of IR than others and I don't know how reflective the F50 pressure plate is. Your first roll of anything other than the Konica should tell you the answer. Also, many modern light meter sensors have IR filters on them as they are a photo-diode type. Your F50 metering may or may not work very well for a proper exposure and you will have to experiment with this also. -- John
April 04, 2001
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