66 



KNOWLEDGE. 



February, 1913. 



of a similar character. The one great drawback attending 

 the use of a pin-hole in place of a lens is the protracted 

 exposure necessary, which so limits ils usefulness 

 that, generally speaking, it is only in still-life subjects that its 

 application is really possible. On the other hand, one of the 

 advantages frequently claimed for the use of a plain aperture 

 in place of a lens is the readiness with which the size of the 

 image may be varied, merely by moving the plate nearer to, or 

 further from it, without the definition appearing to suffer to 

 any appreciable extent 



When employing light of maximum photographic activity 

 the following formulae by Sir William Ab ney may be used : — 



D = 



120 



D and L being measured in inches. Taking nine inches as 

 the distance of plate, the size of aperture would be found : — 



V 9 



120 



3 

 120 



1_ 



40 



inch. 



by so doing, the only 

 alteration being in the 

 amount of subject 

 which is included, and 

 which is dependent 

 upon the relationship 

 between the length of 

 the plate and its dis- 

 tance from the aper- 

 ture. Both theory and 

 practice, howe ver.agree 

 in showing that the 

 sharpest image is ob- 

 tained when a definite 

 relationship exists 

 between the distance 

 of plate and size of 

 aperture, and although 

 diminishing the aper- 

 ture may improve the 

 definition up to a 

 certain point ; beyond 

 this the reverse would 

 be the case, a result 

 explainable on the 

 undulatory theory of 

 light. It therefore 

 appears that the size 

 of the aperture " for the 

 best results " is regula- 

 ted by the distance of 

 the plate from it, and 

 the wavelength of light 

 employed in taking the 

 photograph. We have 

 before us as we write 

 a photograph of a point 

 of light taken with a 

 pin-hole of one hun- 

 dredth of an inch in 

 diameter : the image is 

 shown as a bright 

 centre surrounded by 

 eighteen alternately 

 bright and dark bands 

 (due to diffraction). 

 A second photograph, 

 taken with a larger 

 aperture, shows an 

 image of the same 

 point of light, as a 

 one dark band, with 

 of the first bright one. 



I 



Table 16, giving the 

 diameters of aperture 

 best suited to the 

 distance of plate, has 

 been calculated from 

 these formulae. 



Figure 67. St. Brelade's, Jersey. 



From a pin-hole photograph by Alfred S. Gannon. Diameter of aperture j^-inch 

 distance of plate 6 inches, exposure li minutes. 



bright centre surrounded with only 

 a faint indication of the formation 

 This photograph exhibits the limit 

 to the size of the aperture for that particular distance of 

 plate, as if it be diminished beyond this, each point in a 

 luminous object would give rise to a spot surrounded by 

 alternately bright and dark rings, which would impart a 

 confused appearance to the photograph. The size of aperture 

 that will give the best definition from fulfilling the above 

 requirements has received the attention of more than one 

 eminent writer, and is given by the equation : — 



where D is the diameter of aperture required, X the wave- 

 length of light, and L the distance of the plate from the 

 aperture. 



Table 16. 



We now come to the 

 question of the ex- 

 posure required ; but 

 this need not present 

 any real difficulty if 

 we remember that the 

 rule which governs it 

 generally equally ap- 

 plies here, and that it 

 may, therefore, be 

 obtained from com- 

 parison with a lens 

 aperture of known 

 value. Thus, suppose 

 it is desired to know 

 what exposure is 

 necessary to be given 

 with an aperture of 

 one - fiftieth inch in 

 diameter, to a plate at 

 six inches from it, 

 in comparison with a lens of six inches focus and stop three- 

 quarters of an inch in diameter (f/8). By the rule, the 

 exposures would be as (sV) 2 to (J) 2 or jsjiW to Ai so that the 

 plain aperture requires 1406-25 times longer exposure. 

 Therefore, if the subject required one-fifteenth of a second 

 when using the lens whose aperture was three-quarters of 

 an inch in diameter, by substituting the pin-hole whose 

 diameter was one-fiftieth of an inch, the required exposure 

 would be one minute thirty-three and three-quarter seconds. 

 It is therefore evident that the required exposures 

 may be readily calculated from the intensity ratios 

 of the apertures. In determining the exposure by this 

 method, the distance of the plate should be the same in both 

 cases, as the exposure required varies as the square of this 

 distance. 



In order to avoid any calculations at the time of taking the 



