Quote Originally Posted by scheimfluger_77 View Post
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. [...]
Whoops! That's what I get for working too fast. I was seeing 850 in the question but my scrambled brain was thinking 750. It was a 760 nm I used with only minor difference on the EFKE material. I wish I could go back and edit that post, but we've passed the time window. (Hate it when that happens!)

The Rollei IR400 makes some statement that it "extends to 820 nm" but the curve I've seen looks as though it's already starting to drop at 700 or so, which accounts for having to use much larger corrections with a 760 filter on it. I believe the EFKE goes to the high 700s before it starts to drop. Finding curves and detailed units seems a bit challenging on these films. And I can't say I have seen a definition of "cut-off." It looks to me as though the nominal numbers are some degree of reduced sensitivity, but you might still get something through beyond the nominal "cutoff" at the cost of considerable additional exposure.

So practically speaking, you're correct. Anyway, thanks for catching me at that, apologies for the misleading jabber.

(Alas, Kodak HIE went out to the 900s!)