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I've been interested in telescopes and optics since I was a small boy, but until recently I haven't bothered much with the simplest of all possible optical devices, the pinhole camera. Recently Pixar Animation Studios held a lunchtime class as parts of its employee education programs on pinhole photography, and I decided to take the plunge and actually do some pinhole photography.
My first three reasonable pictures from this camera are shown below:
| Negative | Positive | Description |
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A forty second exposure outdoors, considerably overexposed. |
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A five second exposure outdoors, considerably underexposed. |
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A four minute exposure of my desk, taken indoors but with lots of light coming from my window. Still a bit underexposed, but better. |
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| Basic layout of our pinhole camera |
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The cameras that we constructed were made from cardboard tubing about 2.625" in diameter. We cut off sections about three inches high, and capped the top and bottom with pieces of foam core cut out with Exacto knives. The tops were made to forcefit, the bottoms were attached with glue and a bunch of black tape. A hole was cut in the side, and a simple sliding shutter fashioned.
We quite arbitrarilly cut our film to 3" by 5". The question which immediately arose was "What is the field of view of this camera?" This turned out to be a rather interesting little problem in simple geometry, with a pleasing answer, so I thought I'd just right it down.
The diagram to the right illustrates our basic camera layout. The pinhole
is marked at P, while the film extends along the arc
ABC. You can determine the angle that the film extends by
comparing the film width to the circumference of the tube. In our case,
our film was 5" wide, and the tube had an inner diameter of 2.6".
This means that the film angle ABC is about 229 degrees. The
half angle AOB is then 114.5 degrees.
There are lots of isoceles triangles in this diagram. Notably the triangle
OAB is isoceles, and we know what the half angle is from our
derivation above. This means that the angles OAB and
OBA are equal. Given that, and the fact that the sum of the
angles in a triangle are equal to 180 gives us a value of 32.75.
We know further that the angle BAP is 90 degrees, which means
that the angle OAP is equal to 90 - 32.75, or 57.25. The actual
field of view is the same as angle APC, which is merely twice
that value, or 114.5 degrees in our case.
This number should sound familiar. A moment of thought will show that the field of view of the camera is precisely half the field subtended by the film from the center of the camera. I thought that was kind of neat.
When the noted amateur telescope maker and astrophotographer Henry E. Paul died in 1976, his longtime personal secretary, freiends and family arranged private sales and an auction to be sure that his many valuable books and instruments would go to others with similar interests and hobbies. Thus I came to own a single copy of a curious magazine called the New Photo-Miniature No. 3 (new series), No. 208 (old series), 1935. This particular issue was entirely devoted to pinhole photography.
The following summer, some readers probably saw me at Stellafane with a view camera and tripod taking pinhole photographs. The ethereal, soft quality of the images recorded in this way has long been cherished by photographers. As a camera objective, the humble pinhole has no distortion, infinite depth of field, and more range than any zoom lens. You merely set the pinhole in front of the film at any distance whatsoever, and that becomes the focal length!
The yellowed pages of the New Photo-Miniature cover many interesting details, such as choosing the film and exposure time, making a shutter, and so on. There is a long article by Ben J. Lubschez about fashioning the pinhole itself, in a variety of thin materials like paper and sheet metal.
For telescope makers who check their mirrors with the Foucault test, a pinhole is often placed in front of a flame or incandescent bulb to provide a "point" source of light, with none of the alignment problems of a slit (a more recent innovation). The requirements of mirror maker and pinhole photographer are actually quite similar. An exceedingly small pinhole is not necessarily what is wanted. But the pinhole should have certain other qualities, including a smooth circular rim without burrs (to minimize diffraction effects). And if made in metal, the edges of the opening must be as thin as possible; otherwise the hole will tend toward the shape of a tunnel, with a rapid falloff in off-axis transmission. This would limit angular coverage of a pinhole photograph, and might send uneven illumination to a telescope mirror on the test stand.
Let us turn to the New Photo-Miniature, where the directions are given:
Before we can make the needed pinhole, we must have the needles of the right size. SInce the beginnings of pinhole photography, three sizes have been most generally used, No. 10, about 1/55 of an inch in diameter, No. 11, about 1/65 of an inch, and No. 12, about 1/75 of an inch in diameter... FOr use, the needles should be forced, eye first, into little sticks or holders about the size of a lead pencil, or a fine pin-drill may be used as a handle...
Metal pinholes may be made of copper, brass, aluminum or even silver. They may be made in two ways, using rather heavy, 24 gauge or so, metal or using very thin metal and mounting the finsihed pinhole on thin card or fibre...
If the heavier metal is used, we must first cut a cup-shaped depression in it by starting to drill with a 1/4 inch or 5/16 inch twist drill, stopping just before the drill begins to go through. We must be very careful not to puncture or tear the metal with the drill, but we do want to get the bottom of the depression as thin as possible. After the depression is made, the procedure is just the same whether we use the thin metal or the thick.
The metal is laid on a yielding surface, like several thicknesses of blotting paper or a piece of linoleum. With a blunt rounded point like that of a used lead pencil and gentle pressure, a dent is made in the bottom of the depression or in the center of the pice of thin metal. The metal is turned over and the bump raised by the pencil point is gently rubbed away, even with the surface, with a fine sharpening stone or a smooth file. This results in a very thin spot in the center of the metal. The metal is now turned face-up again and laid on a block of wood or a piece of cardboard. The thinnest center of the metal is now just punctured with the point of the selected size needle. This raises a burr on the back, which is rubbed away with the stone or file. The needle is pushed, not twirled or twisted, through a little farther and the burr is rubbed down. This is all repeated until the needle passes through smoothly to its largest diameter. The needle-hole should now be polished gently with the sharpened point of a toothpick, very gentle because the edge is very thin if properly made. The needlehole should be blackened with India ink or dull black lacquer, but preferably by heat and fumes from sulpur or a burning match. The edge of the aperature must be blackened, above all else, to avoid stray reflections.
Examine the needle-hole with a magnifier and run the needle, used in making, through the aperature carefully to clean out any matter resulting from the blackening process.
For the rubbing down of the burr, various implements may be used; the principle thing is to rub lightly. Fine grained India, Washita, or Carborundum sharpening stones or hones may be used. An old style slate pencil, which is all slate and not cased in wood, is excellent if used deftly. It should be first rubbed down on sand paper or emery paper until it has a blunt bevel on the end instead of a point. Very fine emery cloth is one of the best things to rub down the burr; it should be wrapped around a little block of wood about the size of a domino...
These sure, careful steps stand in sharp contrast to the hit-or-miss procedures mirror makers have generally used to make Foucault pinholes in aluminum foil. Yet when I tried out the method on a sheet of brass 0.025-inch thick, it proved easy and fun to do, and gave exquisitely thin and circular pinholes. I now have a whole set of various sizes, for photographic and mirror making purposes.
Among the old-timers, Rev. W. F. A. Ellison in particular waned against too small a hole in Amateur Telescope Making --- Book One (page 84) because diffraction effects might lead to misinterpretation of the mirror's figure. He and Russell W. Porter merely suggested a fine needle for piercing thin metal. G. W. Ritchey, who made the 60- and 100-inch materpieces for Mount Wilson Observatory just after the turn of the century, preferred to test with a 1/250- to 1/500-inch pinhole, much smaller than an ordinary needle can produce. Honing a needle to make it smaller is described in ATM-2, halfway down on page 89; then the photographer's directions quoted could still be applied.
The size of a pinhole for optimum photographic imaging was investigated by Lord Rayleigh long ago. First he pointed out a plane parallel glass window will meet his 1/4-wave tolerance as a substitute for a lens, provided the focal length exceeds about 20,000 times the square of the aperature in inches. This being so, he noted, one can take away the window and use an opening of the same size instead!
According to his formula, a 1/50-inch pinhole should be used at 8 inches or more, although in photography there is nothing really critical about this. A 1/4 inch "pinhole", which might be used to project sunspots on a screen in a dark chamber should have a throw of 1200 inches or 100 feet; the 11-inch solar image will show diffraction-limited detail likle that seen with a properly filtered 1/4 inch telescope (resolution 20 arc seconds. A heliostat and a long tunnel would be ideal for such an experiment.
A cardboard tube with a pinhole objective atone end an an eyepiece at the other becomes a pinhole telescope. I have tried such an instrument on landscapes by day and on the Moon, planets, and double stars by night, hoping to find out whether more can be seen this way than with the naked eye alone. I'd like to hear from readers who try the same thing.