Quote Originally Posted by Brian C. Miller View Post
Stacking the filters will not help! Either you have a proper filter, or you don't.
Well, the transmission curve of the stack is going to be the product of the transmission curves of the components. B+W claims that their 091 transmits essentially 100% from about 660 or 670 nm upward---two of them should produce a *very* sharp cutoff slightly below that, which would be a kind of "very deep red" filter that passed IR freely, similar to a Schott RG665. I don't know if it would show any IR effects, but looking at the curve I think it's possible. Two 090s (is that the 25A equivalent?) would pass from about 580 nm and probably wouldn't work.

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.
*And* the compensation varies with the time of day, because the ratio of IR to visible changes (highest in late afternoon, lowest at high noon, generally). I never understood why so many people insisted that you needed to use EI 3 or less for IR film when I was getting decent shots at 6 or 12---well, I was doing all my shooting after getting home from work, when the visible light was dim and the IR relatively high. I suppose there could be variance with altitude too. Cripes, why doesn't anyone make an IR light meter?

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.
I've never tried it, but I remember reading that it only worked with HIE and was similar in transmission to one of the far-IR filters. The Rollei film doesn't go very far out of the visible range, IIRC.

-NT