Design, Not a Material
What is an aspheric lens and why would you want to recommend one? Isn't polycarbonate good enough? Or high index? Or Trivex? Many opticians ask these questions because they are confused about what an aspheric lens is and how it benefits patients.
Polycarbonate, high index and Trivex are wonderful materials for ophthalmic lenses. The confusing part is that they are lens materials whereas aspheric describes a lens design. So, polycarbonate, high index and Trivex are all available in spherical and aspheric designs.
The Spherical Lens
A spherical lens is shaped much like a sphere or a ball that has been cut crosswise. The shape of the front is round and uniform. In a plus lens, a spherical design causes the wearer's eyes to look larger than they really are (bug-eyed) and in a minus lens, smaller than they really are. A spherical plus lens is thick in the middle and a minus lens is thick at the edges. The power in a spherical lens begins to change immediately as the wearer's gaze moves away from the optical center, so aberrations and distortion cause poor acuity as the eye rotates around the lens. The higher the lens power, the more the distortion and the worse the off-axis acuity. Aberrations caused by light passing through a spherical lens that cause distortion and poor visual acuity include:
Spherical Aberration — causes a blurring of the image because light rays focus sooner at the edges than rays passing through the center of the lens. This aberration does not cause a big problem because the pupil's small aperture cuts out most of the peripheral rays.
Marginal Astigmatism — caused when narrow beams of parallel rays strike a lens at an angle, creating two focus points, where the light rays come to a point at two different places. The difference between the two focus points equals the degree of astigmatism created.
Distortion — caused when light rays move from the center of the lens to the edges. The rays at the edge increase magnification, causing distortion. The distortion in a high plus lens is knows as a pincushion effect. The distortion in a high minus lens is knows as a barrel effect.
Chromatic Aberration — caused when light moving through a lens is dispersed, or broken up into component colors. The colors with shorter, higher energy wavelengths bend quicker and sharper than the longer, slower wavelengths. So, as the light travels through a lens, different colors are visible.
The Aspheric Lens
The word "aspheric" means "not spherical." So, an aspheric lens is not shaped like a ball. The contours of the front surface change from the center of the lens to the edges. Aspheric lenses are designed to guide light rays through the lens so that they all focus together on the retina, and this design results in flatter lenses. Even in lower-power lenses, such as +1.00, the benefit is apparent in a flatter, less bulgy convex lens. In higher-power lenses, such at +4.00, the flatter profile is all the more evident. Aspheric minus lenses are also flatter. In minus lenses, the aspheric design incorporates a more convex shape into the lens surface so that the edges are flatter. The aspheric plus-power lens flattens toward the edges and the minus-power lens steepens toward the edges. This flatter profile in both plus and minus lenses eliminates the magnification and minification of the wearer's eyes. The flatter lens also makes frame selection easier, as many frames, especially those made in Europe, have flatter curves. Aspheric lenses are lighter, since there is less mass, making eyewear more comfortable. And most important of all the reasons to recommend aspheric lenses is the superior optics. The front curves of the aspheric lens are made so that the power remains consistent throughout the lens and as the eye moves away from the optical center, acuity remains sharp.
Things to Remember
When recommending aspheric lenses, keep in mind that the patient's eyes should be centered as much as possible in the frame, both horizontally and vertically. Decentration of more than 3mm makes the edges thicker, defeating the thinner, flatter and lighter qualities of the aspheric design.
Pupillary distance (PD) should be taken monocularly so that the optical center of each lens is correctly positioned in front of each eye.
Determine the optical center vertically for each eye. This also will position the optical center of the lens correctly in front of the eye.
Prescribed prism must be ground into the aspheric lens. The old method of decentering before edging doesn't work with aspheric lenses.
Aspheric Lens Material
The aspheric design is available in CR-39, mid index 1.54, 1.56, 1.57 and polycarbonate (index 1.59). It is also available in high index 1.60, 1.66 and 1.70. A high index aspheric lens will provide the flattest, thinnest and lightest lens available with superior optics.
Progressive Addition Lenses
Progressive Addition Lenses (PALs) are, by virtue of the power changes in the lens, aspheric. The asphericity of PALs differs from a single vision aspheric in that the progression of power from the distance zone to the near zone is achieved through a succession of geometric shapes. Basically, the distance portion of the lens is an ellipse, becoming a circle at the optical center, changing to a flatter ellipse, a parabola and finally, in the near segment, a hyperbola.
Recapping the Benefits of the Aspheric Lens Design
Superior optics due to changing curves on the front surface allowing consistent power over the whole lens surface
Flatter lens profile
Reduction in plus-power center thickness and minus-power ledge thickness
Wider selection of frames
Reduced magnified and minified appearance of the wearer's eyes
Due to the flatter profile of the aspheric lens, especially if it is a high index, up to 10% of transmittable light can be lost due to increased reflections. Even with a tinted or polarized lens, reflections can be annoying and reduce acuity. A premium anti-reflective coating will eliminate the troublesome reflections, allowing for more comfortable eyewear and clearer, sharper vision.
Fitting aspheric lenses
When choosing a frame to accommodate aspheric lenses:
Choose the smallest possible frame so that the eyes are as centered as possible in the frame.
This decreases asymmetry in the curvature of the front of the lens.
Center the eyes in the frame. The frame PD should be as close as possible to the patient’s PD.
Take monocular PDs to ensure that the optical center is in front of the pupil.
Measure the vertical optical center. This also serves to position the optical center of the lens in front of the pupil.
Recommend anti-reflective (AR) coating. The surface of the aspheric lens sits closer to the eye and the flatter profile causes more reflections than does a spherical lens. The reflections can cause problems ranging from annoyance to distractions to image blocking. AR coating eliminates the troublesome reflections.