code: def MTF50(self, MTFx,MTFy):
          '''
          return object resolution as [line pairs/mm]
                 where MTF=50%
                 see http://www.imatest.com/docs/sharpness/
          '''
          if self.mtf_x is None:
              self.MTF()
          f = UnivariateSpline(self.mtf_x, self.mtf_y-0.5)
          return f.roots()[0f MTF50(self, MTFx,MTFy):
          '''
          return object resolution as [line pairs/mm]
                 where MTF=50%
                 see http://www.imatest.com/docs/sharpness/
          '''
          if self.mtf_x is None:
              self.MTF()
          f = UnivariateSpline(self.mtf_x, self.mtf_y-0.5)
          return f.roots()[0]
docs: return object resolution as [line pairs/mm]
                 where MTF=50%
                 see http://www.imatest.com/docs/sharpness/