Knowledge of Vision

สายตาผิดปกติ คือ สาเหตุของการมองเห็นไม่ชัดเจน เกิดจากแสงจากวัตถุที่เราต้องการมอง เมื่อผ่านเข้ามาในตา ระบบหักเหแสงของตามีขนาดไม่พอดีให้โฟกัสของแสงตรงพอดีระดับจอประสาทตา แสงที่ไปถึงจอประสาทตาเป็นแสงที่ไม่โฟกัส คนที่มีสายตาผิดปกติจึงมองเห็นภาพที่ไม่ชัดเจนไปด้วย

Grouped into two categories:

• By variations in the structure of the eye’s optical system such as a steep or flat corneal curvature and having an axial length of the eyeball that is short or long resulting to unfocused light on the retina. This type of refractive error occurs with certain people, including nearsightedness, farsightedness, and astigmatism which are usually inherited.
• Caused by an age-related condition called Presbyopia in which the lens of the eye loses its ability to focus on near objects. This vision problem occurs to everyone as due to natural aging, organs decline over time.

A refractive condition wherein the optical power of the eye is too much for the length of the eyeball. As a result, light rays are focused in front of the retina causing image to be blurred at distance. This can be corrected with a concave lens moving the image back to the retina.

Myopia is also divided into two major categories:

• Physiologic or Simple Myopia  It is the most common form that is characterized by a mismatch in the refractive power of the eye and length of the eyeball which does not progress with age.
• Pathologic or Progressive Myopia  It is characterized by a continuous growth of the eyeball even until reaching middle or older age. This condition may lead to other eye diseases in the long term.

A refractive condition wherein the eyeball is shorter than normal or the cornea that is too flat. As a result, light entering the eyes are focused behind the retina resulting in blurred vision up close. This can be corrected with a convex lens moving the image back to the retina.

A refractive condition brought about by an irregularity in the shape of the cornea. Normally, the cornea is round but in the case of astigmatism, it is oval like an egg having unequal radius of curvature. As a result, the light rays does not focus to a single point on the retina, creating a blurred image. This can be corrected by a cylindrical lens.

Astigmatism is divided into two types:

• Regular astigmatism is the type of astigmatism where the cornea is curved more in one direction than the other and is correctable with glasses and contact lenses.
• Irregular astigmatism is the type of astigmatism which is not correctable by glasses.It can result from conditions such as keratoconus. Irregular astigmatism is a term used in Ophthalmology but in Physics it has the same meaning as higher order aberration.

 This is a condition which may occur to everyone even with people who are already nearsighted and farsighted. This is part of the natural aging process caused by thickening of the lens thus losing its flexibility to be able to focus on near objects. Symptoms usually start at the age of 40 such as difficulty in reading a book. If you are nearsighted before, you must take off your glasses to be able to see near or bring the material closer. But for people who are farsighted, presbyopia may occur faster than normal.