The measurement of eyebox size is important in the design, manufacturing, and user experience of AR/VR near-eye display (NED) devices. Previous studies have focused on characterization of luminance and the brightness-defined eyebox. We propose to use modulation transfer function (MTF) to define contrast-based eyebox, measured with the rectangular line-pair chart at varying spatial frequencies. We find it is important to examine both brightness-based eyebox and MTF contrast-based eyebox to decide the limiting factor, which depends on the specific NED design and the displayed content.