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Thread: getting started (hopefully0

  1. #11

    getting started (hopefully0

    You could, in theory, obtain a size for the pinhole by very precisely measuring the *brightness* of the image it casts compared to the brightness of the light source used. I have a hunch, however, that you'd get a diameter (after considerable effort and calculation) that was only good to about plus or minus 10%, at best (few light meters are better than 1/3 stop on accuracy).

    You should also be able to obtain a diameter by shining a coherent light through the hole (like a laser pointer) and projecting the image far enough to see the ring-shaped diffraction pattern, and then measuring the diameter of the first ring and distance from pinhole to screen. This ought to be a very accurate method, if you know the wavelength of the laser (not too hard to obtain, as they come in a limited number of choices) and are certain you're measuring the first diffraction ring outside the central peak. I'd expect this to require a projection distance of something like 100 times the "optimum" for imaging, in order to spread the diffraction pattern enough to be easy to measure -- so for a hole around 1/100 inch diameter, you'd need to project it about 16+ feet.

  2. #12

    getting started (hopefully0

    Interesting...

    My first thought was, you're going to get diffraction, so how would you measure a diameter? Then I realized you'd only get pronounced diffraction in the vicinity of the 'ideal' distance. At grossly short of that you might be able to apply image and object and magnification formulae to figure something out. But at very short distances you have the Fresnel diffraction case rather than Fraunhofer.

    I like I-M's idea of a known monochromatic wavelength because you eliminate a lot of unpredictability with light spectrum.

    But to figure out where the minima should be at a given distance, you will be in math up to your ankles already...did you know that the spacing of the diffraction minima are different for circular aperture vs. square (and I assume slit)?

    DIffraction math is so unwieldy that the textbook examples are pretty much the only ones that work out cleanly...and that's based on a number of assumptions and simplifications...you get burned one way or another...Bino, I think, pointed out the microscope photos I took of laser-drilled and electroplated Veco grid apertures were all egg-shaped...is that Heisenberg's Uncertainty Principle? (Which translates to 'Why would you expect better results than the piece of crap instrumentation you used?)

    My brilliant contribution would be to put the mystery apertures in an envelope with a self-addressed stamped return envelope and send them to a friend with the ability to give you reasonable measurements!

  3. #13

    getting started (hopefully0

    Quote Originally Posted by murrayatuptowngallery
    Interesting...
    . . . Bino, I think, pointed out the microscope photos I took of laser-drilled and electroplated Veco grid apertures were all egg-shaped...is that Heisenberg's Uncertainty Principle? . . .
    This suggests a great way to make the elliptical pinholes that are optimum for some photography, such as the oatmeal can type. Gee, we may have to specify major and minor diameters when ordering custom pinholes.

  4. #14

    getting started (hopefully0

    :O)

  5. #15

    getting started (hopefully0

    The chap who originally started this thread seems to have succeeded or given up so I will take it up cause I am in exactly the same position. I have done all that has been recommended here (except for the Einstein stuff) and have a no 10 needle hole in a thin brass plate (f90) tapped to a thin piece of card which is then taped to my 35mm camera body cap hole. All is tapped all over (except the holes) with electrical tape and then tapped in position with tape. I used the aperture to film plane distance as 45mm as suggested above.

    What I was expecting was: unsharp in the middle less sharp at the edges; dark at the edges less dark in the middle. What I get is: even illumination all over and out of focus all over. I am distinguishing between out of focus and unsharp or soft. There seems to be a difference to me. Soft is OK, quite nice, but I seem to be getting out of focus. I have tried two different thicknesses of brass and tin foil but this makes no difference.

    What should I try now. Play around with the aperture to film plane distance by packing up with more or thicker card. Also I thought pinhole size only determined aperture. But it is suggested above that it also affects sharpness profile?

    Thanks. Trying to get this right before I try soemthing bigger.

    Steve Price
    Attached files

  6. #16

    getting started (hopefully0

    35 mm at about 45 mm gives you pretty much a normal perspective with even lighting...you are using a small fraction of the circle of coverage of a pinhole...and you can change it much with th e35mm body construction. You can set it back further into the body possibly with other mounting means until you start to see the camera body itself.

    There is an 'optimal' distance for a given hole diameter, or conversely an optimal hole diameter for the spacing you have it at.

    Have you tried the on-site pinhole calculator for a start?

  7. #17
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    getting started (hopefully0

    According to my chart, a #10 needle is .46 mm in diameter, which is bigger than optimal for 45 mm pinhole to film distance. Everything else is working fine. Try a smaller hole, and you will see a sharper image.

    Nick Dvoracek has been posting some very nice images taken with a 35 mm matchbox camera (shorter than 45 mm) with a .15 mm electron microscope pinhole. .30 mm will give you good results - that is the optimal pinhole size for 45 mm according to Lord Rayleigh (use a constant of 1.9 in Pinhole Designer). Using a constant of 1.5 in Pinhole Designer for 45 mm gives a .236 mm pinhole., which should give you about the best sharpness in the center of the image that you are going to get with pinhole.

    http://www.pinhole.cz/en/pinholedesigner/

  8. #18

    getting started (hopefully0

    Steve,

    I'm thinking that f90 is a bit large. Maybe try a smaller needle. That #10 is about .020" and you probably want something closer to half that size. If I want a hole smaller than the needle I have I just don't push it all the way through. I have a loupe to check the hole with -- often sanded material builds up in the hole so I either blow it our with compressed air or wash it off and check it.

    If you're wanting that vignetting towards the edge then that comes from the image circle projected by a given focal length. Wider angle tends to accentuate that effect. It might be a bit hard to get that effect with 45mm on a 35mm camera. And you can't get a whole lot closer than that because you have a mirror to contend with. Trying to achieve the image we want is what drives us to all this experimentation andcrazy camera building and modifying.

  9. #19

    getting started (hopefully0

    Very helpful advice.

    Made a smaller hole with the no10 needle by only using the tip and not the shaft. Exposure 30secs instead of 6secs. Much sharper image. Jubilation. See attachment - same boring subject for consistency and comparison.

    Earl, are the needle charts you mention on the web? I figure if I know the needle size I needn't try to measure the hole. Thats the mistake I made with the first picture, I guessed the size of the hole.

    My copy of "How to make three corrugated 8x10 pinhole cameras" arrived today. So starting to think big. I will do some more experiments with 35mm first though and post some shots.

    Any other advice/tips gratefully received.

    Regards. Steve.
    Attached files

  10. #20
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    getting started (hopefully0

    I did find a needle size chart on the web, but I don't remember where. Tom Persinger has a card with needles and pinholes on it, if I remember correctly. Here is a snip from my own personal pinhole spreadsheet:

    needle size dia. (mm) dia. (in)
    16 0.25 0.010
    14 0.3 0.012
    12 0.35 0.014
    11 0.41 0.016
    10 0.46 0.018
    9 0.53 0.021
    8 0.61 0.024
    7 0.69 0.027
    6 0.76 0.030
    5 0.86 0.034
    4 0.94 0.037

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