In order for us to appreciate clear vision, rays of light entering the eye must first be refracted or bent by the cornea (i.e. the transparent front surface or ‘window’ of the eye). These rays of light then pass through the pupil (i.e. the black hole) to be further refracted or bent by the crystalline lens inside the eye so that they can be focussed on the retina (i.e. the light sensitive layer at the back of the eye). The retina receives the image formed by these rays of light and transmits the information via the optic nerve to the brain for interpretation.
A refractive error, also known as ametropia, occurs when the rays of light entering the eye are not brought to a focus on the retina. This results in blurred vision and is the reason why corrective lenses (i.e. spectacles and/or contact lenses) are then required. The purpose of the corrective lenses is to alter the way in which the rays of light pass through the eye in order to bring them to a focus on the retina and enable clear vision.
If the rays of light are brought to a focus on the retina without the need for corrective lenses then this is referred to as emmetropia (i.e. no prescription or plano). However, this is actually quite rare as the vast majority of people do have some degree of refractive error.
The different types of refractive error are as follows:
In myopia, commonly referred to as short-sightedness, the eye itself is either longer than normal or the cornea is too steep (although generally it is the former). This causes the rays of light to be focussed at a point in front of the retina. The result of this is that generally near objects are clear whereas distant objects appear blurred.
In order to correct for myopia, minus (negative) lenses are required. These lenses cause the rays of light to diverge, pushing their focus back to the level of the retina.
In hypermetropia/hyperopia, commonly referred to as long-sightedness, the eye itself is either shorter than normal or the cornea is too flat (although generally it is the former). This causes the rays of light to be focussed at a point behind the retina. The result of this is that generally distant objects are clear whereas near objects can appear blurred. However, this really does depend on the person’s accommodation (please see below for more information).
In order to correct for hypermetropia/hyperopia, plus (positive) lenses are required. These lenses cause the rays of light to converge, pulling their focus forward to the level of the retina.
In astigmatism, the cornea is more curved in one direction than it is in the other (i.e. there are two different curvatures to the cornea, each set at 90 degrees to the other). This leads to the commonly used analogy that the cornea is shaped more like a rugby ball rather than a football. The result of this is that vision can become distorted, generally causing objects at all distances to appear blurred.
In order to correct for astigmatism, cylindrical lenses are required. However, these can be minus (negative) or plus (positive) depending on the way in which the prescription is written.
Accommodation refers to the inherent process by which the shape of the crystalline lens inside the eye can be adjusted in order to alter the way in which the rays of light pass through the eye. This allows us to focus on objects at different distances and also change our focus between objects at different distances.
When accommodation is stimulated, the crystalline lens becomes rounder and more spherical. This increases its refractive power, causing the rays of light to be converged and brought to a focus on the retina. Minus (negative) lenses stimulate our accommodation.
When accommodation is relaxed, the crystalline lens becomes flatter and less spherical. This reduces its refractive power, causing the rays of light to be diverged and brought to a focus on the retina. Plus (positive) lenses relax our accommodation.
The ability to accommodate is particularly relevant when it comes to hypermetropia/hyperopia as it is the effectiveness of this mechanism that will determine whether or not corrective lenses are necessary. If a hypermetrope/hyperope is unable to sustain their accommodation in order to keep objects clear, or the effort exerted is causing unwanted symptoms such as eyestrain and/or headaches, then it may be that corrective lenses are required in order to help them.
Typically from about 45 years of age onwards, the crystalline lens becomes progressively more rigid and so less flexible in terms of changing its shape. This gradually reduces our ability to accommodate or focus on near objects, hence why reading and other close work tasks become increasingly more difficult.
This process is referred to as presbyopia and is a (frustratingly) normal age-related change. Unfortunately once it starts it is something that does continue to progress over time but it does tend to slow down or level off later on in life. It also tends to start later in people who are myopic and earlier in people who are hypermetropic/hyperopic.
In order to alleviate its effects, extra plus (positive) power is incorporated into the person’s prescription in order to make near objects clear again. This is essentially what brings on the need for a near prescription (i.e. reading glasses).Back Next