Uniform System of lens apertures

Instead of f-stops, many old American-made lenses are marked in the Uniform System (U.S.). The Uniform System was established by Britain’s Royal Photographic Society in 1881, and was one of the first attempts to establish a standard for lens apertures. It only caught on in the United States, where Eastman Kodak used it extensively on roll film cameras that today often turn up in antique shops and garage sales.

f stop	4	4.5	5.6	6.3	8	11	16	22	32	45
U. S.	1	1.26	2	2½	4	8	16	32	64	128

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