The Principal Sight Defects Explained
How the eye works
The focusing system of the eye consists of 3 main components:
The retina, which is the light-sensitive film at the back of the eye.
The cornea, which is the focusing surface at the front of the eye.
The lens, which changes shape to focus light onto the retina, this is done by contraction and relaxation of the eye's focusing muscle, known as the ciliary muscle.
In order for the eye to see clearly, light from an object must be focused onto the retina. When the eye is focusing on a distant object, the focusing muscles relax and the lens takes on it's longest, thinnest, and weakest state:
When the eye is focusing on a close-up object, the focussing muscles contract, causing the lens to take on a shorter, fatter and more powerful shape. The light from the close-up object is then focused onto the retina:
When an eye is myopic (short-sighted), the eyeball is too long or the focusing system is too strong, resulting in light from distant objects being focused in front of the retina and distant objects appearing blurry as a result:
Close-up objects still appear clear, as light from these is still focused on the retina. The eye's lens does not need to change shape to focus near print as much in a myopic eye as in a non-myopic eye:
A diverging (concave) lens is placed in front of the eye. This diverges light from distant objects so that this light is focused on the retina, and distant objects appear clear:
The term "hypermetropia" means "over-focusing". When an eye is hypermetropic (long-sighted), the eyeball is too short or the focusing system is too weak.
Because the focusing system is too weak, light from distant objects is not clearly focused on the retina when the lens is in it's longest, thinnest and weakest state, and is instead focused behind the retina:
Therefore, in order to see clearly, the focusing muscles have to work to change the lens to a shorter, fatter and more powerful shape, even when looking in the distance:
Light from a close-up object is focused even further behind the retina:
Therefore, the focusing muscles have to work harder still, in order to focus up close:
The excessive effort needed to focus, in uncorrected hypermetropia, can cause headaches, eyestrain, blurred vision and, in children, a lazy eye or squint.
A converging (convex) lens is placed in front of the eye. This converges light from both distance and near objects so that the focusing muscles no longer have to overwork to focus light onto the retina:
Astigmatism, often referred to as "rugby ball-shaped eye", refers to distortion in the shape of the cornea.
The power of a focusing surface, such as the cornea of an eye, depends of the level of curvature of the surface. A more steeply curved surface will be more powerful, and focus light further forwards:
Whereas a less steeply curved surface will be less powerful and focus light further back:
In a non-astigmatic eye, the cornea is equally curved in all meridians, similar to the shape of a football or other perfectly spherical object, and therefore all of the light passing through it will be focused at the same point:
In an astigmatic eye, however, the cornea has a meridian with steeper curvature, and a meridian with flatter curvature, similar to the shape of a rugby ball:
Note that in the above example, the vertical meridian is steeper and the horizontal meridian is flatter. This is not always the case, however, as astigmatism can take on any orientation.
In an astigmatic eye, the light focused by the steeper meridian is focussed further forward, and the light focused by the flatter meridian is focussed further back. This results in blurry vision:
Astigmatism is corrected using a cylindrical lens. These lenses only have focusing power in one direction, and therefore can correct one meridian of the astigmatic eye without affecting the other:
Diverging cylinder lens Converging cylinder lens
The lens is placed in front of the eye and oriented to line up with the meridian to be corrected. This corrects one meridian but has no effect on the other, so that all of the light entering the eye is focused at the same point, as it would be in a non-astigmatic eye:
Astigmatism often occurs together with either long-sight or short-sight, in which case the corrective lens incorporates the required correction for both defects.
As described previously, the eye's own focusing lens is very flexible and able to change shape to focus light at different distances. It does not, however, stay like this forever. As a person gets older, the lens gradually becomes less flexible and more rigid and, as a result, the eye gradually loses it's ability to comfortably focus on near objects. Once a person reaches (on average) their early-to-mid 40's, the lens is no longer sufficiently flexible to comfortably focus, and near objects such as print, start to become blurred.
In a presbyopic eye, distant objects are clear (assuming no long-sight, short-sight or astigmatism), because light from these is still focused onto the retina:
However, because the lens is now more rigid and can no longer change shape, light from near objects is focused behind the retina and, as a result, near objects appear blurred:
A converging (convex) lens is placed in front of the eye when clear vision is required up close. This converges light from near objects, so that it is focused onto the retina and up-close objects appear clear:
Unlike lenses to correct hypermetropia (long-sight), presbyopia-correcting lenses only give clear vision up-close, and make distance vision blurry. They are therefore only used for near vision.
Presbyopia occurs in addition to any long-sight, short-sight, or astigmatism that may be present.
Presbyopia is not an illness, it is a completely normal phenomenon that affects everyone in the world once they reach their early-to-mid 40's. Nor does it in anyway mean that the eye is "past it's use by date", as the eye can still see just as clearly up close as it could previously, as long as near-vision lenses are in place.