JUST as nature sometimes indulges in a reversion to the primitive type, so art finds renewed vitality by a return to first principles. And so photography, which is neither nature nor art, but a hybridised pursuit, may revert back to the original camera of some three centuries ago, when men first observed with wonder the images thrown on a screen by a hole in the shutter of a darkened room. The pinhole camera is simply the oldest form of camera in miniature. To begin again with primitive implements is educative in itself. It teaches us that our highly specialised cameras, our costly sets of lenses and multitude of accessories are merely matters by the way, enabling us to work with convenience and great saving of time. So far as the pictures themselves are concerned, leaving moving objects out of consideration, just as good results are obtainable with an ordinary cardboard box and a tiny needle-prick instead of a lens.
We say "just as good," but in some respects the pinhole will give us better results than the best lens is capable of. First, because there is no distortion. The lines in a pinhole negative will be geometrically true from corner to corner, not betraying any curvature of field or achromatism, or any other of the faults of the ordinary lens. And this with a width of angle which may be as much as 1250, against the 75° of a wide-angle lens. The picture also will be found to possess a roundness and expression of distance values as compared with the flatness of the usual photograph. Many pinhole views may almost be compared for quality with those seen through a stereoscope. For copying, the method is unrivalled, if only time can be spared for the long exposure. To all these advantages it must be added that focussing can be almost ignored. Whether the distance from pinhole to plate be three inches or ten, only the size of picture and angle of view are changed; definition is practically the same.
And now we must explain that the pinhole usually adopted is not a pinhole at all. Photographs may be taken by means of an actual pinhole, but they will be more or less "fuzzy." The hole made by a pin in any substance is irregular in shape, with numerous jagged edges, reflecting the rays of light in all sorts of directions, and prolonging the exposure before sufficient of the correctly inclined rays can reach the plate. What we want is not an irregular tunnel, but a perfectly round hole, with edges so thin that scarcely any rays are intercepted by the sides of the opening. The best apertures are made with thin copper or brass drilled through by needles (7, 8, 10, 12 are the most useful sizes), the surface cleaned with a fine file, and then drilled again. Fine stencil brass is as good as anything, and the needle pricks may be made through pieces about 3/4 in. square, while the metal lies flat on a boxwood block or cardboard. The surroundings of the hole must then be blacked, not with varnish, which would add considerably to the thickness, but by fuming with sulphur or dipping, while heated, into some blackening solution such as nitrate of silver.
If three or more "pinholes" of different sizes have been made, a very simple combined diaphragm holder and shutter may be contrived as follows. Take a strip of thick cardboard, or thin cigar-box wood, about 1 in. wide and 6 in. long. Punch in at intervals of 1 1/4 in. four round holes each 1/2 in. in diameter. Over each of these holes secure with stamp-paper one of the prepared punched metal slips, and make tidy and safe with a further layer of paper. This long strip may slide backwards and forwards over an aperture in the camera front 3/4 in. wide, with a mark showing when each "pinhole" is centred over the aperture. Still better, the set of holes may be punched in a kind of diaphragm wheel. Such a wheel has been designed by Mr. Alfred Watkins of exposure-meter fame, and may be obtained at very low price from the dealers.
We have stated that focussing is a matter of no practical importance with the pinhole, definition being good whatever the distance between plate and camera front. The circle of illumination is another matter. Rays of light which impinge on the plate at an angle have a much greater distance to travel than those which fall on it almost perpendicularly. In other words, the nearer the pinhole approaches to the plate the smaller the circle of effective, even, chemical action. A good rule in practice is that the distance between plate to be covered and aperture should never be less than the length of the plate; and for really good work that distance should be about the diagonal of the plate; for a quarter plate say 5 in., for a half plate 8 in., and so on.
The pinhole does not allow sufficient illumination to use the focussing-screen, and for ascertaining time of exposure we are dependent entirely upon calculation.
Now, the pinhole may actually be taken as the diaphragm of an imaginary lens, or rather infinite series of lenses, because at whatever distance the pinhole happens to be from the plate, this becomes the focal length. The diaphragm value is therefore the ratio between the diameter of the pinhole and the distance from the plate. If we take the size of the aperture made by a No. 10 needle as .02 in., and are working at 5 in., this value =.02/5 or f/250. Mr.
J. H. Noble some years ago worked out a system on these lines, making an allowance for the additional margin of time necessary beyond the rigidly calculated exposure in practice.