The human eye is the best optical device in the world. The performance of this eye is influenced by optical aberrations. Several human eye models were developed to treat and correct this issue. To build and study the optimal and berated human eye, this work relies on the Liou and Brennan eye model. This model is more suitable than other models since it depends on using a gradient refractive index (n) with a spherical lens surface. The effect of entrance pupil diameter on optical aberrations were evaluated programmatically by ZEMAX , computer software (version 14) at 555 nm monochromatic light under the influence of Semi-Diameter values variation by 1.25, 1.5, 2, 3, 4 and 5 mm. The eye's performance was evaluated using the modulation transfer function (MTF), root mean square of the spot sizes (RMS), and wave front aberration. The findings showed that the best image quality acquired at 1.25 and 1.5mm Semi-diameter, which means that under monochromatic illumination, the smallest values of aberrations can be obtained by using a small pupil scale and a small field of view.