The effect of optical blur on visual acuity for targets of different luminances

We studied the effect of 1.25 D of optical blur on visual acuity at luminances ranging from photopic to low mesopic levels. Optical blur reduces acuity at all luminances tested, this reduction being fourfold at a high photopic level (170 cd m-2) and twofold in low mesopia (0.017 cd m-2). The reduction of visual acuity with blur at low luminances is greater than might be expected from a spatial frequency analysis of vision.