This is curious. Strictly speaking you shouldn't be getting any exposure with an 850nm filter if a film's sensitivity cuts off at 820nm.
Originally Posted by DWThomas
In my recent tests I've been working with Rollie IR400 and 80s. I get a speed of about 1 with the cheap 720nm filter I'm using in both films using a C330 and pinhole cameras. Recently I used a 760nm filter on the IR400 and got some pretty stunning results. The asa setting on my Luna Pro? -5 See if you can figure that one out. ;-) I always try to express my exposure needs in terms of the speed setting on the meter, that way I don't have to jockey filter factors in an exposure calculation.