Stacking the filters will not help! Either you have a proper filter, or you don't. If you have two regular red (25A) filters, don't bother with IR. You need #29 (B+W 091) or B+W 092. Or the Hoya R72. The Hoya R72 and B+W 092 are so dark you can barely see through them. The Cokin filter for Ilford has the same properties.
The only film that worked with 25A was Kodak HIE/HIR. Do not bother with any other film with that filter, because it just isn't that sensitive. Also, the speed of the IR response has nothing to do with the film's rating, and it varies between manufacturers.
Another problem is that the IR light reflected varies based on the vegitation. Deciduous trees and lawn grass reflect the most, and conifers and shrubs vary. Once again, Kodak had the best, and it's never been matched. Your results will vary, and will be useless without the right filter.
I have used between 3 and 12 for my IR. I bracket, starting at N, +/-1, +/-2. So five shots per scene.
Recommendation: this isn't the time to use IR. You need to experiment first and find out what works for you. The trees around the Grand Canyon will not be deciduous, and I doubt that you will get good results.
Oh, yeah: the tail end (black part) of E6 can be used as an IR filter. I've done it with Kodak, but I never tried it with the others. It might work. If you have some lying around, go ahead and try it, but you may need long exposure times.
Last edited by Brian C. Miller; 01-05-2013 at 05:21 PM. Click to view previous post history.