How the Eye Works
The eye is functions like a camera. Light enters through the cornea, then passes through the pupil and lens and focuses on the retina. The retina acts like camera film, reacting to incoming light and sending signals to the brain via the optic nerve. The eye is one of the most complex organs in the body composed of many parts. Good vision depends on all these components working normally together.
The tough, outermost layer of the eye is called the sclera. This is the white that we see in peoples eyes. This maintains the shape of the eye.
The front part of this layer is transparent, and is called the cornea. The cornea is a clear dome-shaped window covering the front of the eye. The cornea is about 0.5 mm thick. The cornea is sometimes referred to as the "window of the eye." It provides most of the focusing power when light enters your eye. The cornea is composed several layers of tissue. The outer layer (the epithelium) is the eye's protective layer. This layer is made up of highly regenerative cells that have the ability to grow back within a few days if injured. Laser vision correction surgery is performed on this part of the eye.
The iris is the coloured part of the eye. The iris is an adjustable diaphragm around an opening called the pupil. The pupil is the central black circle that you see in the eye.
The lens is situated behind the iris. The lens is a clear, bi-convex structure about 10 mm in diameter. The lens changes shape because it is attached to muscles, and this focuses light on the retina.
The innermost layer at the back of the eye is the retina - the light sensing part of the eye. The centre of the retina is called the macular. This is the part of the retina that is responsible for fine, detailed central vision. The retina transmits information via the optic nerve to your brain for processing.
Understanding Vision Problems
If you wear glasses or contact lenses, then you have what we call a refractive error. There are three main types of refractive error – near-sightedness, far-sightedness and astigmatism.
Remember your eye works just like a camera. Light passes through the cornea and lens and is focused on the retina. The retina is the light sensitive part of the eye and this in turns sends the image via the optic nerve to the brain. If the light rays are not precisely focused on the retina, the image will appear blurry.
You need glasses or contacts when your eye cannot properly direct light rays onto the retina. If you wear corrective lenses, you may have one of the following common refractive problems:
Near-sightedness or myopia is when a person is able to focus clearly on near objects but distant objects are blurry. This occurs when your eye is too long in relation to the curvature of your cornea. The image is focused in front of your retina.
Far-sightedness or hyperopia is when someone can focus clearly on distant objects but near objects are blurry. This occurs when your eye is too short in relation to the curvature of your cornea. The image is focused behind your retina.
Astigmatism is a type of refractive error that causes blurriness of all images, whether they are near or far. This occurs when your cornea is shaped like an oval or a rugby ball with one meridian being more curved than the other. With astigmatism there is more than one focal point within the eye. Astigmatism often occurs simultaneously with near-sightedness or far-sightedness.
Presbyopia is often confused with far-sightedness but is something quite different. This is the normal aging that occurs in your 40’s as the lens and muscles of the eye lose their ability to focus sharply on near objects.
This makes it difficult to read or see fine details up close. Initially you can partially compensate for this by holding things further away from you, but eventually you have no alternative but to get reading glasses. LASIK cannot correct this problem.
An option that some people consider is mono-vision. This is where one eye is corrected for distance and the other for near. This can be discussed and should you wish to consider this option you would need a contact lens trial to ensure that you can adapt to it, as it does not suit everyone.
How Bad Are My Eyes?
Visual acuity is a measure of the acuteness or clearness of your vision. It is typically tested with the familiar (Snellen) chart using letters which diminish in size from the top to the bottom. The standard test is done at 20 feet or 6 meters. The number 20 is used for visual acuity measurement in the USA as the chart is 20 feet away. In the UK and South Africa we use the number 6, as the chart is 6 meters away. “Normal” vision is often considered to be a visual acuity of 20/20 or 6/6. This means you are able to see the bottom line on the chart 6 meters away (the size of the letters on this line is 8.9 mm).
If you do not have normal vision your visual acuity may be anything form 20/40 (6/12) to 20/400 (6/120). The higher the number at the bottom, the worse your vision.Common refractive disorders of the eye such as myopia, hyperopia and astigmatism are measured in units called diopters. Diopters represent the amount of correction you need to normalize your vision. The more near-sighted, far-sighted, or astigmatic you are, the higher your prescription in diopters.
Your prescription is composed of three numbers e.g. -5.00/ -1.50 x 180
The first number -5.00 identifies your degree of near-sightedness or far-sightedness. The sign identifies whether you are near-sighted (- sign) or far-sighted (+ sign).
The second number -1.50 identifies your degree of astigmatism. The number can be written either with a (+ sign) or a (- sign).
The third number 180 identifies the axis, which indicates the direction of your astigmatism. An axis of 180 degrees, for example, means the astigmatism is horizontal.
Therefore, a prescription of -5.00 -1.50 x 180 indicates that the patient is moderately near-sighted, with a moderate degree of astigmatism in a horizontal direction.
Simulation of refractive error:
The image below give a very rough impression of the degree of blur that might be seen by someone who has a near-sighted refractive error of -1.00, -2.50 and -4.00 diopters.
Presbyopia and Reading Glasses
Presbyopia is the age-related loss of close-up focusing ability. It's a normal process that everyone eventually experiences.
When you have presbyopia, your natural lens can no longer accommodate. Accommodation is a term used to describe the eye's way of changing its focusing distance. To focus on an object, the eye changes the shape of its lens. When we are young, our eye's lens is soft and flexible and can change shape easily, allowing the eye to focus on objects both close and far away. As people enter their 40’s, the lens becomes less flexible and is unable to focus on close-up objects.
Initially it is only very fine print (such as in the phone book) that appears blurry. The blurring is worse in dim light and you may be able to read if the lighting is good. Early on, holding reading material further away from you may help you read. However, eventually you will need reading glasses, bifocals or multifocals for close work.
Keep in mind that while some symptoms may be similar, far-sightedness is not the same condition as presbyopia. Also note that mild short-sightedness counteracts presbyopia. That’s why if you're slightly near-sighted with presbyopia, when you remove your glasses you may still be able to read. Laser vision correction may treat your myopia, but you may need reading glasses for fine print to correct your presbyopia.
Monovision is an option for some people to help reduce the need for reading glasses if they are in the presbyopic age group.
With monovision, one eye (your non-dominant eye) is purposely left mildly short-sighted (e.g. -1.50). The other eye (your dominant eye) is corrected for distance. The brain chooses which eye to rely on when you look at an object. This is a compromise and works well for some people but others cannot tolerate it. So if you would like to consider this option you would need to let your optometrist fit you with contact lenses with this arrangement prior to surgery to ensure that it suits you. At this stage there is no reliable way to correct presbyopia using the laser; however there is on-going research.