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  1. #1

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    Iris Diaphram as pinhole

    Hi to all,
    I am wondering if a high quality iris diaphragm could be used as a multi aperture pinhole. With iris diaphragms being used with quality microscopes, I am wondering if the material can be thin enough and the holes smooth enough. My guess is yes. I think that with one investment, virtually any pinhole aperture could be realized.
    Bill

  2. #2
    Jim Jones's Avatar
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    The smallest iris diaphragm in any of my lenses has a minimum diameter of maybe 1mm and has only four blades. It would be appropriate for about 1 meter focal length. With careful construction, an adjustable three- or four-sided pinhole can be made with a much smaller minimum aperture. A quick check of three blades of about .0012" thick hard material (stainless steel?) suggests a minimum length of maybe .010" for each side of the triangle. Such an adjustable aperture with two fixed and one moveable blade should be fairly easy to make. A four blade adjustable aperture could have a much smaller minimum size, but would require higher precision in construction. These apertures would have more diffraction than a round aperture of equivalent f/number. Since pinholes cost nothing to make and can be made in any size, spending money or time on adjustable pinholes seems more like a fun project than an investment.

  3. #3

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    Any pinhole that is not perfectly circular will produce less than optimal results. Such a pinhole would be impossible to achieve with an iris type aperture.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  4. #4

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    Thank you,
    That is another idea that I can dump into the shredder.

  5. #5
    NedL's Avatar
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    I think there are "zero aperture" diaphragms for use in microscopes, that close all the way? Somewhere I read about making an adjustable pinhole out of three or four razor blades.

    As an aside, I've been wondering about the effects of non-circular pinholes, and whether they might have any artistic value, even if "sub-optimal". An obvious question would be the effect on sun flares, like the number of blades in a normal lens aperture affects the "sun star" flare. But it seems to me you might get softness in one direction or some other effect that might be interesting to play with. Might be interesting to play around with.

  6. #6
    AgX
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    Quote Originally Posted by Gerald C Koch View Post
    Any pinhole that is not perfectly circular will produce less than optimal results. Such a pinhole would be impossible to achieve with an iris type aperture.
    What would be the ill effect of such a polygon compared to a circle?

    Leaving issues of thickness aside (!), one would overlay a pinhole of optimum diameter (mean diameter areawise) with pinholes of other sizes.
    Keeping in mind that the pinhole diameter is calcaluted for a single wavelenght anyway, I would not expect any ill effect.
    The same goes for the distortion imposed by the edgy shape of the polygon.

  7. #7

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    Most of the zero aperture Irises on sale are double irises. Probably resolution disasters. But some microscope iris diaphragms get down to very low diameters with only one iris. They can also be had for less than a new car.

  8. #8

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    Resolution suffers when the pinhole is not circular. The same problem exists for lenses. The image for a 4 blade iris is not as good as one with more blades. For example one of my enlarging lenses has 20 blades in the iris. The effect is similar to having two images super imposed, one taken at f/2 and one at say f/8.
    Last edited by Gerald C Koch; 06-28-2013 at 03:39 PM. Click to view previous post history.
    A rock pile ceases to be a rock pile the moment a single man contemplates it, bearing within him the image of a cathedral.

    ~Antoine de Saint-Exupery

  9. #9

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    PLEngineerinf has a video on Facebook. A motorized iris diaphragm with zero aperture.

  10. #10
    NedL's Avatar
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    Quote Originally Posted by Gerald C Koch View Post
    Resolution suffers when the pinhole is not circular. The same problem exists for lenses. The image for a 4 blade iris is not as good as one with more blades. For example one of my enlarging lenses has 20 blades in the iris. The effect is similar to having two images super imposed, one taken at f/2 and one at say f/8.
    Fascinating. Ralph has shown us that there is one "optimum" for resolution and another for contrast. If we had an elliptical pinhole, optimum for resolution on one axis, and for contrast on another it might lead to some interesting and subtle effects. For example, a photo of a forest of vertical tree trunks might look different depending on whether the camera was upright or rotated 90 degrees due to the direction that contrast is enhanced. I'd be surprised if it was easy to see the difference, but it would be interesting to try it.

    Along similar lines, a few days ago, I saw an image that had a sort of "glow" in the highlights, it caught my eye and I went to look thinking it was going to be a zone plate image, but was just a pinhole. That made me wonder about just adding one or a couple of the zone plate rings ( maybe smaller, "outer" rings ) to add just a bit of that effect to a pinhole image....

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