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HANDBOOK OF PHOTOGRAPHY 



the size of the hole will increase the sharpness — up to a certain point. As the size 

 of the hole approaches one-half wavelength of the light being used (for example the 

 blue Fraunhofer G line), the phenomenon of diffraction will cause the image of a point 

 source to become a series of concentric circles of maximum and minimum brightness, 

 thereby ruining the image. 



For a given wavelength of light and a given screen-pinhole distance there is a best 

 diameter of hole (see page 26.) For example at a screen distance of 6 in. the best 

 diameter of hole is 0.02 in. This amounts to a working aperture of //300 so that the 

 pinhole in this case would have a speed Hsoo of that of an //6 lens. 



Pinhole Camera Construction. — Since this type of camera is occasionallj^ used for 

 pictorial work, the following data will outline the practical angles of pinhole-camera 



d+X/2 



Fig. 1. — Fundamentals of pin- 

 hole camera. Im^age of point 

 source is circular if pinhole is 

 circular. Image at B is larger than 

 image at A; as image becomes 

 larger, definition becomes poorer. 



Fig. 2. — Relation of aper- 

 ture and image size for best 

 definition. The best radius 

 of hole = vX^ where X = 

 wavelength of light to be 

 used and d = distance of 

 screen from aperture. 



construction. A thin sheet of copper or brass may be punctured by a needle and 

 tacked to a larger piece of wood or other material. If the metal in which the hole is 

 punched is comparativelj' small, there is greater likelihood that the piece will not be 

 deformed during the puncturing process. The metal plate may be laid down on a 

 piece of wood, the needle being forced through the center of the metal with a quick 

 tap of a mallet. The hole should then be cleaned up by means of a small file, when 

 the needle is again thrust through the hole. Now the metal sheet should be blackened 

 by holding it over a smoky flame or by dipping it while hot into nitrate of silver. 

 Several pinholes of different sizes may be mounted on a piece of cigar-box wood or 

 other light material and arranged to be pushed in front of an aperture in the front 

 of the camera. Since the exposure time is so long a shutter will not be needed. All 

 that is necessary is to push the desired pinhole to the center of the aperture, and to 

 push it away at the end of the exposure. Sufficient space must be allowed between 

 the several pinholes so that the camera aperture is effectively closed when no pinhole 

 is in position in front of this aperture. 



Table I will be useful in determining needle size. 



Clerc^ notes that a 5-sec. exposure will suffice when using a very rapid modern 

 plate for an open landscape at midday in fine weather with an aperture of 0.6 mm. 

 with the plate at a distance of 8 in. from the pinhole. 



1 "Photography, Theory and Practice," 2d ed., Pitman (1937). 



