It’s not uncommon for people to have two different prescriptions in their eyes – in fact, it’s quite normal. There are a few different reasons why your eyes may require different lens strengths to see clearly.
Anisometropia
The main reason for having two different prescriptions is a condition called anisometropia. This means that the refractive error (need for glasses/contacts) is different in each eye. For example, one eye may be nearsighted while the other is farsighted, or one eye may have more astigmatism than the other.
Anisometropia occurs when the eyes grow at different rates or have different shapes. Slight differences in the length or shape of the eye result in differences in refractive power between the two eyes. This difference usually develops during childhood.
Having an uncorrected anisometropia can lead to amblyopia (lazy eye) in children if the brain starts ignoring the blurry image from the more ametropic eye. That’s why it’s important to detect and correct anisometropia early through comprehensive eye exams.
Astigmatism
Astigmatism is another common cause of needing different prescriptions in each eye. With astigmatism, the cornea (clear front surface of the eye) has an irregular, football-like shape instead of a round shape.
This causes light rays to focus at more than one spot on the retina, resulting in blurred vision at all distances. The degree of astigmatism often varies between the two eyes, requiring different prescriptions to correct each eye appropriately.
Presbyopia
Presbyopia occurs as a normal part of aging, typically starting around age 40. As the lens inside the eye loses flexibility, it becomes harder to focus up close. Reading glasses become necessary to compensate.
However, presbyopia can set in at slightly different rates in each eye. This means one eye may require a stronger reading add than the other. Having two different prescriptions can help optimize vision at all distances as presbyopia progresses.
Eye Injuries or Disease
Certain eye conditions that affect only one eye can also lead to the need for different prescriptions. For example:
- A cataract worse in one eye
- Age-related macular degeneration in one eye
- Keratoconus affecting one cornea
- A retinal detachment or stroke leaving one eye with poorer vision
- An eye turn (strabismus) causing amblyopia in one eye
Treating these underlying conditions can sometimes—but not always—reduce the asymmetry in prescriptions between the eyes.
Normal Biological Variation
Even with healthy eyes, there can still be subtle differences in refractive error between them:
| Right Eye | Left Eye |
|---|---|
| -2.50 sphere | -2.25 sphere |
With such a small difference in prescription, both eyes can still see 20/20 when corrected and function normally. But the asymmetry is enough to require slightly different lens powers for maximum visual clarity.
In fact, it’s more abnormal if your prescriptions are exactly the same in both eyes! Matching prescriptions precisely occurs in less than 25% of people.
When to Be Concerned
While a small difference between your two prescriptions is normal, there are some cases when you should be concerned:
- If there is a sudden large change in asymmetry, this could indicate a new eye condition requiring urgent medical attention. For example, a swift increase in difference between the two eyes could be a warning sign of a retinal detachment or stroke.
- If you have symptoms like double vision, eye strain, or headaches from the mismatch between your prescriptions, you may benefit from vision therapy to help your eyes work better together.
- If there is significant anisometropia in a child, amblyopia treatment may be recommended to ensure the child develops normal vision in both eyes.
Overall, a prescription discrepancy of 1.00 diopter or less is considered normal and non-problematic. Larger unexplained differences or sudden changes warrant a comprehensive eye exam to look for underlying causes.
Correcting Unequal Prescriptions
If you require different lens powers for each eye, there are a few options for correcting your vision:
- Separate glasses for each eye – With this traditional approach, you have distinct pairs of glasses made for each eye. You may alternate between the pairs or wear both simultaneously. This can be awkward cosmetically but works well optically.
- Bifocals – A bifocal has different lens powers in the upper and lower segments. By aligning the stronger prescription with the weaker eye and vice versa, both eyes can look through the optimal part of the lens.
- Progressive lenses – These no-line multifocals provide a smooth transition in power from top to bottom. The prescription can be customized asymmetrically to suit each eye.
- Monovision contact lenses – This involves correcting one eye for distance and one eye for near. The brain adapts to the slight mismatch between the eyes.
- Vision therapy – Orthoptics exercises can train the eyes to work better together through improving convergence, accommodation and eye tracking skills.
Discuss the pros and cons of each option with your eye doctor to determine the best solution for your degree of anisometropia and lifestyle needs.
The Bottom Line
Having two different contact lens or glasses prescriptions is very common—differing by up to 1.00 diopter is considered normal. Anisometropia, astigmatism differences, presbyopia changes, and eye health conditions are reasons your eyes require customized prescriptions.
Work with your eye care provider to correct each eye accurately and optimize your vision. With the right solution, you can see clearly and comfortably even with asymmetric prescriptions between your eyes.
Our eyes are complex organs with many specialized structures that work together to deliver crisp, clear vision. Since these delicate structures develop independently in each eye from conception, some natural variation between the two is inevitable. Having two unique prescriptions tailored to your individual eyes is not a cause for concern in most cases—rather, it allows you to see your very best!
Your optometrist or ophthalmologist can determine if your prescription difference is within normal limits and help correct your vision for 20/20 sight. Don’t hesitate to discuss your options for managing unequal prescriptions to find the optimal solution for your needs and preferences.
With an awareness of the various factors that can cause asymmetry between your eyes’ refractive powers, plus the right corrective lenses or treatment, you can gain a clearer perspective on why your two prescriptions don’t always match up—and how to see your sharpest with this common vision condition.
Our eyes are complex organs with many specialized structures that work together to deliver crisp, clear vision. Since these delicate structures develop independently in each eye from conception, some natural variation between the two is inevitable. Having two unique prescriptions tailored to your individual eyes is not a cause for concern in most cases—rather, it allows you to see your very best!
Your optometrist or ophthalmologist can determine if your prescription difference is within normal limits and help correct your vision for 20/20 sight. Don’t hesitate to discuss your options for managing unequal prescriptions to find the optimal solution for your needs and preferences.
With an awareness of the various factors that can cause asymmetry between your eyes’ refractive powers, plus the right corrective lenses or treatment, you can gain a clearer perspective on why your two prescriptions don’t always match up—and how to see your sharpest with this common vision condition.
