Making a pinhole body cap

Discussion in 'Beyond the Basics' started by TWX, Jan 21, 2020.

  1. TWX

    TWX No longer a newbie, moving up!

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    I ordered a set of five third-party body caps, the intention is to make a pinhole body cap for EF/EFS.

    So if I did the math for non-macro right, the hole size is as such:

    pinhole diameter = SQRT(2.44 · focal length · wavelength)

    Center of the visible light spectrum is around 550nm, or 0.00055mm. Flange distance on a Canon EOS is 44mm, presumably to the back side of the body cap. This yields a pinhole size 0.243mm.

    Assuming the body cap is around 4mm thick, the focal length becomes 48mm, this means pinhole is 0.254mm.

    The 2.44 number came from a youtube video describing balancing the problems of too-small and too-large pinholes. It applies when the subject is sufficiently distant that as a divisor it approaches irrevelance, versus requiring consideration for macro photography.

    So, my game-plan:

    Frst I'm going to take some measurements of the body cap and some general measurements on the camera, confirm my numbers, and attempt to find-center on the body cap.

    Second, I'm going to look through my drill bits including lathe center-drills, see if I have anything whose point is anywhere close enough. If not then I may have to order something. Grainger has small drills in the ~0.250mm size, as low as $10, and there are whole sets on Amazon that are inexpensive with a variety of sizes.

    Third, I'm going to determine if I can make a pinhole accurately in the bare plastic body cap itself, or if I need to make the pinhole in another material and set that into the body cap. I'd rather not use a separate material if I can avoid it.

    Fourth, I'd rather hog-out the cone of material from the inside of the body cap (hence the ~48mm calculation) such that the only thing visible on the front of the body cap is the tiniest of pinholes, so probably use some kind of counter-sink bit to drill down to just below the outer face of the body cap, if I am avoiding using a separate material. If I mess this up I can always make the hole bigger and then use separate material.


     
    Last edited: Jan 21, 2020
  2. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    You can buy a disk with a pre-drilled laser hole. Just drill a larger hole in a body cap and carefully epoxy them together. Saves a whole lot of time and aggregation.
     
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  3. TWX

    TWX No longer a newbie, moving up!

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    What would be the fun in that?
     
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  4. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    As soon as the epoxy sets, you can start shooting. If you try to drill your own hole, you're taking a huge chance the hole won't be ideal and you'll be spinning your wheels.
     
  5. TWX

    TWX No longer a newbie, moving up!

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    Now, if this works, I might look at how short a focal length I can make within the confines of what the mirror movement allows. I have an EOS film camera, I can measure how far the mirror is from the flange, and likewise do the same with my older Digital Rebel XS. This will tell me how much further into the camera body I could set an aperture without mirror-strike, and determine what it would take to make a cone that would reach that far. I might see if the plastic of the body cap is thermoplastic or if it's thermo-set plastic. The latter doesn't melt anymore and thus its shape is fixed, but the former can be heated and deformed, possibly allowing the body cap center to sag-in towards the focal plane before performing the same machining steps to create the pinhole.

    If it's Thermo-set plastic then I'll need to set a cylinder or cone of the right material into the body cap and then machine into that.
     
  6. TWX

    TWX No longer a newbie, moving up!

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    In this case the journey is part of the fun. If I always insisted on an ideal hole I'd've passed-up on a lot of fun along the way.
     
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  7. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    You need exceedingly thin material for the hole. Try drilling an actual round hole in aluminum foil. It's an exercise in frustration.
     
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  8. Derrel

    Derrel Mr. Rain Cloud

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    Making a pinhole body cap sounds like $50 worth of fun for a $5 made in China cap drilled / burned with laser precision. Eagerly awaiting progress reports. LOL.
     
  9. TWX

    TWX No longer a newbie, moving up!

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    I know. To create an image circle large enough for say, 35mm film, I'd need that circle to be around 45mm. 45mm at a distance of around 45mm means the thickness of the hole needs to be the same as the diameter of the hole.

    That said, if I drill the pinhole through several mm of plastic first, then start counter-sinking, I might be able to counter-sink down to where I need to be. We'll see. If not then I can just drill through an appropriate material affixed to the body cap. Trying to make the hole in just the plastic first doesn't invalidate the possibility of mounting something to it if the precisionness fails.

    Besides, it would be pretty cool if I can make it work, to have what looks like a body cap and has only the absolute tiniest of holes a quarter of a millimeter in it, providing a camera obscura effect.
     
  10. Derrel

    Derrel Mr. Rain Cloud

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    I am interested in the home hernia kit. I figure I can do as well as some surgeon with specialized tools. Give me a bottle of whiskey and a pocket knife and some duct tape and I'm good to go
     
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  11. Derrel

    Derrel Mr. Rain Cloud

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    20200121_203601.jpg

    $1.90 for shipping! I think that is outrageous, I mean gasoline is $3.13 a gallon!!!
     
  12. 480sparky

    480sparky Chief Free Electron Relocator Supporting Member

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    I think I bought my pinhole for $6 from ebay. Another couple bucks for a knock-off body cap, a dab of epoxy and I was ready to go.
     
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