Talking about the 1951 USAF Resolution Test Chart

USAF test Chart
Glass chart


The original chart resolves 160 line pairs per millimeter. The entire chart consists of as many copies of this test target as will fit on the film. Each target has five groups of six elements, each with five pairs of horizontal and five pairs of vertical lines. The actual size of the line pairs doubles with every seventh element. To determine resolution, simply identify the smallest element in which you can clearly distinguish all the line pairs, and read the corresponding resolution from the chart. 

Pattern format

The common MIL-STD-150A format consists of six "groups" in three layers of patterns. The largest groups, forming the first layer, are located on the outer sides. The smaller layers repeat the same pattern but are progressively smaller toward the center. Each group consists of six elements, numbered from 1 to 6. Within the same layer, the odd-numbered groups appear contiguously from 1 through 6 from the upper right corner. The first element of the even-numbered groups is at the lower right of the layer, with the remaining 2 through 6, at the left. The scales and dimensions of the bars can be found below.
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Number of Line Pairs / mm in USAF Resolving Power Test Target 1951
Group Number
Element -2 -1 0 1 2 3 4 5 6 7 8 9
1 0.250 0.500 1.00 2.00 4.00 8.00 16.00 32.0 64.0 128.0 256.0 512.0
2 0.280 0.561 1.12 2.24 4.49 8.98 17.95 36.0 71.8 144.0 287.0 575.0
3 0.315 0.630 1.26 2.52 5.04 10.10 20.16 40.3 80.6 161.0 323.0 645.0
4 0.353 0.707 1.41 2.83 5.66 11.30 22.62 45.3 90.5 181.0 362.0 -----
5 0.397 0.793 1.59 3.17 6.35 12.70 25.39 50.8 102.0 203.0 406.0 -----
6 0.445 0.891 1.78 3.56 7.13 14.30 28.50 57.0 114.0 228.0 456.0 -----

Problems with the 1951 USAF Chart

The 1951 chart is not well suited for modern lens analysis. The fragmented arrangement makes it difficult for software to automatically determine the maximum resolution and it does not scale well for testing the corners of lenses where aberrations are more likely to appear. The complex numbering system and use of a look-up chart can be avoided by use of a newer layout chart (the T-21), which labels the groups directly in cycles/mm and is available in the links below from Applied Image. A second version is also available (the T-22) which alters the layout of the bar groups to a mostly straight line pattern, more suitable for scanning. The requirements of MIL-STD-150A are still met because the standard does not specify the pattern group layout.

Edmund Optics - 1951 USAF Glass Slide Resolution Targets

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