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Thread: Minimum focusing distance

  1. #1

    Minimum focusing distance

    Hi all,
    I said I will ask a lot. Then, how do you calculate how much you can get close to the subject and get (pinhole grade) focus? Or everything will be in (pinhole grade) focus all the time. I don't think so, really...
    Thanks in advance.

  2. #2
    It depends on the camera, but most pinholes have good enough resolution close enough that it's never a problem. Here's an image of a tomato slice (backlit) taken 25mm from the camera:


  3. #3
    I've been reading some pinhole theory and got confused. What your focal lenght and pinhole size?
    Amazing shot, by the way.

  4. #4
    500+ Posts earlj's Avatar
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    In every pinhole image, everything in the field of view is in (pinhole grade) focus. Infinite depth of field. There are many posts on this site that complain of the dust on filters placed either in front of or behind the pinhole showing up in focus on the image. Having said that, the calculations for the optimum pinhole for sharpness change when the distance between the pinhole and the object is close to the distance between the pinhole and the film. When people refer to focal length (or focal distance) they are referring to the distance between the pinhole and the film. Here is a paper that proposes some calculations for optimal pinhole diameter for various object to pinhole and pinhole to film distances. Note that there are many people who don't agree with these calculations completely, but the gist of it is that the closer to the camera your object is, the smaller the pinhole that you can get away with before diffraction starts to blur your image.
    "a squid eating dough in a polyethylene bag is fast and bulbous, got me?"
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  5. #5
    500+ Posts dvoracek's Avatar
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  6. #6
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    Hi Ricardo,

    besides the pinhole diameter, "focal" length and subject distance, sharpness also depends on the wavelength (color). As far as I remember, user nicknamed renon performed series of practical tests with various pinhole diameters, and the results roughly corresponded with the Prober-Wellman paper mentioned above. You can find his posts in this forum.

    The world is not black and white. It often looks good in grayscale, though.

  7. #7
    Great. Food for taught. Thank you all for the valuable input.

  8. #8
    500+ Posts Ned.Lewis's Avatar
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    I made a coffee can pinhole camera recently for macro. FL about 4 inches and the pinhole is 0.15mm. I think the Prober-Wellman equation has that an optimum for a subject distance of somewhere between 1-1/4 and 1-1/2 inches, but I have used it much closer than that, down to 1/4 or 1/2 inch. The range of "sharper focus" than a normal optimal pinhole extends out quite a ways, several feet at least. Subjects in the far distance are "fuzzier" than with a pinhole optimized for infinity, but what I find interesting is that they look soft in a different way. The softness produced by a very small pinhole looks different from one that is a little too big or a little too small. It looks different from the normal "pinhole" softness too, I think. It makes me wonder if there are different "kinds" of diffraction...

    Just as some photographers pay attention to "bokeh" and the character of the out-of-focus area, I've been wondering if different kinds of "non-optimum" pinholes could be used intentionally ...
    Some photos: Ipernity
    ( pinholes and solargraphs mixed in among the rest)

  9. #9
    "It makes me wonder if there are different "kinds" of diffraction...
    Just as some photographers pay attention to "bokeh" and the character of the out-of-focus area, I've been wondering if different kinds of "non-optimum" pinholes could be used intentionally ..."

    I truly think you are on to something that bears closer scrutiny and has not been addressed thoroughly in the past. You are most correct in your observations of the subtle differences.
    Sam H.

  10. #10
    I don't know about different kinds of diffraction, but there are, in general, two sources of image softness with pinhole: diffraction effects and geometric effects.

    In my simplistic, non-scientific way of thinking about it, I consider diffraction to happen to the photons that get within about 1 wavelength from the edge of the pinhole. Think of this as a donut-shaped zone around the edge of the pinhole. Conversely, the photons passing through the central portion of the pinhole - the hole in our donut - don't get near enough to the edge to be affected by diffraction.

    As the pinhole is made smaller and smaller, the donut diffraction zone's area begins to outweigh the central donut hole area, since the assumed wavelength of light is a constant.

    But if you make the pinhole larger and larger in size, the diffraction effects shrink to insignificance while geometric effects loom larger, making the image soft.

    The optimal pinhole size is the best compromise between these two effects, balancing geometric softening and diffraction softening of the image.

    I suspect these two image-softening effects - diffraction and geometry - have subtly different effects. For one, diffraction is known to produce interference patterns, and it's also a wavelength-dependent effect, while geometric softening doesn't create interference patterns and is wavelength independent.

    "There was just that moment and now there's this moment and in between there is nothing. Photography, in a way, is the negation of chronology."-Geoff Dyer, "The Ongoing Moment"
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