Dr. Pattama Cherdchukietsakul, Doctor of Optometry
Myopia ( Nearsightedness )
Myopia refers to an image of object at a distance when coming through our eyes, focuses of the object and falls before reaching retina. This will make an image on retina unclear or blurred. When moving closer to the object, an image in our eyes will move closer to retina until it fits well on the retina. We can see the object clearly. Therefore, Myopia means vision is better up close and if the object is far away, it would be unclear. How far we will be able to see clearly is depending on how much the myopia is. The more Myopia, the more blurry vision will be in the distance.
In case an image of object fits well on the retina, and we move the object more closely, our eyes will not be able to accommodate ( Accommodation refers to automatic adjustment of lens power to increase the total light before falling on retina ). When image of object falls behind the retina, this image will be blurred. Naturally, lens will be systematically adjusted to find the best focus. This will enable us to see surrounding things clearly. However, this systematic adjustment will be degraded by average age of 40 years.
To correct myopia by having glasses or contact lenses, the concave lens will generate light before reaching retina. This will enable an image of object in a distance fitting well on retina. It then gives clear image in a distance. When using glasses or contact lenses to correct Myopia, the accommodation of eyes will automatically adjust the focus and allow us to see clearly at all distances.
Hyperopia ( Farsightedness )
Hyperopia refers to an image of object at a distance when coming through our eyes, focuses of the object and falls behind retina. This will make an image on retina unclear or blurred. According to natural of lens, it will be systematically accommodated the lens power. This makes image falling right to retina and enable us to see clearly even though we are having hyperopia. When object is moving more closely, image of the object will fall beyond retina. Our eyes need more accommodation in order to make the object fit well on retina. This may not be able to give the clear vision or we may have eye ache. This depends on each individual age and how much the Hyperopia of the person is.
To correct vision with glasses or contact lenses, convex lens will gather the light to fall well on retina. The image of object at a distance falls well on retina and will be able to see clearly without accommodation system. We can see well in a far distance.
Astigmatism means image of object in one's eyes fall on different points of the object in the visual axis of the eye. This gives clear image of the object in one line and unclear image on the other line. A person with Astigmatism sees numbers 6,8,9,5 and 0 with edges sharp in vertical line, but they are unclear in horizontal line. Therefore, a person with Astigmatism will not be able to distinguish the different in each number.
The types of astigmatism are: both focal lines are located in front of the retina, in different planes, first focal line is in front of the retina, while the second is on the retina, first focal line is in front of the retina, while the second is located behind the retina. First focal line is on retina, while the second is located behind the retina and both focal lines are located behind the retina.
With Astigmatism, vision is blurred both near and far sight. Correcting by eyeglasses or toric lenses will enable images fall on both axis at the same point on the retina, so that we can see objects sharper.
Presbyopia means image of object in the distance focuses well on retina. When the object is moving closer, the accommodation system will automatically adjust the focus so that the image falls well on the retina. However, when approaching old age, the ability of accommodation system will decline. At a time, the Accommodation system will be not enough to read a book in the normal range. This is why most people need to solve the immediate problem by holding a book in length farther than normal to still be able to read it. The best solution is to use optical positive lenses to accommodate the missing power. So that we can still read a book or can work in the usual space we normally have.