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Photography

The Definitive Guide to Pinhole Photography: Part 1

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We all love digital and its versatility, speed and convenience. However, if you're like me, sometimes you yearn to go back to something more pure and simple. What if we go all the way back? No glass, no dials. Just a box, a hole, and a sheet of light sensitive paper. The purest form of photography possible. Sunday, April 28 is Worldwide Pinhole Photography Day, and we intend to prepare you for it.

To celebrate Worldwide Pinhole Photography Day, we'll learn about three different styles of pinhole camera and how to build them yourself for less than five bucks. We'll only be using foam board (corrugated cardboard would also work), heavy duty aluminum foil and duct tape for the materials. These are all available in dollar discount stores.

In the spirit of the 19th century experimental photographers, we also will cover a lot of theory so you can design your own camera and really understand how it work. That being said, you can skip straight to the build section if you'd like.

You'll need a pencil, ruler, set square, and craft knife for the marking and cutting.


1. Get the Paper

Although pinhole camera can be made to use film and even digital sensors, ours will use photo paper. Not the kind you run through a printer, the kind used in a darkroom. This paper is relatively inexpensive, but you'll probably need to order it online unless you have a really good photography shop in your town. Do not open the box unless you're in complete darkness. That's how you'll have to load your camera.

The paper comes in all different sizes. The following measurements in this tutorial are based on the 7x5" black and white multigrade paper I bought. It's exactly 178x128mm. Ilford paper is a great choice, but you can other less known brands for about half the cost.

I found that it's difficult to grade this paper at a particular ISO, but with some experimentation I found that 20-50 minutes of direct sunlight darkens the paper to a sufficient degree as to be "readable."

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The paper actually started out whiter, but the shade was still quite bright.
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The simple setup for testing. I left it like this for about an hour.

Remember that because photo paper is significantly larger than 35mm film, your angle of view or "focal length" will need to accommodate for this. This will affect how we build our camera.

A 35mm film frame is 36x24mm, which is 43.3mm diagonally. My paper is 178x128mm, which is 219mm diagonally; a crop factor of 0.1977x. In other words, I have to use just over five times the focal length of 35mm frame-size to get the same angle of view.

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If this wasn't pinhole, that would be crazy shallow depth of field...

Remember when you're loading the paper into your cameras that only the smoother, shinier side is the one that's photosensitive!


2. Understanding Pinhole Cameras

With no optics, pinhole cameras rely on geometry and the propagating properties of light. They don't require glass for focusing, because light travels in a straight line and focuses itself simply by only being able to hit the medium by passing through the pinhole.

The hole provides a single, multi-angle guide to cover the whole photosensitive medium. The light comes from the source, is reflected from the subject, passes in a straight line through the hole and hits the appropriate place on the focal plane.

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Focusing by literal mechanical blocking!

Thus light from the top right of the scene will pass through the hole in a downward direction and hit the bottom left of the medium, and light reflected from the bottom right of the scene will trace a straight line from where the photon bounced off the object, through the hole, to the top left of the medium.

Because the pinhole only allows light from any one part of the scene in front of it to hit the medium in a linear path, it focuses the entire image with virtually unlimited depth of field. It doesn't care whether the light wave came from three inches in front of it or a mile away, as long as it's tracing that straight line. The only circle of confusion is created by the size of the pinhole itself.


3. Mathematica Obscura (Pinhole Size)

The pinhole can really be any size, but ideally as small as possible to minimise the size of that circle of confusion. The smaller it is, the sharper your images will be. However, there is a limit. Because of the wave-particle duality of light, it travels as a wave, and waves can diffract.

Just as you don't want to close down the aperture in a regular lens too much to avoid diffraction, you don't want to make the pinhole small enough for it to start acting as a diffraction grating.

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Decreasing size: First it passes, then it refracts, and eventually it can't get through! (Eg. the grille on a microwave door).

Fortunately, there's an easy way of working out the diffraction limitation. Two ways, in fact, depending on whether you use the Airy disk method or the Rayleigh criterion method. Without going into the physics of diffraction, the Airy disk method gives higher contrast and perceived resolution, and the Rayleigh Criterion gives higher real resolution at the expense of contrast. Since we perceive contrast more sharply than resolution, I'm using the Airy disk calculation:

$$d = \sqrt{(2.44 \lambda f)}$$

Where \(d\) is pinhole diameter, \(\lambda\) is the wavelength of light and \(f\) is the focal length of the camera.

Of course, there are a large number of wavelengths in the visible spectrum, but we're shooting in black and white so I'm going to pick just one: 550 nanometres, a yellow-green color in the middle of the spectrum. This should broadly work for a variety of subjects, particularly foliage.

Let's run through a simple example. First, we'll convert everything to the same unit. Let's use millimetres. Let's say our focal length is 200mm, which is a "normal" focal length when using 7x5" paper. That's already in the right unit. 550 nanometres is .00055 millimetres. Ok, so 200 x .00055 x 2.44 is 0.2684. The square root of that is about .51, so our pinhole should be .51 (or a half) a millimetre.


4. Exposure Calculations for Multiple Day Exposures

The path of the sun in the sky changes from day to day, with low-sensitivity paper you can create what are known as "solargraphs," which show the various paths the sun takes across the sky over the course of time.

Since the sun moves about 15 degrees an hour, exposure time can be approximately calculable using trigonometry:

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How long it takes for a beam of light to expose a particular point

So we know [latex]f[/latex] and \(d\), and need to find the hypotenuse \(x\), this is where Pythagoras comes in:

$$x = \sqrt{(f^2 + d^2)}$$

Once we know \(x\), we can find the angle across a single point on the film plane:

$$sin({\theta\over 2}) = {d \over x}$$

So \({{sin^{-1}}({d \over x})} = \frac{1}{2}\theta\)

\({15\over 2 \theta} =\) the number of points the sun covers in one hour, \(y\)

\(3600 \over y\) = how much time each individual "pixel" is directly illuminated per day, \(t\)

There are 1200 seconds in 20 minutes, if we take the minimum to get good contrast.

So \(1200 \over t\) is the number of days required to get sufficient contrast!

This is invented based on my own experimentation with the paper I bought; if you use higher contrast, more sensitive paper, or even film, then it should come with ISO ratings and the exposure times should drop significantly.

Of course, this is the exposure level for direct sunlight; reflected light from objects is many times dimmer, so you'll need to compensate. If you point your DSLR at the sun and get 1/8000th, then point it at the ground and get, say 1/250th, then you'll see how much more exposure you need to add to make sure objects other than the sky and roads are exposed.

If all of this is way too much technicality for your liking, don't worry about it. Just find a nice small sewing needle of around 0.5mm (0.02") and have at it!

Now that we know where we are with the theory, let's get to the fun building part!


5. Camera One, Flat Back

This is the most simple form of camera to make, just a regular rectangular box. It will portray the scene as a perfect geometric projection; all straight lines remain straight.

Step 1

To get started, I wanted to create a reasonably, but not overly, wide-angle camera, around a 30mm (35mm equivalent) focal length. To convert this to an actual focal length, I used the crop factor in step one and got about 152mm. For some reason, I ended up going with 160mm, which is close enough.

Knowing the dimensions of the paper, the focal length and the thickness of the foamboard allowed me to create a design. Using the bottom as structure and gluing each of the sides onto that meant it needed to be 10mm wider in each dimension, to take the width of each side:

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The typical freehand scrawls I like to call "designs"! You see the idea though.

Step 2

Mark out the lines in the design using a pencil and ruler, and ensure all lines are perfectly perpendicular using a set square. This will pay off later when gluing it together. Mark the number on each piece after you draw it so that you don't get confused as to which similar-looking piece is which later on.

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Careful to keep the blade vertical. Mine kept wanting to skew.

Use a razor knife with a straight edge and a cutting board to cut them out.

Step 3

Once they're cut out, it's time to cut the hole for the pinhole. Find the exact centre of the front piece and mark it. Then draw a circle around an inch or so in diameter around it. I used 30mm.

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Ready to cut out.

To cut it out, take your knife and cut a cross across the circle, then carefully edge around each quarter. It doesn't matter too much how perfect the circle is. If you end up with an octagon, no problem!

Step 4

Now it's time to glue it all together. I used hot glue and had to be very quick. It would probably be easier to use five-minute epoxy or something similar. PVA (white glue) would also work well, but you may have some waiting to do as it dries quite slowly. Don't glue all of the panels together. You'll need to leave either the back or the top loose in order to load the paper into it!

test
The finished box, glued together.

It would probably be a good idea to try to dry-fit it all together before gluing to make sure that it all fits together correctly. Don't worry too much if there's the odd couple millimetre gap here and there though, we're going to fix that in the next step.

Step 5

While foamboard is cheap, light and reasonably strong, it's not lightproof. Just hold it in front of a window. So we're going to use aluminum foil to wrap it in, ensuring that no light can get through the body itself.

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Lightproofing time!

The easiest way to cover it is more or less how you'd wrap a square gift in wrapping paper. Just secure it as you go with small pieces of duct tape.

Step 6

Once that's done, we're going to make it water-resistant and sturdier with duct tape! You can just go nuts here. Wrap it as much as you like in whatever direction is convenient.

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I love this extra-wide black duct tape.

Finally, once that's all done, is to put the hole back in the front where you cut out the foamboard and create the pinhole.

Step 7

For the flat-back camera, I used a 40mm square piece of aluminum roof flashing, but if you don't have any around, you can just use some more of the heavy-duty aluminum foil. Mine was sanded down in the centre with resin-bonded aluminum oxide sandpaper to make it as thin as possible without actually going through it.

pinhole_13
The centre should feel very flimsy compared to the edges

Step 8

I calculated the ideal diameter of the pinhole to be 0.46mm using the Airy equation above (all measurements are in metres):

$$d = \sqrt{(2.44 \lambda f)}$$

$$d = \sqrt{(2.44 \cdot 550\times10^{-9} \cdot 0.16)}$$

$$d = \sqrt{2.1472\times10^{-7}}$$

$$d = 4.63\times10^{-4}m$$

Step 9

To create a hole this small with the smallest sewing needle I could find (0.6mm), I set a Vernier caliper to 0.46mm, and inserted the needle into the jaw. Assuming that the needle would be unable to enter the jaws any further than where it was 0.46mm in diameter, I took note of how long the needle was from tip to the top of the caliper jaws.

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Admittedly not everyone has a pair of calipers, although digital calipers are cheap these days, and very handy.

Using this method, I knew how deep the needle had to be inserted into get the right size whole.

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Using a tiny 2oz ball pein hammer to gently tap the needle through.

The result was a pinhole that when I eyeballed it with the caliper came out around 0.45mm. Close enough!

Step 10

Once your hole is punched, you need to take the finest grit emery paper you can find and gently sand down the hole to remove any burrs from the punching. These will act as secondary apertures, creating diffractive effects and softening the image. The hole needs to be as smooth, flat and circular as possible.

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Holding the result!

Step 11

Once you're done, tape the pinhole assembly to the inside of the camera, with the pinhole as close to the centre of the hole as you can manage. I also added some light-proofing flaps around the lid to reduce light leaks and improve rain resistance.

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This is where this camera has been for around ten days. Results in part two!

You're done! Time to go shoot!


So Far, So Good

Now we've covered the theory of how pinhole cameras work and the various physical concepts controlling them, as well as the practical workflow of building a basic box camera, you should be able to get started on your own creations!

That's it for the first installment. In the second part, we'll continue the pinhole fun, covering more camera types and how to get your images onto the computer to share.

Questions? Comments? Hit up the comments below!

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