Working Distance.—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.
Exposure.—The pinhole does not allow sufficient illumina tion 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 dia phragm 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. To needle as .oz in., and are working at 5 in., this value = 02 — or f/250. Mr.
5 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.
The time usually allowed for a lens working at f/8 must be multiplied by the figures opposite the distance given.
The Iratkins System.—Mr. Watkins has come to the rescue with a simplified method of calculating these exposures. To each pinhole he attaches a Watkins number which multiplied by the distance from plate gives a diaphragm value. The value is intended for use with Mr. Watkins' Bee meter, but the time given for this f value must be taken in minutes or fractions of a minute instead of the seconds, or fractions of a second, on the meter tables. The Watkins numbers are arrived at by taking as the standard instead of A-• This simplifies calculation of the additional exposure in excess of the theoretical f value always necessary in dealing with pinhole apertures.
The accompanying photograph was taken through a No. 7 needle hole, 5 in. from plate, with an exposure of 5 minutes, at midday on the shortest day of the year—weather some what overcast. It was rather too large an aperture to use at short distance and so the definition is diffuse, but not unpleasantly so. The 8 and to needle holes are the best to employ when the distance is 6 in, or under.
Architecture.—It is in architecture that the pinhole lens is most useful, not.only because it ensures geometrical truth of line and proportion, but also because it conveys a better idea of dimensions. For wide-angle work it is excellent. Even with the best glass lenses, true perspective can only be got at certain distances. By varying the distance be tween pinhole and plate, we can get true perspective almost irrespective of distance. The failure of any focussing screen is a slight disadvantage in obtaining a particular view ; in the absence of the image on the screen we must resort to calculation. Supposing H be the height of the building, D its distance from the camera, h the height of the plate, and d distance between plate and pinhole. Then h H For instance. We wish to photograph the spire of Notre Dame, at Bruges, on a 5 x 4 in. plate 5 in. from the lens, Reputed height of spire 410 ft. How far from the base of the spire must we place the camera ? Naturally, we shall use the camera with the longest side of plate perpendicular ; therefore our equation will be 4 to = D = 820 ft.
or, leaving a few feet for headroom, since we do not wisi the spire to look as if it was just tightly wedged into the picture, say 825 ft. On the other hand when we have stepped back 630 ft. we come to an obstruction, from be. hind which it would not be possible to get a satisfactory view. We are not beaten yet. We only have to increas( the angle by decreasing distance between plate and pin. hole.
5 _ 410 .. d = in. (roughly).
630 This is an extreme case, and it would be better to us( a camera having a rising front when for can substitut( height of pinhole after the front has been raised above bas( of plate. We will suppose the rise of front to have beet one inch. The equation becomes ,„ d = 51, d — 6 30 so that we can leave distance between pinhole and plat( at 5 in. and get a good half-inch of sky above the van( of the spire.
By similar methods we can calculate the breadth of a view in confined spaces.
Some splendid interior pictures have been obtained with the pinhole camera, the only difficulty being the enormous length of exposure. When working with a lens at f/22, very few church interiors can be photographed in less than five minutes. So that, on the Watkins scale, a com paratively light interior could not be photographed on a 5 x 4 in. plate at 4 in., through a No. 8 needle hole, in less than five hours ! The pinhole lens has also been adapted for stereoscopic work, the best arrangement being two No. io needle holes at a distance of 3 in. from the plate.