By Barry Santini
As online eyewear increasingly takes market share away from main street optical, with satisfaction metrics revealing
happy and satisfied customers, it would be easy to conclude that mastering the most optimum way to place prescription lenses in a chosen frame may no longer be worth the time. But nothing could be further from the truth. There’s quite a bit more to know about designing a pair of high optical quality glasses than most people—even those in the industry—think about or are willing to admit.
But this knowledge alone is no longer enough to survive in today’s super competitive market. Today’s opticians also need to understand why departures from excellence are well tolerated for many wearers. Further, opticians must recognize that they only create added value when they can explain, in an approachable and persuasive manner, how premium optics, materials and attention to design really do matter to buyers.
Although many opticians believe that a pair of well chosen, designed and fabricated eyewear has no real competition from online vendors—the burden of proving this to the eyewear buying public does fall squarely on their shoulders. The contemporary skill set of every good optician must therefore include both the ability to deliver an impressive visual experience and the confidence in knowing that anything less will over time, ultimately prove unsatisfactory to the buyer—no matter what they’ve paid. These two experiences—the initial impression of the “Wow” and the longer-term dissatisfaction called the “Whoa”—are nearly impossible to achieve without having full knowledge of the specific preferences and experiences of the wearer.
Combining this extensive personal information with a deep technical facility defines what the eyewear consumer should expect from a main street optical business in the 21st century. To create this new standard, start by arming yourself with all the PDs, heights, pantos and vertex distances you can muster. Then let’s begin to unpack the deep optical knowledge needed to properly design and make a pair of superlative eyewear.
START WITH PROPER OC POSITION
The majority of eyewear made is single vision (SV). And the primary goal in SV glasses is to properly place the optical center (OC) in front of the eye. There are five aspects regarding monocular or binocular consideration to keep in mind when determining OC placement. Here they are in a suggested priority order:
- Eliminate or reduce unwanted prismatic horizontal imbalance at the Major Reference Point, (MRP) which is defined as the point where the wearer’s primary gaze intersects the spectacle plane. For simple rotationally-symmetric lenses, the horizontal alignment of the lenses is complete for an Rx designed for distance vision when made “on PD.” Reading or intermediate glasses involve more considerations. (See sidebar.)
- Reduce or eliminate any vertical prismatic imbalance created by an aniso or antimetropic prescription the Major Reference Point.
- Properly align the complete image shell, formed by the Rx lens, to the far point sphere, or shell in the ametropic eye. This is done by lowering the OC from the MRP 0.5 mm for every 1 degree of pantoscopic frame tilt (Martin’s Rule of Tilt). Large deviations from a normal vertex distance (VD) and/or higher prescription powers may require recalculating how much to adjust the OC. Image shell alignment assumes greater importance with increasing Rx power, frame size and lens sophistication—aspheric, atoric and freeform designs—along with proper choice of base curve in order to maintain good peripheral acuity.
- Reduce the image degrading effects of low Abbe value/high dispersion lens materials, such as polycarbonate and 1.74 index.
- Obtain the most acceptable and pleasing cosmetics in the finished eyewear. This may both include lens thickness and/or prism wedge revealed in the shape of the chosen frame. The simplest way to achieve good cosmetics is to place the OC near frame datum—defined as the midpoint in the vertical or B size of the boxed lens shape.
To position the optical center horizontally, start by measuring the PD: Traditionally done by ruler, pupillometer or digital centration device, the Achilles heel in all these methods is that the result is only an objective measurement. There are some assumptions that are flawed behind the use of pupil center or corneal reflex when determining where the eye’s fovea is actually pointing. Therefore, using the eye’s visual axis is the only standard to employ when ensuring that prism is being properly handled. Tip: An unconventional way to determine the visual axis separation is with an 8 x 24 or 10 x 24 pocket binocular. Nothing fancy needed here, these binoculars can be found online for as little as $12.
Align the binocular tubes to overlap the fields of view into one circle and measure the distance between the eyepieces will deliver a reliably accurate binocular visual axis measurement. This value, along with determination of the dominant eye’s position, will deliver the best subjective PD obtainable. This method is also great for aligning the corridors of progressive lenses optimally.
To place the optical center vertically, start by doting the pupil: This measurement is called the pupil height—which is the height of the pupil above the lowest contour of the bottom eye wire—uses the boxing system of frame measurement as its reference. Remember to keep the facial plane perpendicular to the floor when marking the pupil to reduce parallax. Next, follow the above sequence of OC considerations and calculate where you intend to place the OC. This is the fitting height and is the value you give the lab in the “OC height” box. For rotationally symmetric lenses, meaning a lens whose design is not personalized by position of wear (POW) fitting values such as pantoscopic angle and frame wrap, specifying the (OC) fitting height is one of the most important things you can do to ensure visual satisfaction for the wearer.
Determining optimal alignment of single vision lenses requires more factors be considered than is typically seen. Progressives and segmented multifocals, however, have even more to think, because in these glasses, there are two masters.
A TALE OF TWO MASTERS
In a single vision lens, the MRP for the distance gaze dictates a specific area of greatest optical priority—a single “master” requiring your attention. In any lens providing an additional zone to assist reading, this area becomes one of almost equal priority. So all progressives, lined multifocals and even SV “assist” lenses have prismatic and OC placement considerations that feature “two masters”—one for distance and one for reading. Evaluation is done using the method for single vision lenses, with a few differences:
- The amount of prism considered acceptable must be balanced between the distance and the reading zones. This has been addressed in progressives by placing the prism reference point (PRP) some distance below the fitting point, toward the reading zone. In the absence of prescribed or cosmetic thinning prism, the PRP should feature no prism.
- The actual reading point below the PRP should be estimated in progressives using the lenses minimum fitting height. This is typically 11 mm to 16 mm below the PRP, and calculations regarding net prism imbalance therefore may vary, depending on design.
- Bifocals and trifocals should have their optical centers placed in accordance with the single vision rules. However, if there is high imbalance in the reading area, bicentric grinding (slab off) is suggested in prescriptions of acute power disparity.
- The image degrading effect of low Abbe becomes more impactful in multifocals as the reading zone is almost always located further away from the PRP than the pupil height. Higher hyperopic distance corrections are complicated in this regard by the additive effect that Rx add power has on the prismatic displacement. Conversely, minus Rxs often demonstrate less sensitivity because the total lens power away from the PRP is reduced by the value of the presbyopic add power.
- Cosmetic considerations in hyperopic prescriptions are addressed by thinning prism, calculated at a rate of 0.6 prism diopters base down times the power of the add and measured at the PRP. In most current lab management systems (LMS), adding any prescribed prism will negate automatic thinning prism. You must factor this into your calculations for cosmetic and imbalance considerations.
COSMETICS: THE THIRD MASTER
In both single vision and multifocals, cosmetic optimization was placed last—the position of lowest priority. Yet experience on the optical front lines shows that wearers often articulate cosmetic concerns more than problems with optics. Therefore, a balance must be struck between these two entities, which are constantly at war in every pair of glasses. Finding this balance requires juggling design, material, base curve, prism and even frame choice for each Rx. Superlative eyewear definitely includes getting this balance perfect for the individual buyer, so spend some time calculating the effects of prism, Abbe and power to achieve the best outcome. In view of the higher prices main street opticals typically charge, the eyewear buyer deserves nothing less.
In striving for optimum cosmetics, be sure to include one or more of the following tools. Remember, each tool’s advantage can come with one or more disadvantages:
- Higher Refractive Index Materials - Decreases lens thickness but feature lower Abbe values, which means more chromatic aberration.
- Flatter base curves - Decreases lens thickness but increases the incident angle of peripheral rays.
- Moving the OC back to Datum - Will balance cosmetic lens thickness within the frame, but can increase prism and lens tilt at the MRP for the distance gaze.
- Grinding cosmetic prism - Can offset unwanted wedge thickness, but is the same as introducing unwanted lens tilt.
- Frame Change - Solves many challenges, but may disappoint a buyer whose heart was set on a particular style. Suggesting a frame change is definitely something that online opticals will find very hard to do.
With all the effort we’re putting into getting the optics, comfort and cosmetics right, it remains surprising to many ECPs why the eyewear buying pubic tolerates glasses made with far less care and concern. The answer is—as my friend Tina Lahti told me—is that human vision is squishy
HUMAN VISION IS SQUISHY
Between tolerance to prescriptions in between exams changes to the lack of complaints heard from millions of OTC wearers, to the public’s acceptance of eyewear routinely made off-spec or with poor lens materials and centering, it’s clear that the bump stops defining human visual satisfaction must be truly made of rubber. But just because consumers appear happy with internet specs does not mean you should race online to deliver the same. No, I would argue that online eyewear will provide lots of opportunities for brick-and-mortar opticals to impress the eyeglass wearing public through strategic use of the Wow and the Whoa effects mentioned above:
- The Wow happens when you deliver someone their first real pair of quality specs. The sharpness and clarity are immediately and overwhelmingly obvious.
- The Whoa occurs when consumers try cheaper alternatives and after a honeymoon period reveling in the low cost paid, realize that why pleasure of low price alone is neither long lasting nor compensates for the absence of a more superlative experience, which is the domain of a quality-oriented optical store.
Every pair you make is both an opportunity to impress and an opportunity to reset your client’s expectations. Remember that in every sensory experience, neuroadaptation tends to level the playing field, and people eventually take any perceived improvements for granted. So while you may be satisfied with your current optical game, your clients often will want more. Therefore, never become complacent—because if you do, you’ll place your business at risk.
TRUST AND THE HERO’S JOURNEY
There’s a core constituency of consumers where trust is the single most important reason they remain loyal to their present optical provider. This is common in technologies such as medicine, automotive and computers—all of which are daunting disciplines to master for the lay. Certainly from a consumer’s perspective, there is much about prescription eyewear that is hard to grasp, and this is why many buyers simply choose to ignore the concepts, terms and technology.
But this “ignorance is bliss” condition lasts only as long as the consumer trusts their eyecare professional. If anything in your office is not consistent and fully on point with what a quality superlative eyewear experience entails, that business risks having clients attracted to buy online. When consumers are asked why, they point to unhappiness with perceived value, inattentive action when a problem surfaced or even the inability to deliver a specific product. Clearly, everything is not always under your control.
But if they do try online, industry statistics reveal at least 15 percent to 20 percent will be demonstrably unhappy. Still others may experience a spectrum of dissatisfaction. And how could they not? Online vendors do not have access to any of the important preferential data about fit, personal style, vision and more that is only available in an in-store, in-person exchange. More importantly, online eyewear buyers will eventually have to seek out a brick-and-mortar location to get their eyewear adjusted, repaired, and screws and nosepads replaced.
I know what you’re thinking: Servicing online eyewear feels like one big nuisance, for little to no money. On the contrary, these service opportunities actually represent a chance to potentially become a hero in their eyes. If you are lucky, people with real problems, such as broken frames, lost lenses or a pair that just isn’t working will walk in. These are “heroic” types of encounters, the ones where you can deeply impress the consumer in crisis and set the stage for winning them as a customer for life. With your superior knowledge and optical chops fully at the ready, you can create one of those interactive moments that makes optical life so immensely satisfying.
Be thankful that human vision is squishy, because while fuzzy subjective metrics like “I see fine” and “20/Happy” may be good enough for online buyers, they are clearly not good enough for you and me. Remember that consumer satisfaction will always be a moving target, so you must remain a lifelong learner in order to stay at the top of your game. Sure, some consumers will say that “nothing special” will be good enough. But I’ll let you in on a secret: Everyone is always interested in quality. Starting tomorrow, never forget it is a great privilege to share your wisdom, experience and talent with a prospective buyer, so don’t squander an opportunity to show why main street optical is the home to the “quality difference.”
ADVANCED CONSIDERATIONS IN FITTING HEIGHTS
With the basics under your belt, let’s take a deeper dive into the more advanced considerations for specific lens designs.
PLACEMENT OF LINED MULTIFOCALS
The literature has traditionally taught that bifocal segments be fitted “at the lower lid.” Unfortunately, this is perhaps the least reliable reference to use as a fitting guide, as the position of the lower lid varies widely from individual to individual. A Google search for eye images quickly reveals this. Rather, we should place the bifocal line between 7 mm and 10 mm below the pupil, using our pupil height measurement as the starting point. This will place the optical center of the FT28 mm segment between 12 mm and 15 mm below the pupil—a position not too different really from the reading zone found in a progressive lens. The calculations for prism and Abbe are similar if the distance OC is placed 4 mm to 5 mm above the segment line. For trifocals, traditional wisdom was to place them at the bottom of the pupil, which translates to 1.5 mm to 2.5 mm below pupil center. By recording segment positions in terms of millimeters below pupil center, repeatable multifocal heights will be much easier to both obtain and troubleshoot.
PROGRESSIVES, POSTURE AND POW
In order to reduce the amount of unwanted astigmatism across a progressive lens, traditional and non-digital designs have sought to lengthen the progressive corridor, which lessens both the corridor shape and the surface astigmatism. Unfortunately, depending on the add power and the wearer’s natural posture, this may place the zone of clear reading too low for some people. An alternate solution for reducing astigmatism is to start the progressive corridor slightly above the actual pupil-centric fitting point. This is an effective solution, but its success is tied to how well the patient’s distance correction and their natural posture has been factored for the patient’s individual needs. Too much plus in the distance Rx and/or too much head-up posture will result in unsatisfactory distance vision, particularly for driving. You can correct driving issues as follows:.
- Adjust the distance Rx.
- Lower the progressive fitting height and shorten the corridor length.
- Lower the progressive fitting height and increase the add power.
- Change the progressive design.
On the opposite side of these distance-priority considerations is the need for increased work utility and comfort at face-level computer tasks. Here, shorter and/or faster rise progressive corridor slopes will reduce the need for uncomfortable neck and head adjustments. It should be clear that a lens optimized for the distance environment is often a poor performer at near.
With regard to position of wear, progressive lenses—in view of their varying surface power and “two-master” design—work best if the lens is tilted in and face wrapped a bit in order to lessen the obliquity of peripheral gaze and varying vertex differences. By doing so, undesirable static and dynamic magnification effects are reduced, and wearer comfort is markedly improved.
Recommended fitting values are:
- Pantoscopic angle: 7 to 10 degrees.
- Frame wrap angle: 4 to 7 degrees
- Vertex distance: As close as wearer preference, facial structure and lash clearance allows. Nominally targeted to be 13 mm.
Even though freeform lenses promise to compensate for POW values that depart from the ranges stated above, optimized designs always deliver their best performance when the initial frame fit has been tailored to these values. Even personalized freeform single vision lenses will benefit from adherence to these suggested POW fitting values, especially in the more complex, binocularly challenged and high-powered prescriptions.
If the Rx contains prescribed prism, a compensation must be made for the inherently shifted progressive surface in order to align the reading and intermediate areas to the ray path altered by the prism. The compensation rule of thumb when blocking, not ordering, is as follows: Move the fitting point approximately 0.25 mm in the direction of the apex of the prism. This should be applied to both horizontal and vertical prism in order to ensure optimal binocular overlap of the progressive surfaces. This does not apply to single vision FF lenses. It does apply to so-called “boost” lenses.
KEEP ABBE NORMAL
Experience shows that visual sensitivity to low Abbe materials varies from wearer to wearer. Let’s find out what influences this experience:
1. Types of Chromatic Aberration
a. Longitudinal Chromatic Aberration (LCA): Also known as axial chromatic aberration. This error is seen when different wavelengths of light effectively refract to different focal points and is limited to the incoming pencils of light which lie on axis. The human eye, because it consists of largely homogenous media of similar refractive index and dispersive characteristics, exhibits between 1.5 and 2 diopters of LCA.
b. Transverse Chromatic Aberration (TCA): Also known as lateral chromatic aberration and is often confused with the LCA acronym above. Therefore, best practice is to use the TCA acronym when referring to the transverse type of color error. TCA occurs in off axis pencils of light.
Our perception of the eye’s inherent chromatic aberration is reduced primarily from two factors:
- The eye and brain’s neural processing has evolved to reduce our awareness of color error.
- A reduction of S (blue) cones immediately outside the foveola further attenuates blue awareness and also reduces blue spread, even in high contrast images.
However, and this is important: The eye’s color error is always present in the retina, with the unwanted energy spread from the combination of axial and transverse chromatic aberration negatively impacting image contrast. Now combine this with a low Abbe lens material and overly flat base curve, which increases the obliquity of peripheral rays, you effectively increase the undesirable color spread found in the retinal image. This “perfect storm” of negative color-spread factors can appear to “suddenly” surpass a wearer’s threshold of perception. This may be one reason why polycarbonate and other flat low Abbe lens materials are amongst the usual suspects when diagnosing visual complaints. Therefore, best practice dictates using the highest Abbe value material along with as steep a base curve as best form theory allows to achieve the highest optical performance.
In glasses intended for reading and intermediate/computer, three additional factors can further impact visual acuity and wearer comfort:
- Flatter and/or aspheric lens designs, which are selected for optimized cosmetics.
- Close proximity tasks can present supra-normal, high contrast images—with black text sharply rendered against white or light backgrounds.
- Today’s most advanced and color accurate computer displays contain OLED lighting featuring separate red, green and blue imaging elements, further amplifying color spread effects.
Combine all the above, and the result is a significant increase in the color error inherent in all low Abbe materials. Suggested best practices for near and intermediate eyewear are as follows:
- Ideally, use steeper base curves to reduce peripheral oblique incidence in conjunction with a near-prioritized freeform lens design.
- Choose a high Abbe/low dispersive lens material, which will help mitigate the elevated negative impact on axial and transverse chromatism of modern high contrast computer displays.
- Select an advanced blue light filter to improve wearer comfort and reduce the effects of all chromatic errors on image defocus.
WHY BEST FORM IS RARELY BEST
The empirical and theoretical basis for all best form/corrected curve lens designs can be traced back to researchers von Rohr, Wollaston, Ostwald and Tscherning, whose graphic ellipse is still consulted today to help select the most optimal optical base curve to eliminate peripheral, aka marginal astigmatism. But often overlooked are the specific assumptions underpinning the ellipse’s two displayed values:
- Lens form is limited to the use of spherical surfaces only for both front and rear.
- Prescriptions must be spherical power only—no astigmatism permitted.
- The index of refraction was based on old ophthalmic crown, around 1.53.
- The assumed POW values are zero for both pantoscopic tilt and frame wrap. The fitted vertex distance is chosen to properly align the lens’ optical axis to the center of rotation (CoR) of the eye—and is normally assumed to be 13 mm.
- The optical center of the lens was to be placed directly in front of the pupil.
- The Tscherning ellipse is a graphical representation of the two possible solutions to the quadratic equation that defines a best form lens design, premised on the above assumptions. One solution, the Ostwalt branch, represents front curves of a flatter design that meets these requirements. The other solution, the Wollaston branch, represents curves of a steeper design. The Wollaston branch is named after an early lens researcher, William Hyde Wollaston, whose empirical studies attempted to improve the peripheral definition of early 19th century landscape lenses by trial and error experimentation. Wollaston discovered that steeper curved lenses improved peripheral image sharpness.
Most of the covenants above are not at all consistent with the realities of fitting seen at today’s dispensing desk. Always remember that adjusting the position of the optical center to compensate for pantoscopic angle effectively tilts the lens at the MRP, which will introduce some degree of off-axis error in the wearer’s primary line of sight. Fortunately, freeform single vision lens design was created to correct this error and a lot more:
- Freeform lenses address the reality that prescriptions have astigmatism.
- Freeform lenses account for the use of lens indices different from 1.53.< /li>
- Freeform lenses are the only type of lens to deliver outstanding image quality across the entire lens surface and for a variety of positions of wear.
- Freeform lenses can often place the PRP in a more cosmetically optimum position.
Contributing editor Barry Santini is a New York State licensed optician based in Seaford, N.Y.
The author wishes to thank opticians Marisol Rodriguez, Lou Fullagar and optometrist Dr. Peter Shaw for their help and inspiration with this feature.