I always tell my patients:
“the aim of giving spectacles is NOT to reduce his/her refractive error but only to improve vision”.
Eye glasses are noting but optical instruments to focus light correctly on the sensitive structure of our eye.
Normally, in eyes without any refractive error, objects form a clear & focussed image on the retina, which is the innermost layer at the back of the eye. This is called as an Emmetropic eye.
Some of the important factors which determine this ability to focus include age of the individual, how far or close is the object to the eyes, general health and eye related factors like tear film health, curvature of the cornea (transparent layer in the front of eye), curvature & thickness of the lens inside the eye and the length of the eyeball.
If the front of the eye is more steep or the lens too thick or curved or the eyeball too long, images are then focused in front of the retina. In other words the focal point is too short for that eye and objects closer to that eye appear well focused while those at a distance are blurred.
This is known as Short-sightedness or Myopia. Myopia can be corrected with minus lenses which give a clear image on the retina so things farther away come into focus.
However, if the front of the eye is more flat or the lens too thin or less curved or the eyeball too short, images are then focused behind the plane of the retina. In other words the focal point is too short for that eye and objects closer to that eye will appear more blurred than those at a distance.
This is known as Long-sightedness or Hypermetropia.
Hypermetropia can be corrected with plus lenses which give a clear image on the retina.
Young children have a higher range of focusing powers and can adjust the shape of their lens to overcome small amounts of this hypermetropia. Hence, they do not always require a correction with glasses. However, as we age, this flexibility of lens shape adjustment reduces, causing eye strain for near work beyond 40 years of age requiring glasses for near work. This is termed as Presbyopia.
In some eyes, the front of the eye is an oval shape instead of round or the lens may be slightly tilted in position. In these cases there are two planes of focus instead of one giving rise to image blur. This is termed as Astigmatism and can be corrected with cylinderical lenses placed in a specific axis (given in degrees) to focus the image.
Whatever the optical condition of the eye, wearing you correct prescription lenses is of utmost importance, not only to reduce strain on the focusing mechanism of the eye, but also to allow normal development and functioning of this very intricate optical system.
DNB FICO(UK) F.LVPEI
Senior Consutlant, Pediatric Ophthalmology, Strabismus & Neurophthalmolgy