Light passing through the concave-convex cornea and biconvex lens of each eye produces a crude, realistic image of the scene on the retina. This image is called coarse because, by instrument standards, it represents very poor optical components in terms of spherical and chromatic aberration. The smaller the aperture of the optical device, the sharper the image. In the human eye, this phenomenon is automatically generated by brighter objects. The image is sharpest at the center of gaze because the lens in the eye is adjusted for this area. This portion of the image is located at the fovea, where light obstruction is minimal and the structure of the receptor cells is the most finely divided.