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  1. #1

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    Sort of Strange Question about Pinhole Diameter

    Hello to All,
    It has been years since I made a pinhole image. My "specialty" is emulsion making,in-camera color separation and Platinum/gum on glass. But I have decided to push my practice to the limits by making Color Separations with a pinhole camera. Attached is a diagram of a Tri-color,one-shot camera, which I have. This is a 5x7 camera which contanes 2 pellicle mirrors. The light from the lens takes 3 paths and is filtered through one of three Red,Green or Blue cut-off filter. The distance from the lens to the red filter is 12 inches with the bellows totaly contracted.
    My question is:What is the MAXIMUM pinhole diameter that can still produce an image. I have seen nice pinhole images where exposure time is measured in MONTHS. I am willing to do this, or use strong multiple strobes.
    Regards,
    Bill
    Attached Thumbnails Attached Thumbnails NPC%205x7%20layout.jpg  

  2. #2
    keithwms's Avatar
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    Bill, here is the big issue that comes to mind. As you know, a pinhole's optimum diameter for max sharpness is *very* wavelength dependent, and the 'effective focal length' is likewise very wavelength dependent. If you care about sharpness and want to explore the limits of that, then you might come up with a nifty way to get sharp colour pinhole images by colour sep; I've had some ideas along these lines. This would be interesting because it would not be possible with normal colour films e.g. multilayered films. You'd need to hold the film (or the pinhole) at different distances for the different sep wavelengths... or have different sized holes for the different colours. And then figure out how to bring the colours together in good registry (at an enlarger, I suppose).

    Anyway, that nutty thought aside, there is no limit on the pinhole diam, but obviously the bellows or whatever starts to get very long. If you have a big diameter hole and you put the film close to it, you won't get much focus/collimation at all. I think if you just peruse the online pinhole calculators you'll see that big holes mean big path lengths.
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  3. #3

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    Thank you Keith,
    Sounds like I will need to do this using my monorail view camera. Not the "one-shot". I think that I will try the one-shot approach too. Just to see what happens.
    Just a thought ,though. The cut-off filters on the "one-shot" are very close to the focal plain,not to the lens. Therefor, we are talking white light for most of the distance the beams have to travel.
    Bill

  4. #4
    DWThomas's Avatar
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    I dunno, I love to analyze stuff to death, but this might be one of those "just do it" ideas. Maybe it's the irregularities of homemade pinholes, but my experience is that there is relatively minor variation with a fairly broad deviation from optimum with a pinhole. You might just try a few shots with a middle of the road sized hole and see what it looks like. I would think the location of the filters wouldn't be that important either since each piece of film ultimately only sees the cut off spectrum. This would seem to be an interesting project -- let us know what happens!

  5. #5

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    Quote Originally Posted by keithwms View Post
    ... the 'effective focal length' is likewise very wavelength dependent.
    I could see the focal length of a zone plate camera being wavelength dependent as image formation is dependent on interference, but not a regular pinhole camera. I'm I missing something?

    Denis K

  6. #6
    keithwms's Avatar
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    Denis, the pinhole diam that gives the best result has a factor of wavelength multiplied by focal length in it. There are a few different versions of the criteria; Ralph pointed out in this thread two particular cases, from which I quote:

    Quote Originally Posted by Ralph
    1. The smallest pinhole possible is based on the Airy disc to optimize image sharpness.

    d = sqrt(2.44 * wave length * focal length)

    2. The largest pinhole necessary satisfies the Rayleigh criterion to optimize image resolution.

    d = sqrt(3.66 * wave length * focal length)
    So if you pick a d and then collimate different wavelengths through it, the optimal FLs will be different. I.e. if you keep d constant across the wavelengths red, green, and blue, you'd need for the product wavelength * FL to be constant. Which means that a shorter (bluer) wavelength requires a longer FL, and a longer (redder) wavelengths requires a shorter FL.

    These formulas also show that if you want a large d, you need to go to a large FL.

    That is my (latest) understanding of it, and I'd welcome further clarification from the experts.
    Last edited by keithwms; 12-08-2009 at 10:31 PM. Click to view previous post history.
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  7. #7

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    Quote Originally Posted by keithwms View Post
    Denis, the pinhole diam that gives the best result has a factor of wavelength multiplied by focal length in it.
    I can easily see the image quality being dependent on the pinhole diameter, wave length and selected focal length, but I would still think the focal length is dependent only on the distance between the pinhole and the film plane.

    In other words, if you construct a pinhole camera without the ability to move the film plane relative to the pinhole, you will end up with a fixed focal length camera (i.e. a with a fixed angle of view for the image size of the camera.) I do believe you are correct in that for a fixed geometry pinhole camera you would need to adjust the pinhole size dependent on the wavelength of light you wish to emphasize, or else be willing to sacrifice some image quality.

    Denis K

  8. #8
    nick mulder's Avatar
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    Why not have 3 pinholes very close to each other (optimized in all respects for each color) - L and R (top/bottom whatever) reflected 90deg away from the centre which is left unaffected ...

    Parallax issues could be smaller than the effects of the pinhole size/wavelength issues ?
    Cleared the bowel problem, working on the consonants...

  9. #9
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    Denis, what I am saying is that if one wishes to do colour sep with a pinhole and get optimal sharpness or resolution, you cannot not have optimal results for all three colours at the same d and FL. So you'd have to vary them in concert to get optimal results. Does it really matter? Maybe not :rolleyes: I was just relating a nutty idea I had at one point.

    For fun, consider an apo-corrected lens as a counterexample: it has the same FL across the wavelengths, so no refocus is required for UV through near-IR. The foci at relevant wavelengths coincide. So in some sense, a pinhole is kinda anti-apo

    P.S. That is an interesting idea, Nick. I had more in mind to have a pinhole mount and then three different discs with three diameters, and you'd just swap them out as you do your colour sep exposures.
    Last edited by keithwms; 12-08-2009 at 11:55 PM. Click to view previous post history.
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  10. #10
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    Quote Originally Posted by nick mulder View Post
    Why not have 3 pinholes very close to each other (optimized in all respects for each color) - L and R (top/bottom whatever) reflected 90deg away from the centre which is left unaffected ...

    Parallax issues could be smaller than the effects of the pinhole size/wavelength issues ?
    Let's see...

    Hole size could be optimized for each color.
    It would require much shorter exposures, be easier to make, would be cheaper and easier to source materials with no need to acquire 1/3 reflectance pellicles.

    Hmmm... I'm likin' it.

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