In addition to the limits on the number of defects and their size, the total area of all imperfections must not exceed A*N. Long defects (scratches) are summed up by their width, independent of length. There is no limit on the amount of edge chips, and the concentration of imperfections is limited by the rule that at most 20% of the defects allowance can be concentrated in an area of 5% of the clear aperture.