No. of Needle.
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 1/40 as the standard instead of 1/60. This simplifies calculation of the additional exposure in excess of the theoretical f value always necessary in dealing with pinhole apertures.
Nearest Needle Size.
Good Working Distance.
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 somewhat 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 10 needle holes are the best to employ when the distance is 6 in. or under.
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 between 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/2d=H/D 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?
Pinhole Picture (Taken With A No. 7 Needle Hole On A 5 X 4 Plate).
Naturally, we shall use the camera with the longest side of plate perpendicular ; therefore our equation will be 5/10=410/D.. D=820 ft. or, leaving a few feet for headroom, since we do not wish 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 behind which it would not be possible to get a satisfactory view. We are not beaten yet. We only have to increase the angle by decreasing distance between plate and pinhole.
5/2d=410/630.. d=3 3/4 in. (roughly).
This is an extreme case, and it would be better to use a camera having a rising front when for h/2 we can substitute height of pinhole after the front has been raised above base of plate. We will suppose the rise of front to have been one inch. The equation becomes 3 1/2/d=410/630 when d=5 1/4, so that we can leave distance between pinhole and plate at 5 in. and get a good half-inch of sky above the vane 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 comparatively 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. 10 needle holes at a distance of 3 in. from the plate.
St. Albans Abbey Before Restoration. Print From Wet-Plate Negative.