Figure 1.

USAF 1951 resolution test chart (not to scale). The resolving power of an optical imaging system is found according to the group and element of the smallest 3-line pattern that can be resolved, according to the equation: $resolution=2^{group+(element-1)/6}$. For example the bottom-left target corresponds to 0.445 lp/mm and the top-right target corresponds to 0.500 lp/mm. Reproduced with permission from Buam.⁴⁸