Myopia and presbyopia
Presbyopia affects vision of some short-sighted more of 45: myopia and presbyopia can be subject to refractive surgery.
Myopia and presbyopia: links and differences
Contrary to widespread belief, the myopia does not protect from presbyopia: any myopic older than 45 years is far-sighted. This misconception probably comes from the fact that myopia facilitates near vision... on condition of not not wear or remove its correction by far. Thus, in certain circumstances, myopia can help compensate for presbyopia, but provided that the myopic become presbyopic:
-Don't wear his myopic correction
-doesn't so need to see from a distance when it withdrew its correction (which case returning the glasses or corrective lenses ' caught' again presbyopia).
-has a myopia nor too low (less than 2-D, it does compensate for insufficient presbyopia) or too strong (greater than 3.50 D or 4 d, it imposes a reading too close distance).
Myopia and presbyopia don't offset each other so not really.
Remember to explain these data as the number of myopia (ex: 3 D) corresponds to the inverse of the distance at which the nearsighted eye sees net without effort (this distance is called ' ") Punctum point« ). The myopia the more "useful" to compensate for presbyopia are therefore between-2 D (0.5 m = 50 cm) and-3.50 D (0.3 m = 30 cm). For a 3D myopia, punctume point is therefore 1/3 = 0.33 m 35 cm. There is a small beach not sharpness and the distance of the point (depth of field) punctum, which means that this type of short-sighted to read effortlessly. On the other hand, a myopia of - 8 d does not allow to read comfortably, at least to approach the book around 1/8 = 0.15 m = 15 cm approximately, which is certainly too close to the eye to be comfortable or compatible with playback on screen for example.
Vision to the presbyopic myopes
To understand and compare the vision of presbyopic myopes with a far-sighted Emmetropic (an Emmetropic is a patient who sees from afar without effort), we can make the following observations:
A myopic of 3D (eyeglass prescription:-3D) sees from afar without his glasses as an Emmetropic who would wear his glasses of addition + 3 D for near vision but would look through them away.
A myopic of 3D is a 'like' an Emmetropic which would permanently a correction for vision close to + 3 D. It can read without glasses, but sees blurred by far. When become presbyopic myopes puts his glasses or lenses to see from a distance, he sees so blur... If it does not carry an extra addition to correct the presbyopia (ex: glasses double homes, progressive lenses, in addition to lenses reading glasses).
To avoid fiddling with glasses for the far and for the close, progressive lenses with a myopic-3D is an option. In the glass portion dedicated to the vision of far (top), power (vergence) is negative (-3D) to correct myopia, so that the short-sightedness can see net by far. In the lower part of the glass, the power includes an addition. At a far-sighted with all accommodation, the addition is + 3 D. The power of the progressive lens is then zero in the lower part of the glass of glasses (but the myopia of the patient allows him to read without effort of accommodation). Some presbyopic glasses for myopic glasses are also inverted half-moon-shaped: the lower portion of the glass is removed.
Note in passing that in one presbyopic farsighted, the addition of almost adds to the correction by far: under this addition, the glasses are particularly "magnifying", especially in their lower part.