Fig. 8. Alterations in spatial contrast sensitivity with background level. Sensitivity to all spatial frequencies is reduced as background level is lowered. However, the effect of reduced luminance is less for low spatial frequencies. The predicted spatial frequency cutoff (a measure of acuity) declines with background. (Van Nes FL, Bouman MA. Spatial modulation transfer in the human eye. J Opt Soc Am 57:401, 1967.)