It is still widely accepted to test the resolution of optical imaging systems such as microscopes, cameras and image scanners. For the 1951 USAF, the common 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. However, Opto-Line can delineate to the 8th group, 6th element (456 line pairs/mm).

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