Pinholes and Wire Gauge Drill Bits

[sean_yates]

Has anyone any experience using wire gauge drill bits to make

pinholes for pinhole photography? What advantages and disadvantages

are there? Would pop can material work better than the usual

disposable aluminum cookie sheet?

 

Consulting the detailed chart in Renner's book for focal lengths from

10 to 1000 mm, there seem to be a number of bits in the #80 to #60

range that would produce holes of close to optimum diameter. Would

using the appropriate drill bit and a pin vise create the "wrong"

sort of hole - i.e. more of a tube or tunnel in the material than a

shallow "pin prick" style? Has anyone tried this?

[j.w.]

Sean;

 

Rather than comment directly on the use of wire drill bits, which I have not personally used, allow me to describe briefly the method I use to make pinhole apertures.

 

I use .002" thick (thin) sheet brass. I choose the smallest sewing needle I can find. For pinholes in short focal length cameras, you typically have to go smaller than what the full shaft diameter of a sewing needle will allow, even with the smallest needle.

 

I place a 1" square piece of sheet brass on a hard, clean surface. The sewing needle is inserted eye-end first into a small block of soft wood, to use as a tool without damaging your fingers. The needle point is carefully placed in contact with the center of the brass, and the square of brass is rotated, by flicking the corners of the square with your free hand, while the needle is held perpendicular to the surface. Note that the needle is not turned. Doing so usually "wallers" out the pinhole, because it is difficult to "drill" with the needle perfectly perpendicular to the brass surface. Pressure is applied with the needle, the amount of which is determined best by experience, and by how hard the backing surface is.

 

I stop periodically and examine the sheet brass carefully under a good loupe. When you begin to see a dimple on the bottom side of the brass, you're getting close. Carefully reposition the needle point back into the exact same dimple, and continue the rotation of the brass, and pressure on the needle, until a small hole protrudes through the back of the dimple.

 

Next, using 600 grit emory cloth, carefully polish down the dimple by placing the square of brass dimple-side down on the emory, and using a circular motion and carefull pressure do a few circles of polishing motion. Then, re-examine the pinhole carefully with the loupe, to ensure the hole looks exactly circular, clean and with no burrs or other defects. Do not attempt to completely remove the dimple. Rather, your goal is to polish off the tip of the dimple, where the burrs will be from the needle point poking through the brass. No matter how sharp the needle point, you inevitably have some tearing of the metal; that's what the emory is used to polish away.

 

How do you know when the hole is big enough? First, you can make an approxiate guess at the hole size in relation to the full shaft thickness of your needle, by examining both simultaneously under the loupe. For instance, if you need a hole .025" wide, and your needle's shaft is .050" wide, then stop the hole-making process when the hole is about 1/2 the diameter of the needle.

 

For a more accurate pinhole diameter measurement, an enlarger can then be used, thusly:

 

First, take a metric ruler and tape it to the base of your enlarger. Next, make a mark exactly 1 cm (10mm) long onto clear plastic. You can use the plastic �foils� used to make overhead projector transparencies. Cut out the piece of plastic; put it into your enlarger in the place of the negative. A glass "sandwich" type of negative holder works best, because you can get the plastic line image and piece of brass at the same focal distance. Next, adjust the height and focus of the enlarger so the projected image of the line is covering a known distance on the metric ruler. For instance, the 10mm line may be projected onto 300mm of the ruler. Note this distance measured. You have just determined a calibration ratio, in this case "10/300".

 

Next, without changing the enlarger�s height or focus, replace the piece of plastic with your brass pinhole. You will be able to take the image circle of the pinhole as projected onto the base of the enlarger, and measure it's diameter with the metric ruler. Using the calibration ratio you just measured, you will be able to calculate the diameter of the pinhole. For instance, given the above example of 10mm (real) equals 300mm (projected); let�s say the circle of light measures 15mm across. Multiply the 15mm by the ratio of (10mm/300mm); that is, divide by 30. The pinhole is .5mm in diameter.

 

I find that the skill involved in making "good" pinhole apertures is another rewarding part of the craft of homemade camera making. I would not purchase a so-called "laser drilled" pinhole, not because they are necessarily deficient, but the idea of paying money for what I can fashion myself is against my own personal vision of why I do this for a hobby.

 

Regarding the making of pinhole apertures, a few more thoughts are relevent. First, the issue of light fall-off towards the edges of the image. This happens due to three reasons. The first reason is that the distance from aperture to the corners of the film is longer than from aperture to center, hence the light falls off at a cosine^2 function.

 

Next, when viewed from the corners, your (ideally) 2-dimensional pinhole no longer appears circular, but elliptical. Thus, the apparent diameter of the pinhole decreases as well, along with exposure. The third reason for light falloff depends on the "wall thickness" of your pinhole (how much of a tunnel it is), and how deep your dimple is. You want to use as thin a sheet metal as possible, to keep the tunnel effect to a minimum. Excessive wall thickness, and deep dimple, will exaggerate the elliptical effect out near the corners of the image even more.

 

Another issue concerns the ideal pinhole diameter. Most formulae for calculating the ideal pinhole size are based on the Raleigh criteria, which states that for a given wavelength of light, and distance from aperture to image plane, there will be an optimal diameter which balances the image-sharpening effects of pinhole geometry (the particle nature of light) with the image-degrading effects of diffraction (the wave nature of light). Most formula I have seen on the internet use a value for light's wavelength corresponding to the green part of the visual spectrum, the reason being that our eyes are most sensitive at that wavelength.

 

The problem is that your paper or film negatives aren't necessarily peaking their sensitivity at the same wavelength as your eyes are. You need to use a pinhole diameter optimized for the film medium you've chosen. Panchromatic film, being more blue (short wavelength) sensitive would dictate a recalculation of the Raleigh criteria, and hence a smaller wavelength. Rather than try to work out the math to the N-th degree (or strain the fly-sh*t from the pepper, to use a more colorful metaphor), I merely reduce my optimally calculated pinhole diameter about 20%, for good measure. Its kind of like making a good pot of tea. One spoon for each person being served, plus one for the pot.

 

One other item regarding pinholes. I'm no math whiz, but I suspect the Raleigh criteria is optimized for objects at infinity. For subjects closer in to the pinhole, I suspect one should make the ideal pinhole diameter smaller still.

 

Finally, I've thought about the image degrading effects off-axis, near the corners of the film. Aside from decreased exposure, you would expect diffraction effects in a radial direction from the center of the image, because when the pinhole aperture becomes an ellipse out near the corners, the short axis of the ellipse is now smaller than the optimal aperture you've calculated. In the circumferential direction (in a circle around the edge of the film), you should not see diffration effects, because the long axis of the elliptical aperture is still equal to your optimal pinhole diameter.

 

Most of my cameras have been large format, with angles of view close to the "optimal", which means the diagonal of the film format is about equal to the focal distance. For 35mm, this is about a 43mm focal length. The term optimal is used merely because the human eye supposedly has this relationship between retina width to lens focal length. Also, I used square format, but that's just my personal vision. I do not have any noticable image-darkening in the corners of the print with this chosen angle of view. For wide-angle cameras, however, you will have to accept the light fall-off as part of the properties of the medium you've chosen.

 

I hope this rambling reply is helpful to some extent.

 

Joe VanCleave