A Few Questions?

Discussion in 'Pinhole Photography' started by PBrooks, Jan 27, 2008.

  1. PBrooks

    PBrooks Member

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    Hello, I have a few questions about my current pinhole camera build. It will be a pano type camera that takes 14x36 film or paper, but i will be using film. I built a 16x20 back in college, but would like to do this right so, here is what I have so far:
    Focal length = 38.6"
    Angle of view = 53.3 deg.
    Pinhole dia =.052 or 1.32mm
    f/stop = f743

    How would I recalculate the pinhole dia and focal length to a slightly wide normal angle of view?

    Am I crazy for trying this?

    Where is the best place to get laser cut pinholes in the size I need? (everywhere seems to offer laser cut hole but not big enough)

    Thank you for helping!
    PBrooks :D
     
  2. Greg_E

    Greg_E Member

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    You would use this if you want to make it easy:
    http://www.pinhole.cz/en/pinholedesigner/

    Also don't forget the light falloff at the edges of the strip, so you may want to build this as a curved back design. The pinhole then becomes the center of a circle with a radius the same as you focal length, then just draw the curve, though at 38 inches it will be fairly shallow so you way not care.

    In the diameter hole you want, I wouldn't worry about laser cut holes, I would just buy some good number sized drill bits from http://www.smallparts.com and run them in a high speed device like a dremel tool. Use .001 or .002 or even .003 thick stainless steel for the pinhole material (also available at small parts).
     
  3. DWThomas

    DWThomas Subscriber

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    Hmm -- if you can run Windoze, you might take a look at Pinhole Designer. It's a pretty good little package that can twiddle those numbers around.

    There are a number of sites offering info for various calculations:
    The Worldwide Pinhole Photography Day folks have a resource page. And the F295 group has a lot of info.

    Edit: Great minds operating simultaneously. The classic brass shim stock dimpled with a pin and wet sanded (which surely is described in one of those links) makes a surprisingly good job if one is careful and patient.

    DaveT
     
  4. PBrooks

    PBrooks Member

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    What does it take to join that f295 pinhole forum, I've been waiting?
    Also, What does one do if your on a mac instead of pc?
    PBrooks
     
  5. DWThomas

    DWThomas Subscriber

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    No Windoze, well, I just cruised that WPPD resourced link and I see some interesting stuff such as a web-based calulator that looks, in a quick perusal, like it might offer similar functions. (There's number of interesting looking links in the section of that WPPD resource page under "Specifics.")

    Beyond the dimensional stuff, the Pinhole Designer utility offered exposure reciprocity compensation charts for a number of common films that can be exported to text or spreadsheet files. That seemed cool, although my limited experience found some recent films not listed, and the compensation suggested for one that I tried seemed to produce over-compensation in one of my tests.

    Can't help you with the f295 thing, as I am not a member/subscriber there. I've generally approached my occasional pinhole efforts working from just a few basics and experimenting.

    DaveT
     
  6. rwyoung

    rwyoung Member

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    Angle of view can be calculated as:

    a = 2 * arctan(d/2f) where d is a measure of the film, usually the diagonal as that is the longest part and f is the focal length.

    If we set d=diagnonal of film that is 981.1mm
    Now we can pick the angle of view and solve for focal length. Here are a few:
    (rounding for nicer looking number, do you really care about 849.7mm vs 850mm?)
    60deg => f=850mm
    75deg => f=640mm
    95deg => f=450mm
    100deg => f=412mm

    Now you can compute the hole diameter using d=c * sqrt(f * l) where d is the hole diameter, c=magic constant, 1.9 for some, 1.5 for others, I use 1.5, f=focal length and l=wavelength of light to be focused. With photo paper I usually pick 1.5 for c and 550nm for the wavelength.


    f=850mm, pinhole= 1.026mm
    f=640mm, pinhole= 0.89mm
    f=450mm, pinhole= 0.746mm
    f=412mm, pinhole= 0.714mm

    If you are making the holes yourself, you might be happy just calling them 1mm, 0.9mm, 0.75mm and 0.7mm

    The f-stop should be computed after you have your holes made and remeasured but you are looking at numbers like f850, f711, f600 and f589 roughly.

    And if you need some guesses at the exposure times you can start with:

    tfactor = (fpinhole/f16) ^ 2
    f850 => x2822
    f711 => x1975
    f600 => x1406
    f589 => x1355

    So if you were using photo paper and could call it about ISO 8 speed (film speed, not paper speed, and YMMV greatly with paper) on a sunny day, sunlight over your shoulder and crisp shadows:

    f850 => 2822 * 1/8 = 353 seconds
    f711 => 1975 * 1/8 = 247 seconds
    f600 => 1406 * 1/8 = 176 seconds
    f589 => 1355 * 1/8 = 169 seconds

    You may find that the paper starts to exhibit some reciprocity failure after some point and you will need to make adjustments to your exposure times. If you somehow manage to get film into your giant camera, you will need to consult the reciprocity charts and adjust accordingly or find the Schwartzchild coefficient and fix it that way.

    Anyway, bring along a good book to read while making your exposures... :smile:
     
    Last edited by a moderator: Jan 29, 2008