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Thread: Infrared 101?

  1. #21

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    Quote Originally Posted by DWThomas View Post
    Based on my limited experience, the 850nm filter should be OK with the EFKE film - you might need one stop more exposure for the 850 than the 720. If you run the cutoff that high with the Rollei material, you'll be looking at another 5 or 6 stops over the exposure with a 720.
    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.

    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.

  2. #22
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    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!)

  3. #23
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    Quote Originally Posted by DWThomas View Post
    The Rollei IR400 makes some statement that it "extends to 820 nm"
    Inimitable marketing BS, a unique selling point for Maco stuff

    And that applies to filters, too. If some chinese manufacturer puts "900" on a filter that only means there is a "900" on a filter unless you use a filter from a premium manufacturer, then it will have a pretty sharp cutoff at 900nm. The el-cheapos might be labeled "sort of IR", but they work fine. I used a Cokin IR filter - acrylic stuff - for two years until it was so scratched I replaced it with a good glass filter. It's better, but not by a huge amount.
    The future belongs to the few of us still willing to get our hands smell like fixing bath.

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