Some people imagine that U.S. refers to the " United States," because Kodaks are usually so marked. The letters, however, mean Uniform Standard, and were formerly adopted by the Royal Photographic Society to represent the relative values of the lens apertures. The Society, however, now uses for its standard, numbers indicating the proportion between the stop and the focus of the lens, and this method is now almost universal.

U.S. Nos.

1

2

4

8

16

32

64

128

256

equal

f/4

f/5•6

f/8

f/11•3

f/16

f/22•6

f/32

f/45•2

f/64