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The blurred vision of the myopic

The myopic sees blurred by far (see page: "") Why see the myopic blur? "): the blurred vision of the myopic concerns light sources 'remote', located beyond a distance where the vision is still clear. Indeed, the blurred vision of the myopic degree depends on the distance of the light source observed.

It is conventional to represent the mechanism at the origin of the myopic "blur" by a beam of rays intersecting "in front" of the retina. These incident rays are parallel, as emitted by a distant source (to infinity optical, which for the eye is a distance located beyond 5 metres about). These light rays are initially divergent, emitted in all directions by the source. With distance, they arrive nearly parallel to the level of the eye. So that the eye can have a good visual acuity, it is necessary that the image of a point source is also 'thin' as possible; This will allow to 'split' two point sources close. In the case of an affected eye of myopia, this condition is not met: as the rays intersect in front of the plan of the retina, a divergent beam brush comes "impress" this. the source image is "expanded" in a task seen as "fuzzy".

myopia image fuzzy

Schematic representation of the path of the light rays emitted by a distant point source. A part of the rays emitted by the source is captured by the myopic eye. These rays are almost parallel, and are focused ("intersect") in a plan too ahead of the retinal tissue. They continue their spread in divergence, and the image formed on the retina is more spread out than it would be if the retina was in the plan where the rays intersect.


Close to the myopic vision

If the myopic sees blurred by far, he sees however net without effort (no accommodation) to a 'close', (less than 2-3 meters) distance. It's another way to understand nearsightedness; the eye is "set" to see at a distance over. The myopic eye is like a camera that the update would be fixed and settled on a finite distance (d in the figure). In these conditions, the rays emitted by the source arrive in "divergence", and thereby to converge and focus on the retina. The point at this distance where the nearsighted eye sees net effortlessly accommodation is called punctum point.

myopia and a close source image

A myopic eye sees net at a finite distance, without accommodation. This property allows a myopic become presbyopic to read without effort, as long as d is located at a distance "to read" (30 to 40 cm).

If the nearsighted eye sees net at the distance of the point punctum and blur in the past, he can also see net below the point punctum, in accommodating. For example, a myopic correction is 1 diopter sees net around 1 meter distance. If it accommodates 2 diopters, he sees net at 1/(1+2)=1/3=0.33 meters or 33 cm. A completely presbyopic myopes can no longer fit, and if near-sightedness is less than 2D, it will still need glasses to read comfortably to 35 cm around the age of 60 years. Conversely, a myopic of 8 d has a point punctum located 1/8 = 12.5 cm. To read effortlessly, it must approach his work at this distance. Such myopia is too strong to be useful in case associated with presbyopia.


Away from the myopic vision

The uncorrected myopic therefore unclear by far, IE beyond its point punctum. To correct this vagueness, optical equipment or refractive surgery are necessary. A glass of bezel to concave shape (thicker towards the edge) is indicated; This glass has intended to diverge the incident light rays, as if they came from the punctum point. Rays arriving at the level of the cornea are different, and this difference compensates for convergence excess rays refracted by the myopic eye.

myopia corrected schema

Correction of myopia by a concave glass. By convention, this glass has a power equal to the inverse of the distance of the point punctum, negative. It to diverge the rays, which compensates for excess of convergence in the myopic eye. Optically, everything goes 'as if' the observed image came from the point punctum (concave glass forms a virtual image of the remote objects in his home, which is confused with the point of the myopic eye punctum).

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