INFLUENCE OF CONTRAST ON THE RECOGNITION OF DEFOCUSED LETTERS - A GEOMETRICAL MODEL

We show how contrast (C) affects the recognition of defocused letters. To do this, the maximum distance (threshold distance) at which a subj ect, with an induced refraction of -5.5 D, can recognize a letter was determined. Our results show that when C = 1, the recognition threshol d distance is such that the ratio eta/xi, between the pseudoimage of t he letter (eta) and the blur circle (xi), corresponding to any point o f the letter at this distance is about eta/xi = 2; the exact value dep ends on the difficulty of recognition, in agreement with pre,ious expe riments. This ratio represents the sharpness of the image in a geometr ical treatment of image formation, providing a geometrical criterion f or recognition. Reduction in contrast can be compensated by improving the geometrical sharpness; i.e. bringing the object closer to the subj ect's are point. Our results show that the increment of eta/xi as a fu nction of the contrast is between C = 0.1 and 1.0, We suggest that a s imilar geometrical criterion for recognition could be used for any con trast (at least down to C = 0.1 and defocus > 1 D), provided that the proper value of eta/xi is used in each case. On this basis we propose a purely geometrical model, which agrees well with the data and predic ts the relationships between tolerance to defocus, contrast and visual acuity; including the well-known relation between visual acuity and d efocus when contrast is unity.