Well, I've finally found the reason I was getting no images with my R72 and Rollei IR-400. After reading all your posts, I began suspecting that the problem was with my filter. Many of you used Hoya R72's, while mine is an "bargain" Opteka R72. I acquired a Hoya R72 and ran another series of tests on a roll of IR-400 I just bought. I think my results might interest you. I hope this isn't too long, or have too much detail.
On a bright sunny clear day at about 1500 hrs I shot four series of exposures as Puget Sound landscapes with my N6006, as well as metering with a Sekonic L308S handheld. All exposures were at f/2.8. I also shot 3 unfiltered check shots on autoexposure at ISO 400. The series, with 1 stop intervals, were:
(1) 1/4 to 30 sec with Opteka R72, EV 5 to -2. (2) 1/30 to 15 sec with Hoya R72, EV 8 to -1. (3) 1/15 to 8 sec with a Kodak 87 gel filter, EV 7 to 0. (4) 1/30 to 2 sec with a piece of unexposed & processed 120 Velvia film, EV 8 to 2. Five minute water presoak, then D-76, 6.5 minutes, 68 degrees.
The results were as follows. Calculated ISO's are figured from the Sekonic UNFILTERED ISO 400 reading of 1/500 f/11, EV 16 .
[1] No images on any of the Opteka R72 exposures; clear film. Reciprocity failure didn't help, of course.
[2] Good exposure with the Hoya R72 at 1/30sec f/2.8 (EV8) (8 stops down from ISO 400, calc ISO: 1).
[3] Good exposure with the 87 gel at 1 second, f2.8 (EV 3) (13 stops down from ISO 400, calc ISO 0.01).
[4] Good exposure with the Velvia, 1/30 f/2.8 (EV 8) (9 steps down from ISO 400, calculated ISO 0.5).
From these results and testing with my unmodified D70, it's obvious that the Opteka R72 starts passing light somewhere longer than about 820nm, the cutoff point for IR-400 per Rollei's data sheet, so I never had a chance with this filter. It's probably more like an 87C or 87B or B&W 093. The 87 passes more of the shorter wavelengths, and the Hoya R72 behaves like a nominal R72 that you can find curves for on the net. The Velvia also passes IR and gives rather intresting results. Contrast is highest with the Opteka and decreases as you move through the 87, Hoya, and Velvia. This isn't surprising since scenics are usually either sunlit (high IR) with shadows indirectly lit (low IR), and the sky doesn't reflect back IR as much as it does visible light to the shadow areas; so the Opteka sees only IR. The Hoya and Velvia film can be seen to pass some deep red by looking at a bright light through them.
One final note. Metering with the Sekonic and the N6006 with no filter, the Opteka, and Hoya, I found that meter response with these filters does not relate to the film's sensitivity, probably since the meter sensors are sensitive way deep into the infrared (guess, 900-1000 nm?). So for my use I'm going to meter with no filter then subtract stops, and bracket.
Thanks very much for your interest and comments, which definitely helped me discover where the problem lay.
Rob D