Acuity, more properly visual acuity, measures the eye’s ability to perceive the shape or form of objects. Acuteness of vision is greatest when the image of the object being viewed falls directly on a small depression, the fovea centralis, in the center of the macula lutea, the yellow spot of the retina. The rods and cones, the light-perceiving cells near the surface of the retina, are present in great numbers throughout the retinal surface. The larger the number of cells, and the more closely packed, the greater the visual acuity. The greatest concentration of light-sensitive cells is found in the fovea centralis.