Inconsistent Exposure Information

Discussion in 'Exposure Discussion' started by Lobalobo, Sep 4, 2006.

  1. Lobalobo

    Lobalobo Member

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    Recently, I decided to purchase an inexpensive store-bought pinhole camera to get a feel for exposure and processing. The on-line specs for the (Santa Barbara) camera I bought say that the camera has an f/154 and say: "This is a Baltic Birch, 4 x 5" Pinhole Camera, with a focal length equivalent of 50mm, which is a super wide angle lens for this format. Average exposure in bright, noon-day sun is 4 seconds, using 400 ISO film." The exposure time quoted is *much* longer for ISO 400 film than I expected after using a number of exposure calculators for these conditions, which consistently give exposures of 1/4 second or so even with correction for reciprocity failure, minimal, I guess, for such a short exposure. So why would the manufacturer suggest shutter speeds 16 times longer than the charts suggest? What am I missing here? I do realize that I will ultimately have to experiment to get this straight, but it would be great to be in the right ballpark when I start the experimentation. Thanks.
     
  2. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    Sounds like a typo. I'd say 1/4-1/2 sec. at ISO 400 should be right. 2-4 sec might be in the ballpark for ISO 100, if it's an old style film with normal reciprocity.
     
  3. Lobalobo

    Lobalobo Member

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    Thanks for the lightning fast and informative reply. Given the short exposure time, I am concerned about getting the shutter speed right (as I control it by manually opening and closing a flap). I'll purchase Adox 25 to lengthen the shutter speed sufficiently, even in bright sunlight, but I wonder what you think about using ND filters.
     
  4. David A. Goldfarb

    David A. Goldfarb Moderator Staff Member Moderator

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    If you happen to have fast film and ND filters on hand and can't wait to get some slower film, there's no reason not to use ND filters.
     
  5. Lobalobo

    Lobalobo Member

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    Thanks again.
     
  6. JBrunner

    JBrunner Moderator Staff Member Moderator

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    You could use an ND filter, however at f/154 I'd be hard pressed to guess why you would want to slow it down any further. You would be heading into big reciprocity territory. Also, consider that a pinhole camera has infinite depth of field, so the filter surfaces would be in focus (well sort of, in regard to the focus of the camera), degrading an already soft image.(unless thats part of the effect you seek)

    EDIT -ah, fast film- there's a perfectly good reason- missed that.
     
  7. Ole

    Ole Moderator Staff Member Moderator

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    Maybe the suggested exposure was to ensure decent exposure in the corners?

    A 50mm "focal length" on 4x5" gives vicious falloff. Almost 3 stops less light in the corners than in the center, which brings a 1/2 second exposure up to 4 seconds. Really more like 3 seconds, but then reciprocity failure steps in...
     
  8. Lobalobo

    Lobalobo Member

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    Thanks so much to all. Regarding the ND filter, yes the concern was over fast film and an ability to control a manual shutter pinhole camera. I'll purchase slower film, though, given your advice, and probably not bother with the ND filter for reasons given and otherwise. (Three plus seconds is long enought to make relatively precise adjustments even if just moving a flap by hand.) I expect falloff in the corners and am not too worried about it. My purpose here (at least to begin) is to re-acquaint myself with and teach my daughter about analog photography in the most elemental format I can find, which is a pinhole camera and contact prints. Simple is good, but pure white or pure black negatives wouldn't be much fun, thus my questions about getting a manageable exposure range to experiment with. Thanks again to all.