74. PHOTOGRAPHIC FACTS AND FORMULAS 
Phemenerbed sot dames | eo) age da aii le eles eins eine 2 minutes 
Ooen streets with arc lamps! -y agai 3 minutes 
Open streets with wet roadway or snow ....... 2 minutes 
REIGSE) SUNCEE SCENES |) 2) ay/s 4500) 4: 0le lanes Re A a 6 minutes 
Close street with wet roadway or snow ........ 4 minutes 
Stop APERTURE SYSTEMS.—Various systems have been 
suggested from time to time for numbering the diaphragms 
or stops, but practically only two have survived. In the one, 
and the more general, the F or ratio system, the effective 
aperture is expressed as a fraction of the equivalent focus, 
thus as F: 8, F/8 or f: 8, which means that the aperture of the 
stop is one eighth of the equivalent focus. The other system 
is based on f:4 as unity, and the stops are merely numbered 
in fractions and multiples of this. The following table shows 
the relation of the two systems, the latter being known as the 
U. S. system, or the Uniform System numbers: 
D.S.Ne:, U.S. Net 
gee 1/16 iin ° 
f:1.414 i F216 16 
Rie 4 f222.02 32 
f:2.828 yy, f:28 49 
fe 0.562 fise 64 
f:4 1 reno) 81 
eA 1.56 f:40 100 
Fo.000 2 f:45.25 has 
726 2.25 F256 196 
Ror 3.06 f :64 256 
Ate: 4 fire 306.25 
Fey 5.06 f :80 400 
enles, 6.25 f:90.5 512 
“ee 8 f:100 625 
Stolze proposed f:10 as unit, and Dallmeyer (s35)- The 
Paris Congress also adopted f: 10 as the unit. Zeiss adopted 
