Optical modulation transfer and contrast sensitivity with decentered small pupils in the human eye

Human observers experience a large decrement in visual acuity when a small artificial pupil is displaced from the center to the edge of the dilated natural pupil. This decrement in visual resolution, called the Campbell effect, has been attributed to the retina, the ocular optics, or a combination of the two. Given the uncertainty about the relative magnitudes of these two components over the range of spatial frequencies used in normal vision, we have obtained objective measurements of the retinal image quality and psychophysical measurements of visual performance, with decentered pupils. The contributions of monochromatic aberrations were determined by using double pass measurements of the modulus of the optical transfer function (MTF). For all of the observers, there was a substantial decrement in the MTF with decentering, showing that even when using a 1.5 mm pupil and appropriate spherical/cylindrical refractive corrections, there is a considerable contribution of monochromatic aberrations to the effect. We have compared these optical MTFs with the psychophysical contrast sensitivity functions (CSFs) measured under exactly the same conditions using green gratings generated on the screen of a color monitor. At the low and intermediate spatial frequencies considered (2-16 c/deg), we find the fall in the CSF is much greater than the fall in the monochromatic MTF, with the difference becoming greater as the spatial frequency increases. We show that this discrepancy can be mostly attributed to the effect of transverse chromatic aberration due to the bandwidth of the green stimulus used for the CSF measurements. In conclusion, the combination of the ocular transverse chromatic aberration and monochromatic aberrations accounts for the loss in visual sensitivity found with a decentered small pupil at low and intermediate spatial frequencies.