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  1. #1

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    Impact of non-circular pinhole?

    What happens if your pinhole is not a very perfect circle, or more interestingly if it has burrs or is some other shape like a square?

  2. #2
    SMBooth's Avatar
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    Not a lot from what Ive seen, maybe some different highlights shapes. Burrs which protrude and effectively increase the thickness of the pinhole ( like using a thicker material) could alter things and make for softer images.

  3. #3

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    Not particularly relevant, but at the size a pinhole should be, if you can already identify the shape, isn't it already too big?

  4. #4
    Aron's Avatar
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    it's easy to determine the shape (and size, thus effective ƒ/stop) of a pinhole using a scanner or a microscope. Regarding how much it matters, well, I used to have a 4x5 pinhole camera that had a pinhole with an assymetric and rather undefined shape (crude needle job) and the negatives were good for contact prints and quite acceptable for moderate enlargements. In the most simple case, assuming an infinitesimally thin plate for the pinhole, the distribution of corner vignetting will directly follow the shape of the pinhole.

  5. #5

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    So you might be able to do a heart vignette.

  6. #6
    Nicholas Lindan's Avatar
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    You can also examine a pinhole with an enlarger - put the 'hole where the negative goes and crank the head up. Or you can use a slide projector. Or put the 'hole at the film gate of a 35mm, put it on a tripod, aim it at the wall 10 feet away and illuminate the 'hole with a suitable lamp (60x magnification). Etc., etc..
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  7. #7

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    Using a micrometer to measure by eye (difficult to do) I'm pretty confident that the hole is between 0.25 and 0.3mm. Comparing with a simple ruler at least seems to show that it's significantly smaller than .5mm and it looks at least around 0.3mm on the meter, so I don't believe it's terribly big or at least not more than maybe 25% larger than I was shooting for.

    Anyway, I have a 10x loop and a basic magnifying glass (and I'm very nearsighted... one of the few times it comes in handy) so it's not like I can't get some idea.

    It's still hard to see the exact regularity of the hole of course, but if I hold it up in front of a lamp in the distance I can see a sort of irregular sunstar that makes it look like I have burrs inside the hole (despite sanding etc). I'm unsure if this means there are really burrs, or if an irregular sunstar is just some artifact of diffraction in tiny holes combined with an imperfect circle shaped hole.

    So it's hard to tell but right now I'm going for as close to perfect image sharpness (not resolution) as I can get (which is what mrpinhole calculates, I think). Not surprisingly I didn't get there with crude tools but I'm still curious as to what's most important to improve compared to what I have now.

    For now I used a square cut out of the side of a soda can which is maybe twice as thick as cinefoil which is probably around twice as thick as aluminum foil (which I suspect is so thin I'd have problems with it). I also can't really get it perfectly flat. (Perhaps I should have just let it stay bent and tried to drill the hole without deforming it. After all I'm guessing that trying to flatten it might make it more irregular, though I have no idea how the cylendrical bend might cause problems.

    So it sounds like maybe what I need to improve (in addition to my tools) is the material thickness and perhaps burrs, or at least get more confidence that I don't have burrs. Maybe I should try the electrical zap method on foil? (I do have some capacitors sitting around, hopefully enough total farads to do it.)

    Is half the thickness (such as Cinefoil) good enough or should I get something thinner? I think the coke can is like 0.2mm thick and the cinefoil is 0.1 or something. 0.2 is almost the diameter of the hole I'm going for so I'm guessing maybe that's an issue.

    I'm planning to extend this thing out anyway where an 0.5mm hole should supposedly be used so I'm guessing that will make things a little easier.

    To make things more complicated, the instant film I'm using doesn't really have the best fidelity anyway, at least in the print. (I think the cleared negative should be better.) "Resolving power" is listed as 15 lines/mm. Don't know if that's achievable with a pinhole this size anyway though, especially if not optimized for resolution.

    Anyway, main question now is: Is the material thickness likely to be the biggest flaw in my current pinhole?

  8. #8
    Nicholas Lindan's Avatar
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    The hole will need burnishing with a needle after sanding. A little -very- light twist is all it needs. Then an itty bit of very fine sanding, then another even lighter twist. For inspection try mounting the hole in your enlarger in place of a negative and examining the projection of the hole on the easel - it is the best way that I have found. Ideally the hole has a knife edge to it, but things are never ideal: to examine the smoothness and regularity of the side of the hole examine it with a loupe and glancing light that reflects off the sides.

    The 'sunstar pattern' you are seeing isn't from the pinhole - the pattern is caused by the lens in your eye.

    Pinholes work best at short focal lengths - 40%-50% of 'normal' is about right.

    As to material, I've found 'coke-can aluminum' to be about the best: it is a good hard high grade of aluminum alloy. I roll it backwards around a tube to flatten out the curve. Next best is pie-pan aluminum - a bit thicker and flat, but it is softer so it is harder to get a really nice hole without tearing it.

    The method (I am sure you know it, but repeated here for anyone reading the thread who is just starting out) is: Sand away the paint and lacquer. Then dimple the metal with the rounded tip of a 'knit fabric' needle. Do it against a hard(ish) material like chipboard. Then sand the dimple down - 600 or 1200 grit sandpaper flat on the table, move the metal over the sandpaper - to create the hole. That way you have sanded a (hopefully) knife-edged hole. The thickness of the material doesn't really matter all that much - if it is too thin it can't be sanded without tearing, it is too thick it takes forever to sand down. After sanding, burnish as above.
    Last edited by Nicholas Lindan; 07-06-2012 at 05:37 PM. Click to view previous post history.
    DARKROOM AUTOMATION
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  9. #9

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    Actually I hadn't heard that exact method but I may not have thoroughly read the web page that I was reading. (Something about using a tiny sewing needle to drill the hole with a pin vise. I remember sanding but not dimpling, just drilling the hole with the needle.)

    I went out and bought some goodies this afternoon including an assortment box of sewing needles and a Dremel tool. As an experiment I put the thinnest, sharpest sewing needle in the dremel's chuck, turned it on at 20,000rpm, and without any sort of modification it did a really nice job of drilling some really nice round (aside from the burrs) holes of various sizes in a soda can I tried it on. I also ordered this thing online that is basically a drill press sort of holder for dremel tools, so perhaps I'll see what I can do using that and various grinding bits.

    Using a Dremel drill-press-style might be overdoing things a bit but I keep running into other stuff that I need a dremel type tool for, so I might as well see what I can do with it for pinholes since I have the thing now.

    I also have these old metal Polaroid pack film cartridges that I seem to end up with every time I buy an old Polaroid or Polaroid film back. They have some nice flat sheets of metal in them behind where the negative goes, but they're kind of thick. (Double or more the thickness of soda can aluminum.) Extra thickness might not hurt though if I try to use dremel tool since it ought to cause any sort of drilling or grinding to go more slowly and thus carefully. Not sure what metal they used for these old cartridges though.

  10. #10
    SMBooth's Avatar
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    You want the hole as thin as possible. Drill your hole then sand with 1200 gritt wet and dry paper.

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