Polarized neutron radiography allows the direct and real-space visualization of magnetic field distribution by detecting the polarization change in the transmitted beam. Spin polarizer/analyzer apparatus is important in polarized neutron imaging system, whose polarization efficiency directly affects the imaging quality of magnetic field. In this paper, the relation between the registered neutron intensity and magnetic field distribution is re-calculated after introducing the polarization efficiency parameter. Bender polarizers and 3He spin filters are simulated by Monte Carlo software VITESS to verify the calculation. The quantification ability with low beam polarization is also analyzed by considering the beam monochromaticity and bender geometry. The results will be helpful for processing image data and designing instrument.