Your eyes are amazing tools that take in light and transmit electrical impulses to the brain. Like a digital camera, both are equipped with lenses, but neither really sees anything. Rather, they collect the information to be sent for processing (the brain translating the information for the eye, while the ‘computer’ in the camera processes the digital image). The image that appears on the retina of the eye is transmitted to the brain for processing where vision occurs.
There are two main “lenses” of the eye: the cornea and the crystalline lens. The cornea is a transparent window in front of the eye and is responsible for about two-thirds of the eye’s focusing power. Just behind it is the iris, which gives the eye its color and in the center of the iris is the pupil which allows light into the eye.
Light passes through the cornea into the aqueous body, through the crystalline lens which gives the eye the remaining one-third of the focusing power then passes through the vitreous body before hitting the retina. The retina then processes the light into electrical transmissions to the optic nerve. This leads to the brain which then interprets and creates an image.
People need to wear glasses or contact lenses to help the eye focus when there is a refractive error. The most common type of focusing problem is myopia (nearsightedness). Light coming into a nearsighted eye will come to a focus in front of the retina and be out of focus by the time it reaches the retina. The cause of myopia in the majority of cases is that the eye has grown too large. Myopic eyes can see clearly up close, but objects in the distance are blurry. Minus-powered lenses are used to move the focusing point farther back onto the retina.
Another type of focusing problem is hyperopia (farsightedness). In this case the light theoretically focuses behind the retina. The typical farsighted eye is smaller than average. Hyperopic eyes see better in the distance than they do up close, but neither distant or near objects may be clear. Plus-powered lenses are used to bring the focus point forward to the retina.
Astigmatism is the remaining type of refractive error. The cornea is irregular in shape being more oval than round. Instead of having one focus point like the normal eye, an astigmatic eye has two primary focus points. The more astigmatism there is, the farther apart the two focus points are and the blurrier the vision becomes. Astigmatism will affect both distance and near vision equally. A lens that is curved like a cylinder is used to bring the two focus points together to form one focus point.
The word presbyopia is Greek for ‘old sight’. The crystalline lens inside the eye can adjust its power to change the eye’s focus between objects in the distance and up close. Therefore a young child is able to bring objects very close to his/her eyes and keep the object in focus. Unfortunately, as life progresses, the lens becomes less flexible and cannot change shape as easily. This is known as presbyopia. The early signs of presbyopia are needing more light to read by and/or moving reading material farther away to see clearly. It’s at this point that reading glasses or bifocal lenses are helpful.
There are a few different solutions to presbyopia: bifocals, reading glasses, and monovision. The difficulty for some is choosing the best solution. We’ll try to help here.
A lot of people do not like to wear bifocals (including the progressive lenses or no-line bifocals). Reasons for this vary from difficulties adapting to the way things look through bifocals (especially when negotiating stairs, curbs, etc.) to not wanting to feel/look old. The progressive lenses will take away the line of the bifocal, so cosmetically nobody can tell you’re wearing them. Progressive lenses also have the added benefit of being able to provide clear vision at any distance. True bifocals are great for near and far, but the intermediate range is neglected.
Those individuals who have never worn glasses (or wear contact lenses to correct their distance vision) will need to resort to wearing reading glasses (or bifocals with no corrective power in the top part of the lens). These are available in the drug store, powers ranging from +1.25 for those just starting to need reading glasses to +3.25. Choosing the right lens power is simple: Find the lowest power that allows you to read at the distance you like to read at. Over the years, the power of the readers will need to be increased.
The problem with reading glasses is that they are often not located where you need them to be when you suddenly need to see something up close. For this reason, most people who wear reading glasses will have multiple pairs of them; one for the office, one in the living room, one on the nightstand. Research is underway in the area of presbyopic treatments and although not available at this time, technologies are being developed that will reverse presbyopia and will allow surgery for those that require reading glasses. Monovision may be an attractive alternative for this problem.
Monovision refers to having one eye that is nearsighted (allowing that eye to read) and the other eye being focused for the distance. A minority of people have monovision naturally, but most people who enjoy monovision are doing it with contact lenses or had their eyes corrected with laser eye surgery. One thing is certain: Monovision is not for everybody! Both distance and near vision will be somewhat compromised compared to having both eyes focused at the same distance. The majority of people do not adapt well to monovision, but for those who tolerate the difference between the two eyes, there’s no better way to go.
Who uses Monovision? The most successful candidates for monovision are individuals who:
- Adapt well to change and can accept less than perfect vision.
- Can concentrate on using one eye for reading and the other for distance (some people have such a strong dominant eye that the brain fails to recognize the other eye is clearer at a given distance).
- Do not do long periods of near vision tasks – because most people need to concentrate to see with the non-dominant eye (the eye that is normally used as the reading eye in monovision), it can be tiring if this must be done for several hours a day.
- The best way to experience monovision before surgery is to have a trial with contact lenses. If this is not possible, be sure to discuss monovision with your eye care provider before making your decision.