We speak of refractive disorders, when the optical function of the eye structures, away from what the eye would be “perfect” or emmetropic, denominating anisométrope.
. – Hyperopia: a difficulty of vision occurs, especially near objects, requiring an effort of focus, which can manifest as strabismus in children, eye fatigue, headaches at the end of the day, etc …. From the 40 – 50 years the eye decreases your ability to focus and also appears far vision difficulty.
.-Myopia-produces difficulty viewing distant objects, however, optimal viewing and effortless close objects.
. – Astigmatism is due to the irregularity of the curvatures of the lens of the eye, especially the cornea, either by birth or secondary surgeries, scars, etc …. The objects are displayed blurry or distorted, so far, as of close.
. – Presbyopia: also called “sight”. It is caused by the progressive loss, especially after 30-40 years, the ability to strive for near subjects (reading, sewing …). This is due to the aging of the crystal which increases its hardness, complicated curved capacity and thus increase its potency as a lens.
The eye, as a body, must be understood as a series of biological structures whose primary mission is the “focus” of the image perceived in the retina, that is subsequently transmitted through the visual pathway, the brain area responsible for processing them . That is why we have a series of “lenses” optics, some fixed and hemispherical, as the cornea, and other adaptive as the lens, thanks to contraction / relaxation of a muscle, the ciliary muscle, its curvature varies, ranging and power. This allows, for example read (the lens curvature increases / power), and immediately watch the TV (the curvature decreases crystalline / power).
Another key factor is the size of the eye, especially what we call axial length. The lens system to focus the image in a particular spot on the retina, the “fovea”, and so, if the eye is too large, the image focus in front of the retina, and if the eye is too small the image focus behind the retina, although in this case an action of the ciliary muscle – lens may “overtake” the focus until it coincides with the fovea.
This diagram illustrates the various disorders of refraction, taking into account variations in axial length. A.emmetropic eye: light rays from a distance (parallel), after passing through the two lenses: cornea and lens, converge on the retina. B. myopic eye (larger), distant rays converge in front of the retina instead of rays close images (divergent), able to focus on the retina. C. Hyperopic eye (smaller) away rays focus behind the retina, but increasing the power of the lens cristaliniana (accommodation) is unable to focus on the retina.
There also disorders associated with the curvature of the cornea. The most common is astigmatism, which is basically that the cornea is not perfectly hemispherical, well, part of the object is focused in front i one behind so the image becomes blurred and / or distorted. Astigmatism, likewise, can be combined with nearsightedness or farsightedness. The refractive correction of these defects is placed in front of various types of eye lenses to compensate these defects, and finally allow to focus the image at the fovea. This is achieved with glasses or contact lenses, or “manipulating” the lens system through interventions on the cornea (LASIK, radial keratotomy, etc …), or by entering into the eye lenses (ICL IOL surgery clear lens, etc …). Techniques known as “Refractive Surgery”.