WHO NEEDS VISION CORRECTION?
Who needs vision correction?
Anyone who is finding difficulty in seeing far-away objects or near-by objects needs vision correction. Sometimes, one might want to protect their eyes from exposure to excess sunlight or excess screen-time – in which case there might not be a prescription involved but there are solutions available. You can find more details under Our Lenses
Myopia (Shortsightedness) - also known as Nearsightedness, occurs when the light rays focus much before the retina, which makes distant objects appear blurred. This occurs because of the size of the eyeball being longer, cornea and/or lens being too curved for the length of the eyeball.
Myopia is mostly inherited. It is commonly discovered in children between the ages of 6 and 12 years. This condition aggravates during the teenage years and stabilizes in early adulthood Myopia is nowadays observed in younger children as well, because of excessive usage of electronic devices at an early age. Common symptoms associated with Myopia are difficulty in seeing distant objects like reading road signs & bus numbers, frequent squinting of eyes and headaches due to strain on eyes.
Myopia can be corrected by refocusing the light rays onto the eye’s retina. This can be achieved through prescription glasses and/or contact lenses of negative/minus spherical power. The aggravation of this condition in younger children can be controlled through a healthy lifestyle and reduction of screen time or prolonged near activity.
Hyperopia (Longsightedness) - also known as Farsightedness, occurs when the light rays focus much beyond the retina, which makes near objects appear blurred. This occurs because of the size of the eyeball being shorter.
Hyperopia is most often inherited. Longsightedness, to a small extent is usually present in babies and children because of their small eyeball size . As they grow, the eye size gets corrected and hence the vision also gets corrected – generally by the age of seven or eight.
Farsighted people experience head ache or eye strain and may squint or feel fatigued when performing work at close range. In older people, depending on the severity of hyperopia closer objects become increasingly blurred and distant vision gets impacted as well.
Hyperopia can be corrected by refocusing the light rays onto the eye’s retina. This can be achieved through prescription glasses and/or contact lenses with positive/plus spherical power. The eye is otherwise healthy and hence does not require any special precaution.
Astigmatism is a condition when the light rays fail to form a single focus on the retina which makes all objects – near or far appear distorted and blurred. This happens due to the irregular shape of the cornea in the eye . Astigmatism is caused due to the irregular shape of the cornea and most often is present from birth. It could also develop post an eye injury or disease. Distorted & Blurry images of near and far objects, constant squinting of eyes are some symptoms of astigmatism.
Astigmatism can easily be corrected by refocusing the light rays onto a single focal plane. This can be achieved through prescription glasses and/or contact lenses of negative and/or positive cylindrical lens with axis.
Prescription for glasses is different from prescription for contact lenses. It is best to request the eye care practitioner for both versions, or consult an experienced optometrist, as they would know how to convert the prescription. But it is very important to know what the prescription in hand refers to – glasses or contact lenses.
O.D (Oculus Dexter) refers to the right eye and O.S (Oculus Sinister) refers to the left eye. The correction in each eye is independent of the other eye.
Sphere / Spherical / SPH
This refers to the amount of lens power prescribed to correct nearsightedness or farsightedness. A “-“ or minus number indicates nearsightedness (myopia) and a “+” or plus number indicates farsightedness (hyperopia). The absolute number indicates the severity of the condition. As the name suggests, spherical does a uniform correction along all axis of the eye.
The number is measured in Diopters which is a unit of measure for the refractive power of a lens.
Cylinder / Cylindrical / CYL
This refers to the amount of lens power prescribed to correct astigmatism. A “-“ or minus number indicates correction of nearsighted (myopic) astigmatism and a “+” or plus number indicates correction of farsighted (hyperopic) astigmatism. As the name suggests, cylindrical does a correction only along a particular axis of the eye. The orientation of this axis is defined by “Axis” which is a part of the prescription.
Cylinder power is also measured in Diopters (a unit of measure for the refractive power of a lens).
Axis defines the orientation of the axis along which no cylindrical power would be present. The cylindrical power would be present along an axis which is 90 degrees away from the prescribed axis. Axis is measured in degrees where number 90 refers to the vertical meridian and 180 refers to the horizontal meridian
Pupillary Distance / PD
PD is the distance between the pupils of both the eyes and is measured in millimeters. This measurement is important for fitting the lens onto the frame so that optical center of the lens is in line with the pupil center of the eyes. Not sure on how to measure it? Click here to know more.
PD is the distance between the pupils of the eyes and is measured in millimeters. This measurement is important for fitting the lens onto the frame so that optical center of the lens is in line with the optical center of the eyes. Not sure on how to measure it?
Selecting the right lens is as important – if not more important – than selecting the right frame. A wrong choice of lens will not only affect the aesthetic appeal but will also not provide a comfortable vision correction experience.
Before moving onto the various options in the market, a quick look at the technical terminologies involving lenses:
Index of Refraction / Refractive Index is a relative measure of how efficiently the material refracts (or bends) light. This is again a function of how fast light travels within a material. Hence, higher the refractive index, higher is the light bending property of a material and hence lesser will be the lens thickness.
In the image, observe how the light bends as it travels through the prism – This is because of the material’s refractive index.
Again, in the image, observe how the white light gets split into the component colors (colors of the rainbow). This is because of the dispersion of light.
Higher the refractive index, higher is the light bending property and thinner will be the lens.
Moving onto the materials available at hand, there are broadly 2 materials used for lens making – Glass and Plastic. While the former is no longer predominantly in use (glass provides a far thinner lens but is comparatively heavier than plastics), the latter has seen many more variants evolve in recent times.
|Technical name||CR 39||PC||Hi-Index||Super Hi-Index|
|Thickness||Similar to glass||15% thinner than CR 39||30% thinner than CR 39||40% thinner than CR 39|
|Weight||Lighter than glass||10% lighter than CR 39||Lighter than glass||Lighter than glass|
|Suitable for||Low powers||Low to Medium powers||High powers||Very High Powers|
|Remarks||In-built UV protection, Highly Shatter Resistant|