Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art . at once that the exposure for the latter must be a little over one-halfmore than the former^ For, 20 multiplied by itself is 400; and 25, 625. The ratiobetween 400 and 625 indicates the increased exposure necessary in the former case.— Lea. 17. There is a common idea prevalent that stops to lenses are arranged upon someregular system, and that they are suited for exposures, each one-half longer than thenext larger in sequence; that if 20 seconds be the ri
Wilson's quarter century in photography : a collection of hints on practical photography which form a complete text-book of the art . at once that the exposure for the latter must be a little over one-halfmore than the former^ For, 20 multiplied by itself is 400; and 25, 625. The ratiobetween 400 and 625 indicates the increased exposure necessary in the former case.— Lea. 17. There is a common idea prevalent that stops to lenses are arranged upon someregular system, and that they are suited for exposures, each one-half longer than thenext larger in sequence; that if 20 seconds be the right exposure for any stop, the nextsmaller must have 30, and so on. This, I think, is not the case; it certainly was not inany determination that I have made. For example, a triplet lens showed the followingrelations between its stops: /13J,/18|,/21,/27,/35i,/54. The ratios of the squares of these relations to which the exposures must be directlyproportioned, are: ; ; 441; 729; ; 2916. It will be seen that there is only sufficient regularity to mislead, and not for any realutility. THE DIAPHRAGM OR STOP. 69. A simple slide in front of the ordinary diaphragm, answers a very goodpurpose. The distance between this slide and the dia})hragm depends much on thesize of the diaphragm, and also on the width of the angle of the lens. Fig. 53 gives a front, and Fig. 54 a side view. In the former, A is theordinary diaphragm ; B, a plate for liolding the slide; C, is the slide, workedup and down by means of the screw, D, outside of the lens-tube. In the case of another lens, the diameters of the stops were found to be , , j^l, f x> M>of an inch. The squares of the numerators of these fractions arc respectively 22o, 325, 529, 676,1296. To be in the ratio of one-half longer exposures, these numbers, starting with the first,should be 225, 337, 505, 757, 1135. It will be observed, that after the first three, the difTercnccs are material, and thatthroughout they are so
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