Do you look at your prescription and notice a significant difference between your right and left eye? Or perhaps one of your lenses is noticeably thicker than the other? This is fairly common, and it has a name: anisometropia.

A difference in prescription between the two eyes does not automatically mean that there is a serious problem. However, the larger the difference, the more important the choice of lenses and frame becomes if you want comfortable, attractive and well-balanced glasses.

 

A difference in prescription can make one lens require significantly more optical power than the other.

What is anisometropia?

Anisometropia occurs when the two eyes do not have the same refractive error—in other words, when they do not need the same corrective power.

For example:

• right eye: -1.00 D;

• left eye: -5.00 D.

Both eyes are nearsighted, but the left eye needs a much stronger prescription.

Anisometropia can also involve farsightedness or astigmatism. One eye may be significantly more nearsighted, farsighted or astigmatic than the other.

Why can my two eyes have different prescriptions?

Our two eyes are similar, but they are not perfectly identical. Their length, corneal curvature and optical power may differ slightly. During growth or as vision changes over time, one eye may also change more than the other.

A small difference is often well tolerated. When the difference becomes more substantial, however, the brain has to combine two images produced by different optical systems.

In children, significant anisometropia deserves particular attention. If one eye provides a noticeably less clear image during visual development, it may be associated with amblyopia. This is one reason eye examinations and the prescribed correction are important during childhood.

Why is one lens thicker than the other?

The power of a lens directly affects its geometry.

In the case of nearsightedness

A minus lens is generally thinner in the centre and thicker at the edges. The stronger the myopic prescription, the more visible the peripheral thickness may become, especially in a large frame.

If you have -1.00 D in one eye and -5.00 D in the other, it is therefore perfectly possible for the second lens to look much thicker.

In the case of farsightedness

A plus lens is generally thicker in the centre and thinner towards the edges. A significant difference in farsightedness can therefore also create a visible difference between the two lenses.

What about astigmatism?

Astigmatism adds power that varies across the different meridians of the lens. The axis and cylinder value therefore also influence how thickness is distributed.

The prescription is not the only factor, however: the size and shape of the frame, lens centration, pupillary distance and chosen material also play an important role.

 

Lens geometry changes according to the type and strength of the prescription, as well as the material selected.

Do I automatically need high-index lenses if I have anisometropia?

No—not automatically.

Anisometropia describes a difference in prescription between the eyes. Whether you need a thinner lens depends more specifically on the power of each lens, the frame you choose, the aesthetic result you want and your optician’s recommendations.

For a small prescription difference, a standard material may be perfectly suitable. For stronger prescriptions, a higher index can noticeably reduce lens thickness.

1.60, 1.67 or 1.74: what does a lens index mean?

The refractive index indicates how effectively a material bends light. A higher-index material can produce the same optical power with a thinner geometry, all other factors being equal.

Common indices include:

• 1.50 or 1.56 for many low prescriptions;

• 1.60 as a first level of thinning;

• 1.67 for stronger prescriptions;

• 1.74 for certain high prescriptions when reducing thickness justifies the choice.

However, the highest index is not automatically the best choice. The actual benefit depends on the prescription and the frame. For some prescriptions, moving to 1.74 makes very little visible difference compared with 1.67 while increasing the price.

The right choice is therefore a balance between thickness, optical quality, frame, lens design and budget.

The frame can make a huge difference

With a strong myopic prescription, a very large frame can considerably increase the visible thickness at the edges. A smaller frame, properly centred in front of the eyes and suited to the prescription, can produce a much thinner result—sometimes without changing the lens index at all.

That is why it is better to choose a frame with your prescription in mind, rather than judging it only by how it looks on the display.

At LEONE EYEWEAR, when a prescription may produce a thick lens, we can consider several factors before placing the order: frame dimensions, centration, lens power, material and index.

Lens thickness and image size are two different things

With significant anisometropia, another phenomenon may sometimes come into play: aniseikonia.

Aniseikonia is a difference in the size—or sometimes the shape—of the image perceived by one eye compared with the other. Eyeglasses can influence this difference because lenses can magnify or minify the image.

It is important not to confuse the two issues: making a lens thinner does not automatically eliminate aniseikonia.

When someone with a large prescription difference continues to experience discomfort despite a properly made correction, other parameters can be assessed by an eye-care professional. In some cases, contact lenses can also reduce the difference in magnification associated with eyeglasses because they sit much closer to the eye.

 

A thinner lens and a more similar perceived image are related to different optical considerations.

Why do my glasses make one eye look smaller or larger?

This phenomenon is also related to lens power.

Strong myopic prescriptions tend to make the eye look smaller through the lens, while strong hyperopic prescriptions can create a magnifying effect. If the two eyes have very different prescriptions, this effect may be more noticeable on one side.

The choice of lens, frame and frame dimensions can help limit visible asymmetry, although it is not always possible to eliminate it completely.

Can two lenses be made to look more balanced?

Often, yes. An optician may consider:

• the material index;

• the size and shape of the frame;

• lens centration;

• lens geometry and design;

• the expected final thickness before manufacturing.

The goal is therefore not simply to choose “the thinnest lens”, but to create the best combination of your prescription and your frame.

In summary

Having different prescriptions in the two eyes is possible and is called anisometropia. The greater the difference in prescription, the more visible the difference in lens thickness may be.

High-index lenses—1.60, 1.67 or 1.74, depending on the situation—can reduce thickness, but they are not always necessary. The frame you choose can be almost as important as the lens material.

Do you have a strong prescription or a significant difference between your two eyes? Bring your prescription to LEONE . We can help you choose a frame and lenses suited to your prescription, without charging you for a higher level of thinning when it would not provide a meaningful benefit.

FAQ

Is it normal for one lens to be thicker than the other?

Yes. If the two eyes do not have the same prescription, the two lenses will not necessarily have the same thickness. The frame and centration can also make the difference more or less noticeable.

Is a difference in prescription between the two eyes dangerous?

Not necessarily. Many people have some degree of anisometropia. A large, new or poorly tolerated difference should nevertheless be assessed by an eye-care professional. In children, follow-up is especially important because vision is still developing.

Can contact lenses help with significant anisometropia?

In some cases, yes. Because a contact lens sits directly on the eye, it may produce less difference in magnification between the two eyes than eyeglasses. The suitability and safety of contact-lens wear should be assessed by an eye-care professional.

Scientific sources for editorial validation

1. Tang CY. Thickness and weight of lenses for myopia. Ophthalmic and Physiological Optics. 1990;10(2):159–167. PubMed: https://pubmed.ncbi.nlm.nih.gov/2371061/

2. South J, et al. Clinical Aniseikonia in Anisometropia and Amblyopia. British and Irish Orthoptic Journal. 2020. PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC8269785/

3. Cotter SA, et al., Pediatric Eye Disease Investigator Group. Treatment of anisometropic amblyopia in children with refractive correction. Ophthalmology. 2006. PubMed: https://pubmed.ncbi.nlm.nih.gov/16751032/

4. Zhao L, et al. Evaluation of aniseikonia with an auto-stereoscopic smartphone. 2019. PMC: https://pmc.ncbi.nlm.nih.gov/articles/PMC6944609/

This article is intended for general information and does not replace an eye examination or personalised advice from an eye-care professional.