Effect of Blurring on Perception of a Simple Geometric Pattern

FIG. 1 is a modification of the Poggendorff illusion which has been drawn so that it is clearly visible at large distances. When viewed from a distance the central bar is perceived as being more horizontal and wider than it is drawn. This distortion increases as distance from the figure increases so that distortions of 15° can be found in most subjects. (Similar but much less striking distance and size effects have been noted in standard illusion figures1–5.) The distortion is greater in subjects with poor visual acuity; and it can be increased by artificially increasing the blurring by optical means. The effect does not depend on accommodation, and can also be observed by placing a hair on a fine grating.