Calculations for pinhole projector

Discussion in 'Alternative Techniques & Photo Gallery' started by PlasticSpanner, Oct 5, 2005.

  1. PlasticSpanner

    PlasticSpanner TPF Noob!

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    I am making a projector screen that is very similar to a pinhole camera and would just like someone to confirm my calculations.

    I have a focal legnth of 1265mm and a pinhole of 1.5mm. Will the pinhole be the right size to get a reasonably sharp image and if not how much will I need to change the focal legnth by to use 1.5mm (Hole already machined in a peice of brass!)
     
  2. terri

    terri Administrator Staff Member Supporting Member

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    Gosh, Chris, I have no idea if you're on target here or not. I'll give this thread another day here, but would be glad to move it to the general forum if you'd like - it might get some more looks and responses there. :)
     
  3. PlasticSpanner

    PlasticSpanner TPF Noob!

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    Thanks Terri!


    I suspect this is going to be down to trial and error so I've had a few "pinholes" made up by my engineer friend!
     
  4. terri

    terri Administrator Staff Member Supporting Member

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    Awesome - hope it works! Post your results here so we can see, ok? :D I love the look of pinholes, just one more thing on my "get around to someday" list. ;)
     
  5. Hertz van Rental

    Hertz van Rental TPF Noob!

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    It is possible to calculate the optimum diameter of a pinhole for a given focal length.
    Either side of this optimum - a larger hole will give a brighter but less sharp image; a smaller hole gives a dimmer, less sharp image. The smaller hole gives an unsharp image due to difraction.
    K = optimum diameter of pinhole
    v = distance of pinhole from film plane

    K = square root of v divided by 25

    You can then work out the f-no from v/K
    If you know the speed of the film you are using it then becomes a simple matter to take a light reading and get the exposure time.

    The best way of making pinholes is to get a sheet of brass shim. Place it on a flat surface with a thin sheet of cork or cardboard between (about 1 - 2 mm thick).
    Use a ball punch or any round object of about 10mm diameter. Use this and a hammer to punch a dent in the brass sheet.
    Turn the sheet over and use a fine file to smooth the top of the 'dome' flat and also thin down the brass shim at this point. Takes practice.
    You can then make a precise pinhole in the centre of the dome through the thin piece. A hypodermic needle is the best thing to use as they come in a range of precise diameters.
    It is usually easier to make the pinhole in this way first. Knowing the diameter of the hole, use the calculation above to find the optimum focal length, v.
     
  6. PlasticSpanner

    PlasticSpanner TPF Noob!

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    Strangely enough this is what the engineer came up with & he has nothing to do with photography! :D :confused:

    This bit I could have done with knowing before we started! :lol: Our solution was to use a hardened needle tip in a pin vice on a lathe and offset it from centre to score a tiny circle in the thin bit of the brass! By measuring the offset he is able to produce a reasonably accurate hole!
     
  7. Hertz van Rental

    Hertz van Rental TPF Noob!

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    I stole it from an engineer :lol: It's how you make technical pinholes for the little pinhole cameras they used to stick inside engine cylinders to photograph the wear.
     
  8. PlasticSpanner

    PlasticSpanner TPF Noob!

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    Ahh! So that's probably where he's got the idea from too!:lol:


    A site on the net has a calculator to work out hole sizes and optimal focal lengths but I'm not sure how accurate the data is!

    Apparently my projected image is just over 2400mm in width which may be lighting up the inside of the camera, fogging my picture so I may have to use a few baffles to effectively crop the image near the mirror.
     

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