Professional eye care practitioners have been using contact lenses to aid vision problems for a long time. Typically, the contact lenses worn for everyday issues like myopia have a spherical shape. However, the usage of aspheric lenses has been around for quite some time as well, and their purpose is to increase the depth of focus of the eye: more commonly in the context of multifocal lenses. Manufacturers have developed more aspheric soft contact lenses designed to correct the eye’s spherical aberration and improve vision clarity for contact lens wearers over their spheric lens counterparts.
What is a spherical aberration?
Spherical aberration is the distortion of vision because our eye is a curved sphere and not flat. When light enters our cornea, it gets bent and curved because our cornea is a curved surface, leading to an image that is seemingly curved entering our eye, even if it is not. This happens when the outer parts of a lens do not bring light rays into the same focus as the central part. Images formed by the lens at large apertures make them blurry but get sharper at smaller apertures. This is also referred to as the pinhole effect.
The relationship between spherical aberration and aspherical contact lenses
CooperVision as a manufacturer, mapped out the “average” cornea, knowing that we have spherical aberration-caused visual distortions inherent to the eye itself, and designed a contact lens to cancel out the spherical aberrations that are naturally present. The power profile of the contact lens changes gradually from the central cornea to the peripheral cornea to match the bend and curvature change of the average corneal shape.
This leads to a lens designed to correct your vision ideally at each point of the contact lens — instead of a -3.00 throughout the whole lens, like typical contact lens designs. For example, a -3.00 right in the center with a -2.93 outside and a -2.87 at the periphery. The aim is that each point in space gets corrected accurately through your lenses to help ease how hard your eyes need to work, especially with a varied modern lifestyle of walking while constantly looking at phone screens.
Spheric and aspheric designs differ in the curvature of a contact lens. Aspheric lenses showcase varying curvatures across their surface that include a flatter area, unlike traditional soft lenses with a spherical design. Curvature varies significantly towards the edges with aspheric lenses than in a spherical lens, which has a gradual curvature. Aspheric lenses are also generally more flexible than spherical lenses, where the value of the base curvature is the same at any point on the lens.
Aspheric lenses can correct optical defects and provide sharper vision in the dark and at dusk because light can penetrate and meet at one focal point instead of at multiple points. Although spheric lenses have brilliant imaging properties, aspheric surfaces perform better in severe ametropia because they provide much more precise light guidance. This is especially apparent in the marginal areas of the lenses because the light is directed to the focal point of the visual aid from those areas. This means if you have severe ametropia and have been prescribed to use aspheric lenses, you may find that your vision is wider, clearer, and brighter. These lenses can be helpful for those with low levels of astigmatism during sports or work with computers.
Why are aspheric lenses not common?
Not every case of vision correction would see additional benefits from using aspheric lenses. A study in the Journal of the American Academy of Optometry investigated whether aspheric design soft contact lenses reduce ocular aberrations, resulting in better visual acuity and subjective appreciation of clinical performance than spherical soft contact lenses. They found that with the brands of lenses tested (Biomedics 55 Evolution and CooperVision), the fitting of aspheric design soft contact lenses does not result in superior visual acuity, aberration control, or subjective appreciation compared with equivalent spherical design soft contact lenses.
Another study listed as an article in the Contact Lens Spectrum found no evidence that aspheric lenses optically correct astigmatism. However, one aspheric lens called the Biomedics Premier effectively corrected the spherical aberration that the contact lens power induces and the spherical aberration of the average human eye.
Some eye care patients prefer to opt for aspheric lenses after trying them out because they feel more comfortable, most likely related to improved visual acuity resulting in less eye strain. There is a noticeable difference in how tired their eyes are after a long day at work in front of their screens. This is not a universal experience, however, for all wearers. Each person’s eyes differ substantially, so it’s essential to visit your eye care professional and understand your specific needs better.