70 PHOTOGRAPHIC FACTS AND FORMULAS 
f Se 
hide Se ei E B Ao ar 
20 34 OF, 2 12 4 Be 23 
28 42 40 t3 5 54 17 
38 2). AR) a a a ees 14 
45 66 52 1 15 7 60 11 
56 82 58 2/3 16 8 63 9 
70 103 64 Hs 17 9 66 Zz 
88 130 72 y, 18 10 69 55 
105 154 79 anh 19 a 42 4 
140 206 90 73 20 ‘7 15 3. 
5 257, 100 Y% fA 13 78 2.6 
228 300 115 al 22 14 80 2 
280 410 128 1/6 23 15 84 1.6 
350 575 144 1/8 24 16 88 1.3 
455 670 164 War 25 17 92 1 
560 825 ISO) hf 12 26 18 95 8 
700 1030 204 ae 27 19 98 6 
875 1280 228 ae 28 20 100 5 
To convert Hurter & Driffield into the Watkins system: 
Multiply the H. & D. numbers by 50 and divide by 34. 
To convert Wynne into Watkins: Multiply the square root 
of Watkins numbers by 6.4. 
To convert Wynne into H.& D.: Multiply the square root 
of Wynne’s numbers by 7.7. 
This table gives an approximation of the connection be- 
tween the various systems of determining plate speeds. But 
absolute reliance can not be placed upon it as there is no 
standard system and plate manufacturers differ as to their use 
of the Hurter & Driffield (H. & D.) method, therefore, these 
speeds, as advertised by various makers, are not comparable. 
