We have seen that the eye to focus up close triggers the mechanism of the "accommodation", by which and by the action of the ciliary muscles by varying the thickness of the lens, thereby increasing the horsepower (which remember is a biconvex lens inside the eye). This makes the image of a point close to form on the retina can see it clearly.
Over time, the ciliary muscles, like the rest of the human body, they lose tone and becoming less potent. At the same time, the lens becomes less flexible, yet which the ability to acmodar and thus for near diminishes with age.
Thus we define as presbyopia and optical condition in which, due to the changes caused by age, irreversibly decreases the power of accommodation.
Presbyopia is therefore a physiological and pathological. "Of which does not escape anyone," I add.
Presbyopia begins to manifest approximately between 40 and 45 years, and their symptoms are very clear: It starts being separated reading or close work and it takes more and more light. When the effort is prolonged start dancing letters, etc.
Presbyopia affects both nearsighted and farsighted, thus confirming earlier in the latter than in the former. And of course affects those who have never worn glasses. This is a situation that is often very "shocking" for these people, to notice a rapid deterioration of near vision and usually worry. This should not be a concern because we have said that this is an absolutely physiological thing.
The judgment of the accommodation is not sudden and complete, but that is progressive from small to be revealed at 40, and then continue to rise until approximately there is a difference of about 3 dipotrías between graduation from afar and up close. Normally, a correction of presbyopia should last about two years (sometimes more, sometimes less), so you have to be aware of the need to review, at least every two years.
How to correct presbyopia? The solution is easy. We are in a similar case of hyperopia: formation of the image behind the retina. Therefore it is necessary to place a lens to make converge rays entering the eye. This is done with a lens logically convergent (positive), which is very reasonable if we have seen that presbyopia is a problem of lack of refractive power of the eye.
On the correction to do some precision: Some people are reluctant to use reading glasses for the first time, saying that if they get used to them and can not let go. There's a reason I put of it, but not entirely so. It happens that once presbyopia shows, progress is "relentless", and near vision gets worse and worse, used or unused glasses, with the aggravation of doing an extra effort to the eye and disposition is bear. Man, it also happens that the human brain is an extremely wise, and once they found that wearing glasses can see much better and close more easily, then it is difficult to "convince" them otherwise.
And what kind of correction is more convenient? There are several ways to compensate for presbyopia: half-moon glasses, conventional glasses near graduation, bifocals, progressive glasses or contact lenses.
Who does not require graduation from a distance, or require little, I recommend trying the half-moon glasses, which allow them to look over to the away without having to remove them or bajárselas. The disadvantage is that they apply to "old glasses," or purely aesthetic, but currently makes some of the very latest fashion.
Conventional glasses ("whole") have the disadvantage that when he looked through look closely graduation, which is not seen clearly, being able even to feel certain sensations of vertigo. They can be useful for near vision tabajos long row and specifying a wide visual field, or for jobs that require a closer look at higher or lower levels than usual.
A good solution usually bifocals, which see from afar by the top and near the bottom, no more than a nod naturally. Requires getting used to, but it is relatively easy and should pay special attention to the early days up and down stairs. The disadvantages: It shows the near vision portion, which aesthetically not good (it is a sign of age :-) not allow a clear vision at intermediate distances, forcing approach. They are not comfortable for close in planes above or below the normal forces as awkward positions.
Another good solution is progressive glasses. This type of lens allows culaquier clear vision at distance, simply by varying the inclination of the head, without having to approach what you want to see in near vision. are, in principle, the solution that restores a more similar to what is the visual system before presbyopia. It is very good for whom their work environment is great near vision or who needs change frequently looking away. They have, of course some drawbacks: Side vision is not too good. and certainly worse than bifocal lenses (although progress has been made in this regard, still look "sideways" with a progressive is still quite complicated). Lenses are thicker at the bottom, so that the weight is larger (also in this aspect has been improved a lot, and there are several types of lenses with very light weights). The adjustment is usually more complicated and longer than in a bifocal, although the final results are usually good. Contrary to a widespread view, use your foundation becomes similar to bifocals, ie the top looks for the far and the bottom for near but in between graduation varies progressively and can be seen at intermediate distances. The main problem is still in the price: lenses are usually much more expensive than a bifocal, although at present the great variety of offers you access to economic series that put progressive lenses affordable to almost any pocket.
Another (perhaps the least optically speaking) significant advantage of progressive lenses over traditional bifocals is aesthetics. By not being differentiated the part devoted to near vision, like a normal lens, which "hides" a defect but normal is related to age.
Finally, note that it is very important that when a visual psar, is made clearly clarify what our greatest needs are taken into close view, as this may influence the correctness or type of correction that we recommend professional but, without doubt, the professional and worry of the subject and will be interested in it.