Refractive Errors

In the United States and Canada, the notation 20/20 is registered in terms of feet. In Europe, the notation 6/6 is registered in meters. Both notations indicate vision measured at 20 feet, or 6 meters, from an object. If you have 20/20 vision (6/6), you can see clearly at 20 feet what should normally be seen at that distance. Similarly, with 6/6 vision, you can see clearly at 6 meters what should normally be seen at that distance. If you have 20/100 (6/30) vision, it means that you must be at 20 feet (6 meters) to see what a person with normal vision can see at 100 feet (30 meters) away.

	Emmetropia: In the normal (emmetropic) eye, light rays are focused directly on the retina. Images seen in both distance and near vision are clear.
	Myopia: In the nearsighted (myopic) eye, light rays are focused in front of the retina. Images seen in the distance are blurred, while near images are clear. The myopic eye tends to be elongated.
	Hyperopia: In the farsighted (hyperopic) eye, light rays are focused behind the retina. Images seen in the distance are clear, while near images are blurred. The hyperopic eye tends to be short.
	Astigmatism: In the astigmatic eye, the cornea and/or the lens is irregularly shaped. Light entering into the eye is scattered, and images are distorted.
	Presbyopia: The presbyopia eye occurs with age. As the lens of the eye loses its flexibility to accommodate (focus) at near images, bifocals become necessary. Presbyopia may be present in conjunction with other refractive errors listed above.