The Evolution of Single Vision
By Barry Santini, ABOM
Release Date: January 31, 2017
Expiration Date: January 27, 2022
Upon completion of this program, the participant should be able to:
- Learn the design evolution and reduction of compromise in single vision lenses for vision improvement.
- Understand how SV lens materials and treatments further interact to deliver the best of lens design.
- Know how freeform, position of wear and design morphing can personalize design.
Probably no topic is more confusing, misunderstood or under appreciated than how the choice of lens base curve can impact the optics, fit and cosmetics of a new pair of prescription glasses. In single vision lenses, it's rarely thought about since a finished lens is typically used, out-of-the-envelope, and the base curve is whatever base curve the manufacturer or lens designer has chosen. That leaves the ECP completely out of the decision. Advanced free form SV lenses can deliver vision as good as or, in some cases, better than contact lenses. We'll describe both the improvements and the reasons why every eyeglass wearer inherently wants, and today deserves, the panoramically wide, sharp vision possible from this state of the art lens technology.
Barry Santini, ABOM, graduated from New York Technical College in 1975 with an AAS in Ophthalmic Dispensing. He is a New York State licensed optician with contact lens certification, is ABO certified and was awarded an ABO Master in 1994. As sales manager for Tele Vue Optics from 1987 to 2003, Santini developed his knowledge of precision optics and has been an owner of Long Island Opticians in Seaford N.Y., from 1996 to present. In addition, Santini is an amatuer astronomer and lecturer and plays bass trombone in the Brooklyn Symphony.
This course is approved for one (1) hour of CE credit by the American Board of Opticianry (ABO). Technical, Level II. Course STWJHI656-2
Probably no topic is more confusing, misunderstood or under appreciated than how the choice of lens base curve can impact the optics, fit and cosmetics of a new pair of single vision prescription glasses. In single vision lenses, it's rarely thought about since a finished SV lens is typically used, out-of-the-envelope, with its base curve being whatever base curve the manufacturer or designer had chosen. That left the ECP completely out of a vital part of the optics decision.
For a long time it's been the norm to target only the optics, fit or cosmetics as a top priority, which immediately demotes the remaining two. But by doing so, you are constantly entering into some significant compromises. Today, with the availability of advanced free form single vision lens designs, accepting any compromise is no longer your only option. The challenge facing eye care professionals today is therefore to learn how to achieve the optimal balance between optics, cosmetics and fit for every patient and every pair of eyeglasses.
When asked this question at lectures: "How many of you are using free-form single vision lenses, particularly with position of wear measurements?" ECPs respond with few hands raised. Why is it that ECPs are using finished and surfaced single vision lenses of a basic design that is essentially unchanged for almost 100 years? Today it's time for both eyeglass wearers and eye care professionals to begin benefitting from the superiority of lenses that fully utilize the tremendous computing power, lab processing capabilities and base curve flexibility inherent in advanced free form single vision lenses. If both professional and consumer fully understood the true value of these lenses, the questions surrounding both need and cost would no longer be front and center.
Just as free-form progressives have shown that wearers can "just see better", single vision free form lenses offer eyeglass consumers an improved degree of sharpness and field of view that far surpasses conventional finished and traditionally surfaced lenses. In fact, advanced free form SV designs can deliver vision as good as or better than contact lenses. We'll describe both the improvements and the reasons why every eyeglass wearer inherently wants, and today deserves, the panoramically-wide, sharp vision possible from this state of the art lens technology.
How We See - A Tale of Two Cameras
The human eye evolved to become a hybridized optical design: Starting as a wide angle camera optimized for sensing motion to help make food, fight or flight decisions, it also developed the capabilities of a high resolution camera, designed for in-depth, detailed target assessment. But even as our eye fixates on an object of interest, it actually never stands still for long. Rather, it engages in saccades— rapid push-pull movements— designed to place the. high-resolution foveal region in a position to best evaluate all the details of the intended target. Even during fixation, the human eye still doesn't rest, continually performing smaller, mini-movements of 0.5 degree and 1 second duration known as micro saccades. These smaller scale excursions help gather additional high-resolution information while simultaneously preventing the fading of our visual perception, thereby maximizing the persistence of vision.
The eye's dual camera construction also helps to explain the dual nature of our visual perception: We have evolved to prioritize having sharp central vision combined with motion-sensitive side vision, wherein the eye reacts to changes in light and movement by rotating the fovea to linger, scan and evaluate any target of interest. Next, let's take a look at the how our eye learned to triage where it would focus its attention.
The Cone of Visual Attention
Although the eye's total field of view, central vision plus peripheral, can stretch to over a 160 degrees, the sharpest view of the eye is located in the foveal region located within the macula. But these regions only subtend fields of view of 0.5 to 2 degrees and 15 to 18 degrees respectively. The eye therefore must rotate off the facial plane to scan objects of interests with the fovea. The brain then integrates this higher-resolution information into a bigger picture called the cone of visual attention. This cone subtends approximately a 50-55 degree wide field of view, and is commonly referred to in photography as the eye's normal perspective. But the eye also resists rotating beyond approximately a 15-degree angle to scan any object without initiating head movement to re-center. This means the eye uses a primary field of view of 30 degrees, within which it routinely travels, nestled inside a larger cone of attention totaling 50 to 55 degrees. This fact is, in part, one of the reasons that lens designers start with prioritizing sharpness within a 30-degree field of view when choosing surface-curves.
So What Lens Curve Should We Start With?
The frustrating experience of looking through lenses with narrow fields of view is not unfamiliar to consumers today. Having to make frequent head movements in order to see peripheral objects clearly is innately uncomfortable, and you need look no further than the experience of first-time progressive wearers to understand and appreciate their adaptation issues. Perhaps nothing is more revealing of how desensitized ECPs may have become to the importance of having a wide and sharp field of view than how reflexively they respond to peripheral vision complaints with "Don't worry - you'll get used to it."
But to a lens researcher in the early 1800s named Henry Wollaston, the whole idea of tolerating the poor peripheral vision of the flat-base eyeglass lenses of the day (minus lenses flat on the front, plus lenses flat on the back) was unacceptable. What Wollaston discovered was that by 'bending' a lens into a meniscus-tear form, the field of sharp vision noticeably increased from the restricted 5mm area/10 degree angle common to these flat form lenses. Wollaston subsequently went on to apply his more steeper meniscus form to lenses for the newly emerging technology of photography. His curved photographic lens became known as the "Wollaston Landscape Lens," acknowledging its superiority for filming wide panoramas and landscapes. It even became a piece of standard equipment in the toolbox of a 19th century photographer's arsenal of lenses. Meanwhile, recognition and adoption of Wollaston's meniscus lens form for eyeglasses was overshadowed by excitement in Thomas Young's discovery of astigmatism. Thereafter interest in Wollaston's improved peripheral optics, curved-lens form laid fallow for almost 100 years until a Parisian oculist named Ostwalt in the late 1890s rediscovered it.
But Ostwalt found Wollaston's lens curves way too steep to be practical to manufacture, and became amongst the first researchers to use a scientific approach in designing a series of improved field of view lenses which featured flatter curves. By relying on mathematical calculations, Ostwalt helped transition lens design from a 200-year legacy of empirical art to one based on a solid, scientific foundation that created lenses delivering both sharp central and peripheral vision.
Just before the advent of mathematical ray tracing, the optimal approach for obtaining best peripheral performance lenses, aka 'best form' lenses, was found empirically to require using an ocular curve of 6 diopters. This 6D rule of thumb, aka Vogel's Rule, worked well because it accomplished 2 things: First, it kept the front curves steeper in profile, reducing the obliquity of peripheral gaze angles. This reduced effective lens tilt and and therefore oblique astigmatism. Second, it delivered a curved image plane— or shell— which both approximated the arc of the macula while also closely following the arc of the fovea as the eye turned about its center of rotation.
The Advent of Scientific Ray Tracing
The first lenses to employ the added precision of individual ray tracing belong Dr. Moritz von Rohr of the firm of Carl Zeiss. Dr. von Rohr designed a series of eyeglass lenses delivering pinpoint sharpness across a 60-degree field of view by eliminating the vision-degrading error of off-axis astigmatism. Why did Von Rohr choose 60 degrees? Because he wanted to more than cover the eye's full cone of attention, thereby ensuring absolutely top performance for his new Punktal lens's premium asking price. Wearers would no longer have to uncomfortably turn their head to point their lens's sharper central region toward what they wanted to see. But Punktal (for pinpoint) lenses required that each prescription use its own specific radii for the front and rear surfaces, making them both costly and time consuming to manufacture. With WW I interrupting the supply of these fine German lenses, the timing was right for American companies to explore not only how to deliver Punktal's performance in a quicker and more affordable manner, but also spurred on the development of efficient crown glass manufacture in the United States.
Birth of Base Curve and Corrected Curve
To overcome Punktal's high cost and long fabrication times, lens designers Tillyer at American Optical and Rayton at Bausch & Lomb begin to explore the idea of creating a series of corrected curve lenses using three new approaches:
- They grouped nearby lens powers together to make them on a single curve, thereby reducing inventory and tooling.
- They reduced the area targeted for absolute pinpoint correction to a 15 degree angle/30 degree FOV, which matched the general limit of the eye's rotational excursion before head movement is Initiated. (Note: The exception to this 15 degree head movement rule is encountered during reading, where the often eye rotates down and across a page between 30 and 60 degrees, respectively.)
- They allowed for a certain amount of power error (0.24D) and/or astigmatism error (0.12D) in the peripheral field beyond the macula. These aberration limits were rationalized both by the ability of the eye to accommodate small power errors and that the accepted precision of the exam room has been traditionally no finer than 0.12D.
Limitations of Corrected Curve
But corrected curve lens design required accepting some trade offs from the zero-astigmatism goal of Zeiss Punktal lenses in order to reduce the number of curves and shop tools required in fabrication. Further, moderate to stronger cylinder powers could not be fully included in the target optimization using simple spherical curves for the front and rear lens surfaces. Because of this, optimal performance could only be realized when the lens was correctly placed in front of the eye. This 'orthogonal' position requirement was met only when:
- The lens's optical axis passed through the eye's center of rotation, which ensured the lens's image shell, was properly aligned for all gaze angles of the eye.
- The lenses were fit at an assumed distance from the eye's center of rotation. Designers used a fitted lens vertex distance of approximately 13.5mm; a value arrived at after measuring many eyeglass fittings and averaging the data to a mean value. The eye's center of rotation was considered to be an additional 13.0mm behind the cornea, whose thickness was averaged to 0.5mm. This brought the assumed lens-to-center of rotation (CR) distance, to a total of 27mm.
Today, we know that assuming a fitted VD of 13.5mm and an overall CR distance of 27mm was a bit over simplified. The designers did not include the impact on the CR distance by type of ametropia, axial length variations or individual fitting preferences (lash clearance, sinus sensitivities or other personal cosmetic requirements)
The Steep Price of Steeper Curves
Wollaston, von Rohr and other best form adherents had all found steeper curves performed better when it came to specifying the form of lenses offering good peripheral clarity. But steeper curves make even steeper demands on thickness, weight, and glazing. Additionally, steeper curves create changes in magnification, unfavorably impacting perspective and wearer comfort. Today, one way to help reduce lens thickness is by choosing a higher index of refraction material. With higher index's greater light bending power, flatter curves are possible for any Rx power. And flatter curves help to reduce bulge, thickness, weight, magnification and glazing difficulty. Higher index lenses to the rescue!
Except for one problem: The greater light bending power at the lens periphery meant off-axis light bundles are refracted to a greater degree than conventional plastic or glass This simulates an increase in the effective obliquity or tilt of the lens surface, which further increases peripheral astigmatism. Therefore higher index materials should be placed on steeper base curves, not flatter, in order to ensure good off-axis performance, which negates a lot of higher indices promised benefits.
This is why the aspheric surfaces we find on higher index lenses became the norm: By using the surface astigmatism of aspheric base curves to counteract off-axis astigmatism in the lens periphery, Aspherics help to reduce the steeper curves required when using higher index materials while maintaining peripheral clarity. All the benefits of high index lenses are retained, including bulge, thickness, weight and glazing difficulty reduction, along with the unwanted magnification changes caused by steeper front curves. All these benefits can be realized just by using aspheric surfaces on the front curve. What if we could use them on the back curve as well?
Fabulous Free Form
Free Form lens technology goes beyond what simple molded front side aspheric surfaces can offer. Free form manufacturing, applied just to the rear surface of spherical front curve ophthalmic lenses, can deliver significant advantages to widening the field of view for any single vision lens wearer. Besides overcoming the inability of best form lenses to fully correct the off-axis errors in moderate to stronger astigmatic prescriptions, free form single vision lenses allow the full variety of materials to accessed in an economical manner. Further, departures from assumed values for the fitting parameter of pantoscopic tilt, wrap angle and vertex distance, aka position of wear, or POW, are easily and completely integrated into the optimization process. This results in optical performance that no out-of-the-envelope SV lens can come close to touching. Ultimately, the gold standard for balancing optics, thickness and cosmetics would be using both the front and rear surfaces in a fully optimized, single vision free form lens. The front surface could be either a molded aspheric a custom-made free form surface. Additionally, when frame or cosmetic considerations require alterations from conventional base curves, only using an advanced free form SV design can ensure that maximum acuity, comfort and utility are delivered to the wearer.
By using a digitally optimized, true free form lens design, eye care professionals finally have a tool that allow less compromise between optics, cosmetics and fit when creating the best pair for wear. Instead of being restricted to using a traditionally manufactured lens design's base curve, advanced free form lenses can offer the ability to change the designer's target base curve up to 2 diopters in either direction. Of course, the exact curve available may depend on the index of refraction you select. It's a true balancing act, worthy of bestowing artisan status to those opticians who learn how to do it well.
An Umbrellic View of Lens Placement
Fitting A Frame With Free Form
An alternate approach is to straddle the extra thickness of a prescription lens within the frame eye wire, balancing the amount of lens extending beyond either side of the eye wire. But sometimes a chosen corrected curve or aspheric lens will not be available in a base curve appropriate for obtaining the desired bevel placement. What can an optician do to best balance lens thickness, curve, optics and glazing considerations and deliver the best overall pair of eyewear?
Understanding Marked Curve vs. True Curve
For labs to calculate the curves necessary to generate the proper prescription power, the need to know a lens surface's actual refractive power. For semi finished lenses, the front or base curve became the lens's marked curve. To convert to true curve, you took the index minus 1, and divided by 0.53 to arrive at the surface dioptric curve if measured with a conventional lens clock. Remember all vocabulary related to the surface curve's radius is referenced to a 1.53 index. It is the true curve that opticians needs to know when determining what lens will best fit a frame's bevel curve.
It is helpful to imagine a lens's image shell to that of an open umbrella, with the umbrella's handle representing the lens's design pole. With this in mind, the goal of the optician becomes twofold: First, place the pole of the umbrella so it intersects the eye's center of rotation. This will align the umbrella's canopy (image shell) parallel to the eye's far point sphere as uniformly as possible throughout all gaze angles. Next, longitudinally position the umbrella's canopy, i.e., the lens's image shell, to be coincident with the eye's far point sphere. Deviations here result in power errors that opticians incompletely address when they perform a vertex distance compensation. Why incompletely? Because vertex compensations only address variations in lens to cornea distance, leaving unaddressed variations in eye's center of rotation distance behind the cornea.
Positioning the Pole
In fitting a conventional corrected curve or aspheric lens, the optician must ensure the lens's design pole, the handle in our umbrella analogy, intersects the eye's center of rotation. This has been usually accomplished by employing Martin's Rule of Tilt, whereby every degree of pantoscopic tilt requires tilting the lens's optical axis to compensate and ensure intersection with the eye's center of rotation. Since the lens' optical axis, by definition, passes perpendicularly through the optical center— the OC— simply moving the OC downward 0.5mm for every degree of pantoscopic tilt brings the lens's image shell in proper alignment with the eye's far point sphere. Martin's Rule assumes a cornea vertex distance value of 13.5mm, so significant deviations in fitted vertex will require recalculation of this rule of thumb.
Free Form and Position of Wear
If you are not routinely measuring vertex distance, along with pantoscopic tilt and frame wrap angle, you should start. Although most free form designs utilize default fitting-values gleaned from averaging fitting data, the individual values found by the hand of the expert optician truly matter here. Personalized free form designs requires using personalized fitting value, and, along with the lens design engine computations using dynamic merit weighting arrives at a free form lens surface that places the lens' image shell in alignment with that of the eye's far point sphere. This compensation can be even further refined using additional factors, including lens size, shape and ED. These last three factors, along with vertex distance, help to map the eye's cone of attention and gaze angles onto the surface of the lens. This results in the surface adjustments needed for correcting the most significant deviations in shell alignment, reducing aberrations.
Personalized free form designs will therefore require additional consultative and fitting time. Local, independent optical offices are often the best situated venues to allocate the time needed for these measurements and discussion of choices. While online is possible in a simplified sense, the direct, hands-on approach makes for the best results. The rewards enjoyed by client and consultant help separate the independent optical from the short cuts larger competitors often employ.
The Magnifying Effect of Base Curve
The radius of curvature of a lens's front curve, aka its base curve, along with the lens's thickness, help to determine its cosmetic and magnifying effect on both the eye and the retina. The steeper the front/base curve results in greater lens thickness and increased magnifying effect. The combined influence of the front curve and lens thickness is called the lens's shape factor, and it can play an impactful role in a wearer's comfort and perceptual adaptation to a new Rx.
Myopia, Astigmatism and the Petzval Condition
As lens engineers achieved success in correcting peripheral astigmatism for larger gaze angles up to 30 degrees, they noted the resulting Petzval image shell was routinely found on the hyperopic side of the eye's retina. This was considered ok since the eye can compensate for small amounts of hyperopic defocus through accommodation. Conversely, if the Petzval surface was found on the myopic side of focus, the eye couldn't compensate through accommodation, and this approach was avoided.
But there's a possible latent problem here: Vision experts today have theorized that a hyperopic defocus in the peripheral retina area outside the macula may be a suspect in the growing epidemic of myopic progression seen around the world. Put another way, the best designed lenses i.e., the ones with the sharpest correction for peripheral clarity, may in fact be an unwitting accomplice to the global increase of myopia.
The Gorilla in the Periphery: Abbé
Despite what corrected curve, aspheric and today's advanced free form designs can deliver in superior peripheral clarity, the one aberration that cannot be fully overcome through lens design is chromatic error. This is almost wholly dependent on the dispersive properties of the lens substrate, and is described by an inverse quotient known the material's abbé value. The lower the Abbé, the greater the amount that colors are dispersed, and the greater the smearing and blurring of edges with its impact on acuity. The total effect of Abbé on peripheral acuity is a combination of the total diopter power, fixation distance from the optical center, Abbé value of the material and the obliquity of the incident rays. As color error increases when you look away from the optical center, some opticians choose to prioritize OC placement over all other fitting considerations.
But they would have chosen unwisely. By focusing strictly on OC placement in front of the pupil, without regard for pantoscopic tilt or vertex distance, opticians can overlook the important alignment of the lens' image shell with the eye's far point sphere, further compounding the cumulative negative effect of Abbé on peripheral acuity. The best approach for prioritizing peripheral clarity is therefore to:
- Ensure ideal image shell position through proper lens alignment.
- Select the most favorable Abbé value material yielding the thickness desired.
- Tweak the base curve choice using a fully POW compensated lens design.
- Fit a properly selected frame to deliver the best acuity across the entire lens surface.
Understanding why prioritizing OC placement in front of the pupil is not wholly correct will begin to stop the use of the phrase "taking an OC height." Why? Because what you're really after is determining the pupil height. It's what the optician chooses to do with the OC's position relative to the pupil height that separates the masters from the apprentices.
Match Base Curve? Choose Wisely
THE SV OPPORTUNITY
Even though Boomers make up a significant part of the population, there are more single vision lens wearers as part of the spectacle wearing population, about 52% of lenses. The current US population over the age of 18 is about 250M people, of which 64% wear glasses and 11% wear contacts. This demographic gets a new frame on average every 2.2 years, plus just about 10% buy only lenses. The latest VisionWatch report for 2015 (a Vision Council survey) reports 52% of prescription purchases were single vision. That says that we have a 41.6M pair SV lens market. What does it mean for each office?
If sold equally over the 35K locations in the US (which it's not), each office would sell 1,189 pair/year. In an office open 250 days a year; that's about 5 pair a day. What's the SV lens opportunity in your office?
We can segment this further by gender and income. Certainly, lens technology appeals somewhat more to men, while income affects both genders equally on the choice of the latest products. Understanding the wearer helps you to be more effective and successful when suggesting better products that answer both defined needs and wants.
What about the Doc's initial prescription? Should all the lenses be adjusted for position of wear and viewing distance? Probably not, because low lens powers, when compensated, are often changed very little compared to moderate to stronger Rx's. As a result, these changes may not be noticeable to the wearer. That means prescriptions over ±1.00D and cylinders over -1.00D are ideal for free-form's promised improvements.
How big an actual opportunity is there? If ±1.00 to a -1.00 cylinder make up just over 20% of all Rx's, then the rest represent an 80% opportunity. So, next time you're deciding whether to order a single vision finished lens from your stock house, or a conventionally surfaced single vision lens, why not consider using personalized single vision lenses, adjusted for personalized POW values and the intended range of use. Lastly, when a patient requests their PD because they are considering purchasing eyewear online, educate them on the differences and the opportunities for a more personalized lens that will deliver superior vision and comfort.
Move From SOS
Staying with the same old stuff—aka SOS—can feel warm, comfortable and familiar. With most single vision wearers rarely appearing to have problems that are not directly related to the original Rx, who can blame ECPs for asking "Why try to fix something that ain't broke?" The answer is that people don't know what they really want until they've been given a opportunity to try. Think about the IPad: Who was really asking for a tablet computer with no USB, expansion or printer ports less than 7 short years ago? But who can really live without one today? This is the story unfolding in office after office that understands, embraces and promotes the superior vision inherent in advanced free form single vision lenses.
For example, new SEIKO Superior Single Vision (SV) lenses can be ordered customized using position of wear values and also, for the intended focal length of use. While an advantage of stock lenses is fast, low cost and convenient, it doesn't separate you from the competition when others around you do the same thing. SEIKO Superior SV lenses can be ordered with POW fitting measurements in an "F", Far Priority; "B", Balanced or "N", Near Priority lens design. As a result, these lenses are created for the intended focal length of use. To understand more about how this extension of an evolution of single vision technology works, visit 2020mag.com/CE. Should you use lenses personalized by focal length? Lenses that celebrate an optician's craft are good. If one understands that lenses have specific design attributes for the way in which they are worn (POW), and also that an intended focal length can make the way that they work better, just seems to me.
To borrow a phrase... "You've come a long way baby" is right for single vision, however, using all that has been achieved is not well understood. Advanced free form SV lenses can deliver exceptional vision. These improvements and the technology delivered so seamlessly by your laboratory are the reasons why every eyeglass wearer inherently wants, and today deserves, the panoramically wide, sharp vision possible from this state of the art lens technology.