Measurement of Visual Acuity with Blurred Tests

Abstract

WHEN testing visual acuity with Snellen's current type, we use black letters printed upon a white ground, and the transition from black to white is as abrupt as can be. If the patient's eyes are emmetropic, or hyperopic without presbyopia, or myopic and corrected, the retinal image of the test remains sharp, and visual acuity is limited only by the structure of the retina and the aberrations of the eye. On the contrary, when the patient's eyes are myopic and not corrected, the retinal images are blurred and visual acuity is poor. This relation of visual acuity to ametropia has been studied by W. Swaine¹ whose measurements one of us has shown to be in fair agreement with theory². But further experiments Were necessary.