2 SIMPLE PSYCHOPHYSICAL METHODS FOR DETERMINING THE OPTICAL MODULATION TRANSFER-FUNCTION OF THE HUMAN EYE

The foveal optical modulation transfer function (O-MTF) of the human eye for a pupil 8 mm in diameter was determined at 1-23 c/deg by using two new methods. The first method was based on the comparison of the effects of quantal and added spatial noise on grating contrast sensitivity at each spatial frequency. The rationale behind the comparison is the fact that quantal noise is not affected by the O-MTF of the human eye, because individual quanta cannot be blurred by the point spread function of ocular optics. In the second method we measured spatial contrast sensitivity for gratings whose area decreased but illuminance increased in proportion to spatial frequency squared. For such gratings the variation of contrast sensitivity with spatial frequency (f greater than or equal to 0.5 c/deg) is solely due to the O-MTF of the human eye, because the effects of grating area and retinal illuminance (quantal noise) on contrast sensitivity are independent of spatial frequency. The two methods provided identical O(MTF)s. The foveal optical modulation transfer function of the human eye for an 8 mm pupil was found to be O-MTF = [1 + (f/11.1)(2.5)](-1), where f refers to spatial frequency in c/deg. The equation means that the OMTF is constant at low spatial frequencies, becomes reduced to 50% at 11.1 c/deg and decreases thereafter with a slope of -2.5 when plotted in double logarithmic coordinates. At 1-16 c/deg our OMTF showed less image degradation than any foveal OMTF of the human eye for 6-8 mm pupils reported so far in the literature.