Just to note ... someone upthread suggested the 87 is a 730 cutoff; I was adding 5 or 6 stops to use a 760 filter -- I would expect 1, at most 2, stops would handle the difference between using a 720 and a 730.** Of course, if you are traveling into reciprocity failure, still more exposure will be necessary; that could be dealt with by lowering the EI for the initial reading or by a separate compensation adjustment. Best I recall, my exposures used with IR400 were in the sub-1-second range and I don't recall thinking much about reciprocity failure, although at 1 second and above some films might be getting into that problem. (Nothing anything like pinhole exposures where a cloudy day and f/275 or the like might make a compensated 30 second exposure into 3 minutes with the compensation.)[ . . . ] At the point where your #87 filter starts passing light the film has lost almost all sensitivity. Not quite though, and I think Dave's suggestion to add an additional 5 or 6 more stops of exposure (beyond what you would do for the R72) should be about right. So I'm suggesting an EI of around ~0.3 for this combination. [ . . . ].
Just to note ... someone upthread suggested the 87 is a 730 cutoff; I was adding 5 or 6 stops to use a 760 filter -- I would expect 1, at most 2, stops would handle the difference between using a 720 and a 730.** Of course, if you are traveling into reciprocity failure, still more exposure will be necessary; that could be dealt with by lowering the EI for the initial reading or by a separate compensation adjustment. Best I recall, my exposures used with IR400 were in the sub-1-second range and I don't recall thinking much about reciprocity failure, although at 1 second and above some films might be getting into that problem. (Nothing anything like pinhole exposures where a cloudy day and f/275 or the like might make a compensated 30 second exposure into 3 minutes with the compensation.)
** To note, I only needed one additional stop to go from a 720 to a 760 with the late, lamented Efke IR film. The Rollei IR400 spectral response is already down at 720 and apparently drops like a rock above 720,
Andrew, I have seen specifications for this film claiming 820nm too, but I think that number is optimistic; something of a marketing value mostly. Unlike Efke IR820 which had a response which was basically flat all the way out to around 820nm (and then fell off sharply), the Rollei film's response peaks in the high 600nm range and then declines rapidly and steadily after that. Now the spectral plots (if you can believe them) do show some extremely small sensitivity at just over 800nm or so, but it is so low there compared to the peak that you just aren't going to see much response out there at all. So the film isn't really going to give the same response with an 87 filter at all.Rollei IR is supposed be sensitised up to 820, so one should be able to get good results with the 87.
... lenses I've seen have a "little red R" next to the infinity mark on their focus scales to allow for a correction. That correction, I'm guessing, is valid only for infinity focusing...
... so what does one do in a close-up situation (where relying on depth of field may be insufficient)? ... Can someone explain how to use IR film with a view camera?
...Also, how does IR film differ from non-IR films; does it solve any of these concerns? Is my thinking just nonsense?
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