Underlying all of these choices, though, will be certain technical factors that your optometrist will consider. These will determine how well the lens fits on the cornea of your eye — the transparent, protective outer layer of the eye — and the vision correction it provides. One feature in particular that your optometrist will consider is the curvature of the lens — of which there are two types available: conventional spherical contacts and aspheric contact lenses.
But what exactly are these two types of lenses, what are the differences between the two, and how will your optometrist decide which is best for you?
What is spherical aberration?
When a lens has a perfectly spherical shape, which a conventional spherical lens does, something called spherical aberration occurs. This means that parallel rays of light pass through the central part of the lens, and are focused at a different point than the light rays that pass through the edge of the lens. This results in multiple focus points for your eye, which ultimately means your vision will be blurry.
There is some natural spherical aberration in the eye due to the shape of the cornea and crystalline lens (which is a transparent, elastic structure that helps to refract light to focus on the retina), and when a soft spherical contact lens is placed on the cornea, it adopts the shape of the cornea. This leads to an increase in spherical aberration and reduced quality of vision — though it’s your optometrist will make sure this is corrected by picking a lens with a curvature that ensures all rays of light focus on the same point.
What are aspherical contact lenses?
By contrast, aspherical contact lenses don’t have a perfectly spherical shape. This type of lens has a changing curvature across the surface of the lens, and so there is a gradual change in power from the centre of the lens to the edge. In contrast, spherical contacts have a constant curvature and power across the lens surface.
The curvature of an aspheric contact lens deflects light rays in a way that corrects spherical aberration. This reduces blur, improves depth perception, enhances contrast sensitivity, and provides clearer vision.
How are aspherical contact lenses different from spherical lenses?
Generally, these two lenses differ in the following ways:
- An aspheric lens has varying curvature across the surface of the lens rather than a uniformly spherical shape
- Aspheric contacts can correct spherical aberration and reduce the blurring of vision. They can provide sharper, clearer, and brighter vision in some people. On the other hand, spherical contacts conform to the shape of the cornea and add to the spherical aberration present in the eye, due to the natural shape of the cornea and crystalline lens.
- Aspherical contacts can help improve visual acuity (clarity) in low-light conditions following cataract surgery.
Are aspherical lenses right for all contact lens wearers?
Whether aspherical lenses are right for you really depends on your eyes — and, importantly, the amount and type of spherical aberration you experience. This type of lens provides better vision in some, but not all people. To find the right lens for you, your optometrist will assess your eyes and decide whether aspheric contact lenses are recommended.
In general, people with prescriptions between +3 and -6 are unlikely to benefit from aspheric lenses. In fact, using aspheric lenses on these prescriptions may even lead to an over-correction of spherical aberration and worsening of visual acuity. On the other hand, aspheric lenses can benefit people with high refractive errors (more severe instances where the shape of your eyes keep light from focusing at the same point on the retina) by reducing visual blur.3 The aspheric design may also benefit patients with low levels of astigmatism.1, 3
Still unsure which lens may be best for you? Visit our contact lenses page to learn more about the different types of lenses available, or book in a chat with one of our friendly Specsavers optometrists.
1. The Vision Care Institute. (no date). Aspheric Contact Lenses: What’s the Deal? [Online]. Available at: https://www.jnjvisioncare.ae/sites/default/files/public/ae/documents/aspheric_ccontaact_lenses.pdf [Accessed 10 October 2019].
2. American Academy of Ophthalmology. (no date). How to Choose an Aspheric Intraocular Lens. [Online]. Available at: https://www.aao.org/eyenet/article/how-to-choose-aspheric-intraocular-lens [Accessed 10 October 2019].
3. American Academy of Optometry. (no date). Visual and Optical Performance of 55 Aspheric vs. Spheric Contact Lenses. [Online]. Available at: https://www.aaopt.org/detail/knowledge-base-article/visual-and-optical-performance-frequency-55-aspheric-vs-spheric-contact-lenses [Accessed 10 October 2019].
4. Semeraro F, Romano MR, Duse S, Costagliola C. Quality of vision in patients implanted with aspherical and spherical intraocular lens: Intraindividual comparison. Indian J Ophthalmol. 2014;62(4):461–463. [Online]. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4064223/ [Accessed 10 October 2019].