Effect of Pupil Diameter on the Retinal Image Quality Calculated >From Double–Pass Point–Spread–Function Measurements

Abstract

Abstract: : Purpose: To quantify the effect of pupil diameter on the ocular modulation transfer function (MTF) and contrast sensitivity (CS) calculated from point spread function (PSF) measurements. Methods: The double–pass ocular PSF of the dilated left eye of 5 subjects (age = 19 to 30, less than 0.25D cylinder and sphere) was measured using an ocular point spread function analysis system (PSFAS, Topcon Corp, Japan). The PSFAS uses two apertures (artificial pupils) to control the diameter of the entrance and exit pupil. Following the technique of Artal et al (1995), to include the effect of odd aberrations, the double–pass PSF was measured with unequal entrance and exit pupil diameters. From the double–pass measurements, the PSFAS software calculates the single–pass ocular PSF, ocular MTF, and convolutes the single–pass PSF with simulated Landolt rings to estimate the contrast sensitivity curve. On each subject, measurements were performed with a 1.5 mm diameter entrance pupil and 2,3,4,5,6 mm diameter exit pupils. The value of the spatial frequency that reduces the MTF by 50% (MTF50) and the cutoff frequency (MTF0) were used as measures of the retinal image quality. Results: The average value and range (in parenthesis) of MTF50 in cycles/degree for 2,3,4,5,6 mm pupils were: 12.7 (11.3–14.5), 13.2 (10.2–16.6), 12.4 (8.5–16.5), 9.3 (5.9–13.5), 7.48 (4.8–10.7). For the same pupil diameters, the values and range of MTF0 in cycles per degree were: 37.4 (32.8–41.9), 51.5 (48.9–55.3), 63.8 (58.5–76.2), 68.1 (53.2–86.4), 61.1 (50.4–78.4). Near–diffraction limited performance was obtained only for the 2 mm diameter pupil. Conclusions: Even though aberrations significantly affect the MTF for pupils 3 mm and above, the retinal image quality remains approximately constant for pupils less than 5 mm in diameter.

Keywords: refractive surgery: optical quality • visual acuity