Eccentric photorefraction (EPR), which is used to determine the refractive error of eye, is greatly relied on the crescent pattern of the pupil image or on the intensity gradient in the principal meridian across the pupil. The vignetting image of the pupil, which is formed by a camera lens aperture with an eccentric light source, and contains a lot of refractive information of the subject eye, has been studied by many authors expecting to get more accurate measurement. In this paper, we have investigated the formation of the crescent image of the pupil. A mathematical expression that calculates the relative intensity in the whole pupil is derived. The intensity distribution is shown in a topography figure that gives a more clear understanding of the pupil image. Based on the discussion of the point light source, light sources are then further extended to other shapes such as line, circle or rectangular form for more general cases, simply by integrating the point source over the scale of the actual geometric form. The analysis and calculation presented are based on Gaussian optics for simplicity. The investigation results show that line source with 0 eccentricity at its one end is suggested in the EPR, in which the intensity slope is much sensitive to the refractive states.