The conversion of the square-wave transfer function into the corresponding sine-wave transfer function (i. e. MTF) via the Coltman's formula was performed by computer simulation. We expanded the Coltman's formula which is used for correction up to a maximum of twenty-five (25) terms, and then compared and discussed the difference of the results obtained using various numbers of correction terms. Our results showed that the use of first four (4) terms of the Coltman's formula was sufficient for correction, judging from the standard deviations of the resultant MTF values. A unique phenomenon, however, was noticed. Namely, if the first eight (8), eighteen (18), and twenty-three (23) correction terms were used, the MTF values might be reversed when the transfer function is still present at high spatial frequencies. If we set the MTF value at zero spatial frequency to unity (different from normalization), the results were the same as those corrected MTFs without reversion. However, when the correction terms up to five (5) or fourteen (14) were used, the reversion was present, even if the MTF value at zero spatial frequency was set to unity. Therefore a full consideration must be taken when using Coltman's formula, provided this phenomenon exists. Moreover, our calculated results showed that the MTF value at zero spatial frequency still could not be unity, even if the correction terms were up to 812. We concluded that the Coitrnan's correction expression is a function having considerably slow convergence.
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