There are several abnormalities and defects of vision in the human eye. The most common defect which is found in many people is termed as astigmatism. Astigmatism is a defect caused by an irregular curvature of the cornea or lens, i.e. they may curve more in one direction than in another so that vision becomes indistinct along certain planes. Special cylindrical lenses are needed to correct this defect to some extent.

Man has two eyes, whose visual field overlap. The eyes can focus on the same object at the same moment. Each eye receives a slightly different image from the other one. The brain is able to interpret the images together and enables us to see one three dimensional object. This is called stereoscopic vision. Such vision gives us a better perception of size, depth and the distance of the object from us.

Stereoscopic vision is important to predatory animals such as cats, eagles and hawks as good judgement is necessary to capture prey. Animals that are hunted need a wider field of vision to detect the approach of predators. For example, rabbits have eyes set high on the sides of the head. They have wide visual fields but restricted stereoscopic vision.