View Full Version : U.S. vs Inernational aperture scales

18-Jan-2012, 15:48
It's got to be in here somewhere, but I don't seem to be using the search engine properly.
Anyhow, is there a list comparing the old U. S. system with the curent International numbers? My (elderly/senile?) memory seems to tell me that f:8 is the same for both systems.

Steve Smith
18-Jan-2012, 16:01
I think that f16 is the same for both.


Ole Tjugen
18-Jan-2012, 16:04
"US" stands for "Universal System" or something like that, not "United States". so the other one is not "international".

"f:8" refers to an opening of 1/8 of the focal length, making it a dimensionless number.

US 16 is equivalent to f:16 (IIRC).
US 8 - f:11
US 4 - f:8
US 2 - f:5.6
US 1 - f:4

Remember that f:4 was about as fast as lenses got at that time. A few superfast Petzvals were f:3.5 or even f:2.8, but f:4 was a reasonable starting point.