With a test object such as that just described, every point in the object plane should be reproduced exactly in the parallel image plane. If this condition is secured the lens has a "flat field," but if not the image suffers from "curvature of the field," and when received on a flat plate cannot all be sharply defined at the same time. If you cannot get a direct pencil and an oblique pencil in focus at the same time the existence of curvature is manifest. If the field is curved with a plane object it may be flat with a curved object, but that is not "flatness of field" as commonly understood. The form of the field varies with the distance of the object, hence the test object should be placed at a distance likely to be used in practice. It need not be far away, for, as a matter of fact, absolute flatness is only of vital importance when copying flat diagrams, etc. An ideal condition of flatness can only be secured with highly-corrected lenses that produce perfect image points.