Making a pinhole body cap

[TWX]

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.

 

[480sparky]

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.

 

[TWX]

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.

What would be the fun in that?

 

[480sparky]

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.

What would be the fun in that?

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.

 

[TWX]

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.

 

[TWX]

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.

What would be the fun in that?

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.

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.

 

[480sparky]

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.

What would be the fun in that?

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.

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.

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

 

[Derrel]

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.

 

[TWX]

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

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.

 

[Derrel]

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

 

[Derrel]

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

 

[480sparky]

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.

 

[stk]

I made a pinhole cap many years ago. Drilled a large hole in the cap, cut out some aluminium from a soda can, used a pen to make an indent on the aluminium and used fine sandpaper on this dent until I had a small hole. Then just tape the aluminium to the cap.

Maybe I was lucky, but it worked fine on the first try.

 

[weepete]

0.25mm is very small, you'll need a very light touch not to snap it just with the pressure and have almost no feedback. You'll also need to be careful you don't load it and jam the flutes with swarf, multiple small touches would be the way to go.

 

[petrochemist]

The formula there is for the optimum diameter of the pinhole. Whichever size you make 'focal length' will be determined purely by how far the pinhole is from the sensor (with a 4mm thick pinhole as you describe it will be the whole range from 44 to 48mm). Body caps are far too thick for reasonable results, the pinhole should be in something as thin as practical.

In most cases the wavelength range is big enough that you'll be quite a way from optimum for some of the image whatever you do. I've never tried imaging at a 'single' wavelength it might give much sharper results than we are used to for pinholes, but there's little enough light to deal with as it is.

Even the smallest drills I have are bigger than the optimal size at body cap focal lengths but they do work in a very crude way. I did this ~10 years ago with a drill bit between 0.35mm & 1mm in diameter (probably the smaller end but I'm not sure) direct in the body cap. Hand holdable 'images' were possible with the pop up flash at ISO 1600 - the subjects were recognisable but less so than on the CCTV footage show on TV cop shows! Vignetting & distortions from the imperfectly drilled hole were very noticeable - I didn't quite get it central either.

If you want to 'drill' your own the normally recommended approach is to flatten out a piece from a drinks can, make a dent in it with a needle, then sand (finest available wet & dry) the rear side gently till the raised section just brakes through. Then fit this metal pinhole to a body cap with a larger hole drilled in it (1/4" to 1/2" works well). (EDIT: I see @stk has already described this technique - I should have read page 2 replies before posting) I couldn't be bothered to go through this process so brought a cheap laser drilled on instead, for my MK2 (then added a lensbaby version to try the 'zone plate' variant as well).

I've only used them on a handful of occasions, on an APSC DSLR, and an IR converted MFT body, both were too soft for frequent use. At some point I'll give it a go on FF, probably mounting via a helicoid to give a zoom control. Pin holes are one place where sensor size makes a huge difference, at some point I'll try them on my 5x4. Diffraction at f/>128 doesn't totally ruin the image on that, the zone plate probably falls in the aperture range used by many LF photographers - I think it was around f/64