The sharpness of images photographed in this way, when the conditions are properly adjusted, is quite comparable to that of images given by soft-focus lenses, and particularly by anachrom atic lenses.
A considerable angle of field can be covered by a pinhole, which can thus be advantageously used for photographing monuments in cases when it is not possible togo a sufficient distance The one disadvantage of this process is the relatively long exposures required. This, how ever, is of considerably less account now that conditions for the best results, which explains why the rules formulated by different experi- menters show a certain amount of inconsistency.
For photographing distant objects, the op timum distance F from the aperture to the screen, or to the photographic plate which may be substituted for the screen, should lie between two limits calculated respectively by multiplying the square of the diameter d of the aperture by 625 (Abney ; Dallmeyer), or by 1,250 (Colson ; we have at our disposal plates of such extreme 39. Making a Pinhole. It is not very easy to obtain commercially metal plates having calibrated holes with clean edges suitable for This rule, which is partly experimental and partly theoretical, may be expressed thus 625 F< 1250d2 pinhole photography. It may, therefore, be worth while briefly to describe how such an aperture may be made.
Experience has shown that there is no appreci able difference between the results given by pinholes of circular apertures and those of square apertures. One can, therefore, proceed to construct one as follows (Malvezin Gabriely).
On a piece of card draw two lines in the form of a cross (Fig. 14). At their point of intersection cut out a circle of about in. diameter. Outside this circle stick four needles into the card (shown by the black circles), the diameter of the needles corresponding to that of the desired aperture, making sure to push them in far enough for their uniform cylindrical parts to be actually in the card. Next, fix with glue four bands of
about I in. width cut in metal foil (extra thin copper) or in aluminium leaf, as shown in the figure. There will now remain a square aperture with true edges, the length of each side being equal to the diameter of the needles. Remove the needles and protect the whole thing by another card with a piece cut out of the centre, glued on to the first. Blacken the exposed surfaces of the cards.
In order to make a circular aperture in a piece of metal foil resting on a strip of soft wood or on a piece of lead, a needle may be used as a punch. The needle should be stuck through the centre of a cork along the axis of the latter, cut off flush with one end, and the other end of the needle then cut about 1/25 in. from the other surface of the cork. This projecting end must be in the parallel cylindrical part of the needle. Now rub the protruding end on an oilstone until a plane and polished end with sharp edges is obtained. The hole is then made in the metal foil by giving the top of the cork a sharp blow. The edges of the hole thus made should be examined with a strong magnifying glass, and, if necessary, made perfectly smooth by means of the finest emery paper. To render it permanent, the metal foil may now be mounted, as described above, between two cards.
The aperture made in this way may be fixed to the front of a camera or any light-tight box which can be loaded with a plate or film. A 'card running in grooves makes a sufficiently good shutter, since the necessary exposures are