The 1951 USAF resolution test chart is a resolution test pattern conforming to MIL-STD-150A standard, set by US Air Force in 1951. It is still widely accepted to test the resolving power of optical imaging systems such as microscopes, cameras and image scanners, although MIL-STD-150A was cancelled on October 16, 2006. The pattern consists of groups of three bars (small Ronchi rulings) with dimensions from big to small. The largest bar the imager cannot discern is the limitation of its resolving power.