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Why see the myopic blur?

The universal symptoms of myopia is blurred vision by far, IE beyond a distance that is sometimes a few meters (low myopia), sometimes much less (a few tens of centimeters).

Why this blur? One sometimes reads the image formed in a myopic eye is located in a close-up too towards the retina. This is true, but may induce a false perception of the propagation of light and image formation mechanisms (see introduction on the) formation of images by eye).

Light and retinal image

The light does not image form until a "photosensitive" fabric does not interact with it. The light is a transport of information when interacting with the photosensitive retinal tissue, be converted into nerve impulses which give sensation to the brain perception of an image. As long as the light does not interact with matter, there is no image. The retinal photoreceptors (rods and cones) contain light-sensitive molecules (ex: rhodopsin) which absorb the energy of the light particles (photons) and convert them into nerve impulses through electrochemical changes. As long as the light that is refracted by the cornea and the lens has not reached the layer of photoreceptors, there are no retinal Imaging.

"" This seems obvious, but the statement of the "image formed in front of the retina ' implies that the image could form 'ex nihilo '. In a myopic eye, there is only an image, that which is formed on the retina. It is the sharpness of the retinal image which affects the sharpness of the image seen at the level of the Visual areas of the cerebral cortex.

Retinal image of the myopic blur

In fact, when the light emitted by one or several distant sources is focused by the cornea and the lens, there is a plan (called focal plane), where the image of the emitting source will be the sharpest possible (as long as you place a "screen", or a light-sensitive fabric as the layer of photoreceptors of the retina in the plan). Ahead of and behind the optimal plan of collection, the collected image will be more blurred. The focal plane is defined as the plan which focuses the light from a distant source.

One of the conditions for a clear vision is that the image captured by the retina is clear. If the retina is located in the focal plane of the lens 'cornea' couple, then the perceived image is the sharpest possible. So, the distance between the retina and cornea (axial length) determines the sharpness of the retinal image.

The following illustration shows a "Emmetropic" says look, for which the retina is in the focal plane of the horny + crystalline. The image of distant sources is clear, because each source point (component these distant objects) is focused on a one-time basis (or almost, because the pupillary diffraction prevents the formation of a truly one-off image) on the retina. In plans located forward or backward, the image that would be perceived by the retina (if she was actually in one of these plans) would be more blurred.

plan not myopic eye of the image

Retinal image in an Emmetropic eye formation. Terms of collection of the image (the retina) is located in the cornea and the crystalline lens focus plan (plan number 3). In other plans, a retina would collect a more blurry image: the defocus enlarging the image formed from each of the sources in a 'disc' even greater as you move away from the focal plane. The superposition of these discs creates a blurring of the image in the end. 


Myopia is linked to a mismatch between the plan which is located in the retina, and the focal plane of the cornea/lens couple. This plan is behind the focal plane (in farsightedness, he is forward). The perceived image is even blurrier as the eye grows (the collection of image plan back).

myopia plan myopic eye of the image

The retinal plan of a myopic eye (where is located the fovea) is located at the back of the plan where to focus the light emitted by distant sources. The retinal image formed in this plan is blurred.


Myopia and punctum point

Distant sources emit light usually represented as "spokes". They emit light in all directions, but the portion captured by the eye of these rays is small, and due to the distance to the source, these rays can be regarded as parallel, and focused in the focal plane of the cornea and the lens. This one is in front of the retina to a myopic eye. For the closest sources, the incident rays are more parallel but slightly divergent, and this divergence is responsible for their focus Back of the focal plane. There is a distance for which the rays of a near object focus on the retina. This distance is the 'punctum point' of the myopic eye.

myopia blurred and viewing distance

The distance from the observed source determines the degree of image seen in a myopic blur. Upstairs, the source is far away (to infinity), the rays emitted by each source within the image and picked up by the eye are considered to be parallels. If the eye gets closer to the source, this parallelism is reduced is the rays are captured in "divergence." To a certain degree of divergence, the focus of these rays occurs naturally on the retina: 'near' vision is clear, effortless to accommodation of the lens. The distance between the eye and the points of this plan (punctum point) is related to the degree of myopia.

The figure of correction of myopia (ex:-1.50 D) corresponds to the inverse of the distance (in meters) of the plan of the punctum point. If the eye must be placed 50 cm from a source to start to see NET, myopia is 1 / 0.5 = 2 diopters. For a no myopic eye (Emmetropic), the punctum point is located at infinite distance. Myopia is equal to 1 / ∞... either 0 D!


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