Sharpness discrimination for foveal targets.

A briefly flashed pattern never appears quite sharp, and visual acuity is reduced with short exposures. These observations led to an examination of the effect of exposure duration on sharpness discrimination. A foveally seen edge appears just-discriminably blurred when its edge light distribution is changed from being sharp to conforming to a ramp whose width (increasing from 0% to 100% of maximal luminance) is approximately 1 arcmin. When the exposure duration is reduced, the ramp width for minimal blur increases, rising by factors of 2-4 for exposures as short as 30 msec. This change is not due to a shortage of light. Threshold blur discrimination is not affected by retinal image motions of up to 1 deg/sec. Temporal combinations of sharp and blurred borders always worsen performance, and multiple brief presentations do not give so good a threshold as a single longer one.