The USAF 1951 resolution target was created due to the need to assign numerical tolerances to optical systems and photographic processes. The resolution range depends on the quality and functioni of the system or process being tested. In the standard Air Force 1951 target, the change in pattern size is in a geometric progression based on the sixth root of 2. To simplify, the number of lines per millimeter doubles with every

sixth target element. These six elements are known as a group, and the group heading, such as -2, is the power of 2 to which the first element is raised to express the number of lines per millimeter in that element. In other words, 2 to the -2 power (2^-2) is 0.25 line per millimeter, 2 to the 0 power (2^0) is one line per millimeter, and so forth.