It is thought to aid the choroid layer in supplying the retina with nutrients and in transporting oxygen and carbon dioxide. The optic nerve centers the retina in the lower quadrant of the back of the eye. The area of sharpest visual accuity of the retina is in the macular disk. The fovea centralis is a tiny pit in the center of the mocular disk. It is the area of maximal optical resolution.
Most birds have single fovea.
The avian lens is softer than that of mammals - a very important feature in rapid visual accommodation. Ciliary muscles change the shape of the lens to focus on near or far objects, a process known as accommodation. Predatory birds such as owls have long, tubular eyes and cannot readily accommodate their eyes to focus on very near prey. They often have to move away from their prey to better see it. The vitreous body is a clear, jelly like substance that fills the eye between the lens and the retina.