PINHOLE PHOTOGRAPHY Photography by means of a pinhole camera, that is, a camera in which (see A) instead of a lens there is a small hole made by a needle in a metal plate, card, etc. If a pinhole is pierced through one side of a light-tight box, the light rays proceeding from any object (say a candle flame) placed in front of the hole will form an image of the object on the opposite wall of the box inside ; the size of the image depends upon its nearness to the camera, and its distinctness or definition depends upon the size of the hole. Photographs obtained through a pinhole possess a pleasing softness of outline, distortion is absent, and any angle may be included upon the plate, a wide angle by having the hole near the plate (see A) and a narrow angle by having the hole farther from it (see B). On the other hand, the definition is not critically sharp, and the necessary exposure is so long that it is impossible to photo graph moving objects. Within certain limits a pinhole has no plane of focus, as proved by the fact that results practically as good as one another can be obtained by having the sensitive plate at different distances from the hole.
Any camera may be used for outdoor pinhole photography, as may also any light-tight box, but the former has the advantage that the dis tance between pinhole and plate is easily altered, remembering that, while this is not necessary for focusing, the shorter this distance the greater the angle of view, and consequently the more of the subject included upon the plate. Por indoor work, failing a regular camera, any light-tight box C may be used, the plate being supported by means of a printing frame or drawing pins.
The pinhole must be circular, with perfectly clean edges without the slightest burr ; at its edges the thickness of the material must be less than the diameter of the hole. The presence of a burr on the edge would give precisely the same effect as photographing through a tube, namely, to limit the angle included. A pinhole of in. diameter bears the same relation to metal in. thick that a I-in. lens bears to a tube 4 in. long. The reflections from the interior of the hole in a comparatively thick plate of metal would also interfere with the brightness and definition of the image. Tinfoil is the material often used, but it is very fragile when mounted up in place of the lens. Many prefer to use thin brass, in which a boss (see D) has been made by means of a steel punch. The boss is rubbed or filed down and the pinhole made through the thinnest part (see The " pin "-hole is best made with a needle, which should be inserted gradually and alternately from both sides of the hole so as to obviate burr. Any other way of making the hole may be adopted as long as the result is a circular hole with clean edges.
The effect of the distance between plate and hole has been referred to. The diameter of hole should vary with the distance, and the rule proposed by Sir W. Abney is very useful in this connection. To find the diameter of the hole, divide the square root of the distance by 120.
To find the distance, multiply the diameter of the bole by 120 and square the result. Thus, assuming the distance to be 25 in., the square root is 5, and this divided by 120 equals -glz in., which is the diameter of the required hole. In the case of a hole in. in diameter, the dis tance will be 9 in., since A x 120 3, and (3 x 3) = 9. But extreme latitude is possible because of the great depth of focus.
With regard to determining the diameter of the pinhole, it may be measured more con. veniently if pierced with a needle of standard size The following table is based on Abney's rule :— The following tables of needles, their sizes and diameters, are due to Messrs. Alfred Watkins and E. J. Wall :— Por most practical purposes pinholes of in., in., and in. will be sufficient, and they can be used at any distance from 1+ in. to 3o in., although theoretically the respective distances are 5 in., 2+ in., and 1+ in. The largest hole will be useful for all out-door work, the medium hole for architecture, both interior and exterior, and the smallest for copying and short focus work.
The pinhole having no definite plane of focus it is obviously unnecessary to focus on a ground glass, but the photographer will need to know the amount of view included upon the plate. The amount of light coming through the pinhole is generally so small that it will not be easy to examine the image on the ground glass. By far the most practical plan is to take a piece of millboard just large enough to slide into the grooves in which the dark-slide is usually carried, and in the card to cut an aperture the size and shape of the plate used. If a half-plate is used in a half-plate camera there will be no need for the card, but smaller plates than the camera usually takes are often used for ex perimental pinhole work. Turn back the focus ing glass from the camera, slide in the card (if necessary), direct the back of the camera towards the view, and look through the pinhole from the front. It is then possible to judge the suitability of the pinhole, and, by racking in or out, the amount of view included. Then swing round the camera, insert the plate, and make the exposure.
The usual method of exposing is to fit the pinhole in a pill-box arrangement, using the lid as the cap, or to fit a flap or revolving disc over the pinhole. In the first of the tables the ratio aperture (" stop value ") is given for a certain extension of the camera ; if any other extension is used the ratio aperture must be found by dividing the distance between pinhole and plate by the diameter of the pinhole. The usual law relating to exposure holds good, namely, that the exposure varies as the square of the aperture number ; thus, if a lens working at f/8 requires a quarter of a second, with f/600 the exposure would be x 600a f x 60o x 600 = 8 x 8 = (approx.) 1,400 secs. = (approx.) 24 minutes.
The calculation of the exact exposure fortun ately does not matter so very much, as, owing to the smallness of the aperture, there is an immense amount of latitude.