Some improper points of system illuminance formula in the optics textbook and optical software have been found and analyzed, and more practical formula have been deduced. On the assumption that an optical lens is a passive transfer system, after analysis of the original formula the relationship of the off-axial field of view with the relative illuminance is cos4ω'. When the field angle in image space becomes smaller, the energy will be enlarged, which is impractical in a passive transfer system. Then, by taking entrance pupil as reference, we recalculate the illuminance formula and obtain a reasonable one. Software simulation validates the new function. And the illuminance of image plane off-axial fields is proportional to the cos4ω of the objective field angle.