GiarreBlog.Com

Because the corrective effect of a positive lens increases with distance, it corrects a higher hypermetropia than is indicated in diopters.. Ignoring the very small differences that derive from this, the real hypermetropia is measured with a thin lens put on the principal plane of the eye.

If instead the lens is put at 14mm from this plane, its focal distance will have to be 14mm longer, so that a real hypermetropia of 4.0 D will have to be corrected by a lens with a focal distance of 250 + 14 = 264 mm which in diopters is approximately +3.79D.

A formula that gives these same values is the following:

Where ? = Power of the lens;

where A = Real ammetropia of the eye;

where d = distance of the lens from the principal image plane, expressed in meters.

Related Article: Measuring hypermetropia

The measure of total hypermetropia is represented by the power of the positive lens with which, at relaxed accomodation, the eyesight reaches its maximum sharpness.
The accomodation is never completely relaxed and in normal conditions we could measure only the manifest part.

The lens that measures the hypermetropia is the one in which the focus coincides with the remote point of the eye at relaxed accomodation.
The manifest hypermetropia is measured by the strongest positive lens that achieves the best vision. In fact, for vision at a distance, of a positive lens of 1.25 D 10/10 is obtained and the result is the same with 1.50 D but with 1.75 D vision reduces to 9/10 - 8/10, the strongest lens that permits the best vision is the one of 1.50 D.

In reality the best measurement of manifest hypermetropia is obtained by the strongest lens when both eyes are employed in the vision. Measurement of the latent hypermetropia is done with the difference between the strongest lens that gives the best vision and the one that corrects the manifest hypermetropia; therefore, if the manifest part is of 2,00 D e in cycloplegy we find 3,00 D, the latent hypermetropia is 1,00 D.

From the calculation point of view this reasoning seems correct, in practice one has to take into account that cyclopegy, paralyzing the muscles, show a hypermetropia that should not be considered such, because when the cycloplegic wears off the muscle tone is restored and the apparent hypermetropia of 1.00 D disappears.

Related article: Acquired Hypermetropia

Excluding the hypermetropia caused by an insufficient development of the eye, there is one known as “acquired” which can result in:

1. Increase of the refraction index caused by generic diseases.

2. Reduction of the eye’s refractive capacity caused by age;

3. Extraction of the chrystal

The acquired hypermetropia becomes evident at the age of 50-55 when the emmetropic eye becomes hypermetropic; or if the eye was first slightly myopic, the quantity of hypermetropia first neutralizes the myopia, then exceeding the myopia value it becomes finally a slight hypermetropia. The causes of acquired hypermetropia are not exactly and completely known; with certainty one can say that the preponderant part of it is caused by the increase of density of the chrystal. This increase is twofold: in the refractive sense and also in the volumetric sense, causing an increased use of the external faces of the chrystal, being less curved, which gives an inferior total power.

In any case, whatever the reason, it is known that at a certain point in life, the eye starts to lose a bit of its potential and this reduction continues year after year, following the table below:

Related article: Accomodation spasms.

Is the condition when a hypermetropic exercises, for a distant vision, more accomodation than available.

If the hypermetropia is 2,50 D and the exercised accomodation is also of 2.50 D but no part of it is released, it is called latent hypermetropia ; if instead for the same defect 3.50 D of accomodation power is required, one speaks of accomodative spasms.

Generally the accomodative spasm is caused by an astigmatism, or by an uncorrected hypermetropia; sometimes the spasm cause the hypermetropic to become apparently myopic. Often the term “accomodative spasm” is used in those cases in which a large part of the hypermetropia is latent.

Articolo correlato: Relative Hypermetropia