Visual Acuity Depends on Perceived Size

Increasing (or decreasing) the convergence angle can shrink (or expand) the perceived size of an object without changing its retinal size. The present report deals with the question of whether such a change in perceived size affects visual acuity. We investigated the effects of perceived size on the legibility of letters, using a telestereoscope which can control the observer’s convergence angle while keeping the size of the retinal image constant. We demonstrated that letters do become more legible (illegible) as perceived size expands (shrinks), although their retinal size stays constant. Then, we measured the ‘threshold’ size of the convergence angle, at which letters became legible from illegible, for several retinal sizes (0.1 to 0.2 deg) of letters. The result shows that the threshold decreases as the retinal size of letters decreases, so that the minimum retinal size of legible letters depends on the convergence angle. This implicates contributions from the size perception at higher levels of the visual system to the determination of visual acuity.