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Ilana Cohen, Dave Wang, Howard Howland; Limits to visual acuity at low light levels and contrasts. Journal of Vision 2004;4(11):50. doi: 10.1167/4.11.50.
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We wished to investigate the effects of luminance, contrast, defocus, pupil size, and higher-order aberrations on visual acuity. In particular, we wished to know if pupil size was optimized for maximum acuity (Land and Nilsson, 2002, pp. 103). To do this, we conducted acuity experiments at low light levels where vision is photon-limited at low contrasts (Rose, 1973, pp. 33–34). We first measured the wave aberrations of 24 subjects and determined visual acuity at a screen luminance of 6.2 cd/m2 across a range of contrasts while measuring pupil size and defocus. We then determined visual acuity across the same contrasts while using a 1.5mm artificial pupil. Pupil size did not change significantly with contrast and hence was not optimized. The artificial pupil improved acuity at high contrast and decreased it at low contrasts, as compared to viewing with natural pupils. No significant correlation was observed between the magnitude of higher-order aberrations and acuity under all conditions, nor did the magnitude of higher-order aberrations correlate with the improvement in acuity at high contrast due to use of the pinhole. We conclude that the most significant factor in degrading contrast acuity is second-order defocus.
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