Reasons For Wearing Spectacles

There are number of reasons why spectacles are required, one or a combination of the following:

Short sight
The eye ball is too long spherically in relationship to the focussing of the eye. This is called myopia as the focal point which should form on the back of the eye (retina) is formed shorter than the length of the eye and requires a (-) sphere to correct (the lens will be thicker at edges and thin in centre, i.e. concave).

Long sight
The eye ball is too short spherically in relationship to the focussing of the eye. This is called hypermetropia as the focal point which should form on the back of the eye (retina) is formed longer than the length of the eye and requires a (+) sphere to correct (the lens will be thinner at edges and thicker in centre, i.e. convex).

Emmetrope
When the eye is the normal size eye, no need to correct (plano or zero) emmetrope. (no prescription required)

Astigmatism
The eye is more like a rugby ball shape rather than a football shape, called an astigmatism, this needs a cylinder to correct it and at a particular direction called axis. This cylinder prevents the images being distorted in that direction by the rugby ball shape eye.

When an astigmatism is present the lens will be uneven thickness' in different directions (thinnest edge is at 90 degrees to thickest edge). A toric lens is a plus or minus lens with a cylinder, a combination of long sighted and astigmatism or short sighted and astigmatism.

Prism
If the image is formed too high/low/left or right of the retina then the image needs a push to form it exactly on the retina. A prism pushes the image onto the retina, once the correct size and direction are found (can be used for muscle imbalance correction).

Presbyopia
As people get older the crystalline lens becomes less flexible causing reading to become more difficult. This in early stages can be corrected by holding the reading text further away from the eye. Eventually arms won’t be long enough, requiring an extra amount of (+) sphere called reading addition to help focus closer.

There are various solutions for presbyope’s:

• Separate reading only spectacles
• Bifocals, distance above line, reading below line
• Varifocals, designed to give infinite number of working distances depending on head movement