Tolerance to Defocus at Low Spatial Frequencies: Effect of Luminance

The effects of optical defocus on spatial contrast sensitivity are frequency dependent: defocus is detrimental to the visibility of high spatial frequencies, while low spatial frequency sensitivity is relatively less affected by the eye’s refractive state (Charman, 1979; Green and Campbell, 1965; Tucker, Charman and Ward, 1986). One would expect that vision at low luminance levels would be relatively unaffected by defocus, because the range of visible patterns is limited to lower spatial frequencies. However, Bedell (1986) reported that the contrast sensitivity loss produced by +1.25 D of defocus extended to lower spatial frequencies when the luminance was reduced. Similarly, the eye’s optical quality has a greater effect on low frequency sensitivity at low luminance than at high luminance levels (Coletta and Sharma, 1994). Experiments with acuity targets of fixed contrast also imply a decrease in tolerance to defocus at low luminance (Simpson, et al., 1986). Tucker and Charman (1986) measured acuity for gratings of 80% modulation as a function of focus and found that the through-focus acuity functions became narrower at mesopic luminance, even though the acuity limits shifted to lower spatial frequencies.