If you have a strong prescription, it's best to invest in lenses which helps you see whilst keeping your eyes looking their normal size.
This guide explains the differences of aspheric, spheric and high index lenses to help you decide which are best for you.
What is the difference between aspheric and spheric lenses?
Aspherical spectacle lenses use varying curves across their surface to reduce bulk and make them flatter in their profile. Spherical lenses use a singular curve in their profile, making them simpler but bulkier, especially in the centre of the lens.
Example of a + (convex) shaped lens for correcting hyperopia (far-sightedness) in both a spherical and aspherical profile. Notice the flatter and thinner shape of the aspherical lens using the varying curvatures.
What prescription type do aspheric lenses benefit most?
Aspheric lenses are especially beneficial if you have a strong plus (+) Sphere prescription of more than +5.00 dioptres.
Far-sighted (+) spectacle lenses are thickest at their centre; therefore, the flatter aspheric profile is much more aesthetically pleasing. Flatter and thinner aspheric lenses will fit a broader variety of frames, giving you more choice for your next pair of glasses.
Near-sighted (-) spectacle lenses are thickest at their edges. Aspheric lenses are less beneficial for minus prescriptions, but they'll still reduce edge-bulk and appear flatter than a spheric equivalent. Ideal for reducing the weight of your lenses overall.
Thanks to their undulating curves, aspheric lenses reduce the aesthetic magnification of your eyes for a more natural sort of look.
Should I get aspheric lenses?
If your prescription is fairly mild, say less than + / - 2.75 dioptres, aspheric lenses won’t be that much thinner. However, they’ll look flatter and sleeker.
What are the advantages of aspherical lenses?
- Good for strong + prescriptions
- Better aberration for clearer vision
- Better peripheral vision at the lens edges
- Available in CR39, polycarbonate or Trivex
- Less eye-magnification for a more natural look
- Broader frame choice due to their thinness
- Better vision from reduced magnification
- 30% thinner than spheric equivalents
- Lighter in weight
- Flatter surface
What are the disadvantages of aspherical lenses?
- More expensive than spheric lenses due to complex manufacture
- Require lens coatings (anti-scratch and anti-glare) to prevent reflections from their flatter lens surface
- Require accurate pupillary distance and ocular height measurements to minimise distortion from their varying curvatures
- Require frames with lens holes that align centrally with your pupils
- Less robust than their spherical counterparts, making them more prone to damage.
- While these lenses reduce spherical aberration, they may introduce other types of aberrations at the lens periphery, which can affect peripheral vision.
Is aspheric the same as high index?
No. Aspheric lenses refers to the lens profile (cross sectional view), whereas high index refers to the lenses refractive index. Unlike single curve (spheric) lenses, aspheric lenses use varying curvatures in their profile. High index lenses denotes the lenses ability to bend light. With the same dioptric strength, high index lenses are thinner than standard index lenses.
However, aspheric lenses are typically made with high index materials to enhance the benefits of thinner, flatter lenses. Lenses over a 1.67 refractive index are often aspheric.
Spherical or aspherical denotes the profile of a lens. Spheric lenses use a single curve in their profile, whilst aspheric lenses use varying curves. These varying curves make your lenses thinner and generally flatter which makes them look nicer and prevents your eyes being magnified.
Refractive index denotes a lenses' efficiency at bending light. The higher the refractive index, the better it bends light, the thinner a lens can be. In a strong prescription, 1.67 high index lenses will be thinner than 1.50 standard index lenses.
Combining aspheric & high index
Say you need a +6.00 Sphere dioptre. High index 1.67 lenses will be thin, but aspheric high index 1.67 lenses will be even thinner and look visually flatter. As a rule of thumb, the stronger your prescription, the higher refractive index you’ll need.
Are all progressive lenses aspheric?
Yes. Progressive lenses (varifocals) feature varying curvatures and thicknesses within the lens, therefore they’re all considered as aspheric. As they provide a spectrum of focal power from the top the lens to the bottom, differing thicknesses are required.
What are double aspheric lenses?
Double aspheric lenses (bi-aspheric) are designed for strong minus (-) prescriptions which utilise aspheric curvatures on both their front and rear surfaces. These lenses have an enlarged field of view which provides greater vision and reduced distortion at the edges of your lenses.
Slightly more costly than single aspheric lenses, this lens-type are even thinner and even lighter weight. Ideal if you’re heavily far-sighted and your prescription is stronger than +6.00.
Are aspheric lenses more expensive?
Aspheric lenses are more expensive due to how they’re made. Becuase they use more complex curvatures, aspheric lenses are more difficult to produce than spheric ones, hence their increased cost. (Around 20%.)
Who needs spherical lenses?
Spherical lenses are best suited for those with mild prescriptions with a Sphere power of less than +/-4.00 or and Cyl power less than +/- 2.00. As they use a single curvature, they tend to be made from 'stock lenses' which makes them much cheaper and faster to produce and fit to your glasses.
Summary of spherical vs aspherical lenses
- If you have a strong prescription, aspheric lenses will be flatter and thinner than spheric ones. However, they’re more costly due to their more complicated production.
- Aspheric versions of high index lenses are even thinner but have the aesthetic benefit of looking flatter and sleeker.
- Aspheric lenses are approximately 20% more expensive than spheric equivalents, whilst double aspheric lenses are approximately 50% more expensive.
- Aspheric lenses use varying curvature on the front of the lens to reduce bulk, whilst double aspheric lenses do this on both the front and the rear of the lens.
- Aspheric lenses are more prevalent in high indexes such as 1.61, 1.67 and 1.74 designed to reduce thickness for strong prescriptions.
- Aspheric lenses are commonly available in lightweight materials such as polycarbonate and Trivex, but can also be made from CR39 (regular lens plastic.)
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