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HANDBOOK OF PHOTOGRAPHY 



where / is the length of the camera from pinhole to plate. This formula leads to the 

 values given in Table VII. 



Table VII. — Optimum Size of Pinhole 



/.in 



.1, in 



Relative aperture 



Size of image point, in. 



10 

 0.02(5 

 //380 

 0.008 



In the fourth line of Table VII is given the size of the spot of light falling on the 

 plate from each separate object point for the stated values of / and A. It will be 

 noticed that the image spots are in every case less than one-third as large as the 

 pinhole itself and are not much greater than the normal sharpness requirement for 

 ordinarj' photography. The relative apertures are very low (less than //200) hence 

 a long exposure is required, but" with a fast plate in bright sunlight, even this will 

 amount to only a few seconds. The entire absence of distortion for any size of angular 

 field is a very great advantage afforded by the pinhole camera, in addition to its 

 evident cheapness and simplicity. 



The variation in illumination over the field of a pinhole camera would be equal 

 to that given by the cos'* <t> law expressed in Table V, page 20, if the pinhole is pierced 

 in a plane thin sheet of opaque material. If the edges of the pinhole have been raised 

 by the insertion of the pin or if the sheet material has considerable thickness, then 

 there will be some vignetting of the oblique beams, resulting in still less light at the 

 corners of the picture. It is unlikely that a pinhole photograph would be satisfactory 

 if a field much greater than about 90° were attempted. 



Perspective Effects of Photography. — To understand the nature of photographic 

 perspective,' we need merely consider the tracks of rays from all parts of an extended 

 scene into a simple pinhole camera. No change in the ray directions occurs at the 

 pinhole, each ray continuing along its rectilinear path until intercepted by the photo- 

 graphic plate. The picture formed on the plate is then necessarily a "correct" 

 representation of the three-dimensional object projected upon a two-dimensional 

 plane surface. A pinhole is considered here instead of any real lens for simplicity and 

 to be sure that distortion or other optical aberrations shall be absent. 



However, such a picture although "correct" would not look pleasing unless it 

 conformed to a number of long-established conventions. The first is that the plate 

 plane shall be vertical. This corresponds to holding our eyes so that we look directly 

 forward in a horizontal plane. If we tilt our head and- look upward at a high building, 

 we see the vertical sides of the building appearing to converge to a point in the sky, 

 but we do not ordinarily realize that this is so, and we object strongly to a photograph 

 taken on a nonvertical plate which shows this effect. On the other hand, a deliberate 

 exaggeration of this appearance may be used to suggest great height in a building. 



The second convention that must be observed is in the distance of the camera 

 from the object. To look with our eyes at an automobile end-on from a distance of 

 3 or 4 ft. shows the near end disproportionately large and the far end relatively small. 

 However, in a photograph, this disproportionate magnification of various parts of the 

 picture can appear unpleasant or even absurd, and it should be avoided by always 

 placing the camera as far as possible from the object to be photographed. To yield 

 a picture of the required size, a lens of sufficiently long focal length must, of course, 

 be used, and this is one of the chief objections to small hand cameras equipped with 

 fixed short-focused lenses. A good rule is never to place the camera closer than 10 ft. 

 from a person or other medium-sized object which it is desired to photograph. 



