Visual acuity has been measured in terms of the reciprocal of the angle in minutes subtended by the individual lines in a parallel-line test object at the limit of perception of the lines. Both clear and opaque lines in this test object were of equal width. It was found that the limit of perception of the lines in such a test object depends upon the orientation of the lines. Visual acuity, measured with the lines passing diagonally through the visual field at an angle of 45 degrees to the horizontal, is between 10 and 20 percent lower than that measured with the parallel lines passing vertically or horizontally through the visual field.
G. C. HIGGINS and K. STULTZ, "Visual Acuity as Measured with Various Orientations of a Parallel-Line Test Object," J. Opt. Soc. Am. 38, 756-756 (1948)