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    Refractive Errors

    By John D. Ferris, FRCOphth
    Squint Clinic
    Amblyopia, Pediatric Ophth/Strabismus

    Normally images will be focussed clearly on the retina at the back of the eye. However if the lens in the eye is too thin or the eye is too short the focal point for the eye is too long. This is where the term ‘long-sightedness’ comes from. You may also hear the term hypermetropia used which is the same thing. It means that things in the distance will not be very sharp and things up close will be even more out of focus.

    Young children have incredible focusing powers and can change the shape of their lens to overcome some of their long-sightedness. However due to the strain this puts on the muscles in the eye it is not sustainable for long periods and can cause strain and cause the eyes to tire easily. So, by correcting the long-sightedness we allow the muscles in the eyes to relax so that the eyes can see a clear image without having to strain.

    Normally images will be focussed clearly on the retina at the back of the eye. However, if the lens in the eye is too thick or the eye is too long the focal point is too short for the eye. This is known as short-sightedness or myopia. It means that things close to you are in better focus and objects appear blurred the farther away they are.

    Correcting short-sightedness gives a clear image on the retina so things farther away come into focus.

    In a normal eye with no refractive error all light rays meet at the retina to give a clear focussed image. An eye like this will have a round or spherical front surface. In an eye with astigmatism the front of the eye is more oval or rugby ball shaped. This means that light rays coming from certain directions will focus just in front of or just behind the retina causing a blurred image.

    Astigmatism along the horizontal and vertical axes are the least visually disabling. However, astigmatism can occur in any direction affecting light coming in at different angles which causes even more blur.

    Republished, with permission, from