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 4 8 16 32 64 128

sources

Comparison of the U. S. and f. Systems of Marking Stops

The U. S. (Uniform System) of marking stops for photographic lenses is based on the area of the opening. This system is generally used on rectilinear lenses.

The f. system is based on the diameter of the opening, and is used on anastigmat lenses.

The following table shows the comparison of the two systems, and the exposure value of the various markings.

U. S. f. Exposure
Value
4 = 8 = 1
8 = 11.3 = 2
16 = 16 = 4
32 = 22.6 = 8
64 = 32 = 16
128 = 45.2 = 32

f. 6.3 is a marking found only on anastigmat lenses; its equivalent is approximately U. S. 2.5. Its area is 61% greater than f. 8. This means if 1/100 of a second is the correct exposure, under certain conditions, at f. 8, the correct exposure, under exactly similar conditions, will be 1/161 of a second at f. 6.3.

Kodakery, October 1913, page 25.

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