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Eye Information



More Vision Topics

Why an Eye Has the Rx it Does?

Why does an eye have the Rx it does?
There are many reasons a person’s eye has the prescription it does, but the focus here will be mainly on the shape of the eyeball. If no other factors are in play, a person’s Rx will depend solely on the shape of their eyeball.

Perfect vision is said to be 20/20 or Plano, needing no corrective eyewear, eyeglasses or contact lenses. Plano is no prescription in an eye. If a person has no prescription > 0.0 or < 0.0 diopters, which is the measurement used for eyes, then the person is said to have 20/20 vision. A person with 20/20 vision is said to have perfect vision. Vision is measured and determined using the Snellen Chart with its basis being, a person is able to clearly see details when viewing them at 20 feet away.

Why do I have a minus Rx?
If a person has a minus Rx this means the person hypothetically has too much plus Rx in their eye causing the light rays entering through the pupil, the eye’s window to the world, to come to rest at on focal point before reaching the retina located at the back of the eyeball. The minus prescription cancels out the additional plus or extra corneal material of the eyeball at the front part of the eye.

If when an eye doctor performs an eye examination, refraction, on a person’s eye the result is a +2.0 diopters then a lens with a -2.0 diopters will be necessary to bring the eye to Plano. As previously discussed, Plano is no prescription.

Why do I have a plus Rx?
If a person has a plus Rx this means the person hypothetically has too much minus Rx in their eye causing the light rays entering through the pupil to come to rest past the retina falling on one focal point behind the eye. The plus prescription cancels out the additional minus or “missing” corneal material of the eyeball at the front part of the eye.

If when an eye doctor performs an eye examination, refraction, on a person’s eye the result is a -2.0 diopters then a lens with a +2.0 diopters will be necessary to bring the eye to Plano. As previously discussed, Plano is no prescription.

Why do I have a plus Rx in one eye and a minus Rx in the other eye?
It is not uncommon for a person to have a different prescription in each eye. Accommodation is easily accomplished by the eye doctor prescribing a plus Rx for one eye and a minus Rx for the other. With the technology available today there are no adverse effects making the differences in the two lenses obvious. This means eyeglasses are cosmetically pleasing to the eye.

Why do I have a minus and plus Rx in one eye?
It is not uncommon for a person to need two different prescriptions for one eye. However, this is usually due to the aging process resulting in presbyopia. A presbyopic eye is one that has difficulty seeing at a distance and needs help bringing the eye into focus to see objects far away clearly. At the same time, the presbyopic eye will need help with viewing detail up close due to the muscles in the eye not working as well as they used to. Even the muscles attached to the eye get weaker as they age, but with a little help from corrective eyeglasses or contact lenses clear vision is possible.


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