What are refractive errors?

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Refractive errors are disorders of the eye that prevent images from being properly focused on the retina, which results in blurry vision. These defects may be caused by disorders in eyeball length, the shape of the cornea or the transparency of the lens. Refractive errors include to myopia, hyperopia and astigmatism.  

Anatomy of the eye and how vision works

A good example to understand this better is a film being projected at the cinema. The projector has to focus correctly to obtain a sharp image on the screen.  

The eye works in the same way. The light passes through the cornea and the crystalline lens just like it passes through a projector lens to focus the image.  

To focus properly, the distance between the lens and the screen has to be correct. If the distance between the projector and the screen is too close or too far, the image on the screen will be blurry. 

The cornea is a spherical transparent surface at the front of the eyeball that allows light to enter and focuses the images on the retina to make them sharp. The lens is a transparent structure inside the eye that changes shape to help focus on near objects. 

These natural lenses (the cornea and the lens) have a smooth, uniform curvature. When this curvature is not smooth or uniform, the light that enters does not refract correctly on the retina (the back of the eye), which means the images are out of focus.   

Having adequate lens power and correct eyeball length is known as emmetropia (normal vision). But when any of these factors (generally the length) changes, the image is not properly formed on the retina, which causes ametropia (blurred vision).  

Substantiated information by:

Jorge Peraza Nieves
Mireia Hereu

Published: 21 October 2020
Updated: 21 October 2020

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