Refractive (= focussing) errors, and how spectacles and contact lenses work. (20)

The normal eye that needs no spectacles or contact lenses:

So, in the normal eye, the rays of light from each point on the object being viewed converge to a single point on the retina which is the layer lining the back of the eye which detects the image.

The rays of light from different parts of the object are focussed to different points on the retina so that an image of the object is produced on the retina. In fact the image on the retina is upside down and back to front;
however, because the brain has all our lives "seen" objects upside down and back to front, it appears normal to us and we are able to use the image to interact accurately with our surroundings.

The most common cause of blurred vision is focussing error (= "Refractive error"):

Short-sight:

When people are said to be "short-sighted" (= Myopia) this is almost always because the eye is longer than normal so that the focus of light from objects is in front of the retina so that rays of light from each point on the object are spread over the retina in a "blur circle" which means that the image is blurred:

Long-sight:

When people are said to be "long-sighted" (=Hypermetropia = Hyperopia) this is almost always because the eye is shorter than normal so that the focus of light from objects is (theoretically) behind the retina. This means that, once again, rays of light from each point on the object are spread over the retina in a "blur circle" which means that the image is blurred:

Astigmatism:

Sometimes the focussing problem is not simple myopia or hypermetropia, but rather the transparent window on the front of the eye (the "cornea") is rugby ball shaped instead of the more normal spherical football shape. This is called Astigmatism, and it causes rays of light in different meridia of the image to be focussed at different distances:

Presbyopia (= age-related requirement for reading spectacles):

Even if our vision is clear when young, as we move towards middle-age the natural lens loses its elasticity and the eye becomes unable to see near things clearly without reading spectacles. This is called presbyopia.

Correcting the above refractive errors with spectacles:

All of the above refractive errors (myopia, hypermetropia, astigmatism and presbyopia) can be corrected with spectacles or contact lenses. These work by altering the directions of rays of light before they enter the eye, so that after the incorrect focussing mechanism of the eye has finished acting on the light, the rays of light are focussed correctly on the retina.

So that's OK then ? Not really - both spectacles and contact lenses cause certain problems:

It may be that in a few years' time we'll look back at the concept of spectacles and consider it ridiculous that we perched lenses on our noses to correct the vision. Just a thought.

So neither spectacles nor contact lenses are ideal. The ideal situation is when the eye is focussed as clearly as possible as soon as the eye is opened each morning, without the need for spectacles or contact lenses.

The next post will talk about the ways in which we can correct the focus of the eyes so there is no longer any need for spectacles or contact lenses.

Or for more information contact the St. Albans Medical Centre, U.K. on telephone: +44 (0)1727 812925 or email: Contact@StAlbansMedicalCentre.com