Blurring of the retinal image and contrast thresholds in the fovea.

Under carefully controlled experimental conditions, the contrast thresholds of visibility at the fovea of a point light source for a white background having a luminance of about 12 mL were determined for various degrees of out-of-focus images. Compared to the threshold for sharp imagery, the threshold was markedly increased with increase in blur of the retinal image. The mean results of three observers showed that this increase was about 10 times for an image of white light out of focus by 1 diopter, and this threshold increased as the square of the dioptric power of the out-of-focus imagery. The results have implications in visual-search procedures.An equation, based upon (1) retinal response to a uniform luminous density in the blurred image, (2) a minimal effective angular size of the blur disk size, and (3) an asymmetry factor probably related to the character of the spherical aberration and possibly to the Stiles-Crawford phenomenon, was derived and was found to describe the data adequately. Two exposure times (1/2 sec and 1/5000 sec) and three different spectral distributions of the light from the point sources gave essentially the same results. For three subjects the minimal angular size of the blur disk was of the order of 7 to 8 min of arc in diam. This minimal size may be partially accounted for by spherical aberration but a physiologic area effect also may be present.