Frieser's formula was obtained as the line spread function for photographic films and papers, using two flux theories and introducing modified Kubelka-Munk's two costants, S ' and K ', where, Frieser's k is given by 2 (2 S ' K '+ K '²) ^-1/2/log e . Calculated silver density curves of knife-edge exposures were obtained, by calculation and graphical method, using this line spread function and the characteristic curve. The linear portion of this calculated curve moves parallel in proportion to log E -log E_F , and this relation coincides with the experimental results. Using this relation, the value of k can also be determined. The evaluation value for line-width reproduction, d , was defined as the density on the linear portion of calculated density curve at edge-point produced by the knife-edge exposure, E_γ , which is the optimum exposure required for correct width reproduction when the width of image is perceived in the field of microscope (×50). Using d as a medium factor that correlates psychological width to physical width expressed by k and sensitometric parameters, a line-width reproduction equation was obtained by log E_γ -log E_F = log 2- d/γ where, r is the slope of linear portion of the characteristic curve. It was shown that the method to estimate the optimum exposure for line-width reproduction of single line original, using this, is simple and has practical usefulness. With the average value of d (=1.20) for various phototypesetting films and papers, or with the proper value of d for the material, if desired, the optimum exposure can be estimated easily using only sensitometric results.