Refractive Error - Classification

Classification

An eye that has no refractive error when viewing distant objects is said to have emmetropia or be emmetropic meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object in this case is defined as an object 6 meters or further away from the eye. This proves to be an evolutionary advantage by automatically focusing the eye on objects in the distance because it allows an individual to be alert in, say, a prey-predator situation.

An eye that has refractive error when viewing distant objects is said to have ametropia or be ametropic. This eye, when not using accommodation, cannot focus parallel rays of light (light from distant objects) on the retina.

The word "ametropia" can be used interchangeably with "refractive error" or "image formation defects." Types of ametropia include myopia, hyperopia and astigmatism. They are frequently categorized as spherical errors and cylindrical errors:

• Spherical errors occur when the optical power of the eye is either too large or too small to focus light on the retina. People with refraction error frequently have blurry vision.
  • Myopia: When the optics are too powerful for the length of the eyeball one has myopia or nearsightedness. This can arise from a cornea with too much curvature (refractive myopia) or an eyeball that is too long (axial myopia). Myopia can easily be corrected with a concave lens which causes the divergence of light rays before they reach the retina.
  • Hyperopia: When the optics are too weak for the length of the eyeball, one has hyperopia or farsightedness. This can arise from a cornea with not enough curvature (refractive hyperopia) or an eyeball that is too short (axial hyperopia). This can be corrected with convex lenses which cause light rays to converge prior to hitting the retina.

• Cylindrical errors occur when the optical power of the eye is too powerful or too weak across one meridian. It is as if the overall lens tends towards a cylindrical shape along that meridian. The angle along which the cylinder is placed is known as the axis of the cylinder, while 90 degrees away from the axis is known as the meridian of the cylinder.
  • Astigmatism: People with a simple astigmatic refractive error see contours of a particular orientation as blurred, but see contours with orientations at right angles as clear. When one has a cylindrical error, one has astigmatism. This is caused by a deviation in the shape of the cornea, a shape other than spherical. This defect can be corrected with refracting light more in one area of the eye than the other. Cylindrical lenses serve this purpose.
  • Presbyopia: When the flexibility of the lens declines typically due to age. Individual would experience difficulty in reading etc. This causes the individual to need visual assistance such as bifocal lenses.