By Barry Santini

Knowing the effect of frame fit, or Position of Wear (PoW) measurements, in compensated freeform lenses is key for today’s optician. While PoW measurements improve overall lens performance in freeform compensated lenses, the lensometer reading will appear to fall outside of ANSI tolerances relative to the original Rx. The compensated power of the lens as received from the lab is not varying from the original Rx, but instead produces the effective power to achieve the original Rx in an as-worn lens. This is an important concept that remains a mystery for many ECPs. Read on to learn why compensated lenses are checked against the lab verification values and not against the original Rx.

POSITION OF WEAR: THE EARLY YEARS
The basics of how a spectacle lens was assumed to be properly placed in front of the eye—its position of wear—began with research done by the pillars of ophthalmic history. Names such as Abbe, Glancy, Gullstrand, Ostwald, Rayton, Tillyer, Tscherning, Von Rohr and more were together the scientists that created the founding principles of best-form ophthalmic lens design. But these principles were created in an age when extremes in fashion and frame design were unheard of, and therefore, extremes in frame size, lens shape or fit of the spectacles were unheard of. Around the turn of the previous century, custom made prescription glasses were fitted using a system that became known as the datum fitting system. Using the datum system was approached this way:

1. Bridge fit: The proper fitting bridge for a patient nose was determined. This “bridge size” value was then benchmarked for that wearer. If it was a 22 mm bridge, then pretty much any 22 mm bridge would fit in similar fashion.
2. Lens size: The binocular pupillary distance of the patient was then determined. The target lens size sought would attempt to reduce or eliminate all lens decentration. For example: Starting with a binocular PD of 66 mm, a found bridge size value of 20 mm would be deducted, resulting in a lens size as close to 46 mm as possible.
3. Frame fit: With the bridge and lens size values in hand, the style of the frame was chosen next. In the age of the datum system, frame style selection was much more limited than it is today. The position of the temple attachment, dictated by frame style, was chosen to best fit the facial width at the attachment point. If various end-piece extension styles were available, these would be reviewed to help optimize facial fitting. By the late 1930s, novelty in frame fashion was starting to find favor with those seeking eyewear to help communicate their personal sense of style.
4. Vertical centering and lens tilt: As with most frames of the 1920s, ’30s and most of the ’40s, these shapes were predominantly symmetrical, featuring ovals, rounds, pantos, hexagons and octagons. No matter what shape was chosen, the goal was to center the eye vertically as much as possible. In almost all cases save the panto shape, the pantoscopic tilt of the lens plane was zero. In the case of the panto shape, with its novel, high-mounted temples pioneered by the American Optical Ful-Vue version, some small vertical adjustment to the optical center might be made, depending on the Rx and the degree of frame tilt.
5. Vertex distance: During the datum age, most vertex considerations centered around frame and eyelash clearance, with prescription power compensation only being made in the higher dioptric powers.
6. Frame wrap: Universally, almost all frame fronts were close to absolutely flat, aka zero wrap angle. This ensured the best optics were realized with the lenses commonly used during the datum fitting period.

FASHION IN THE AGE OF DATUM
After the introduction of optically superior corrected curve lenses starting in the 1920s, changes in the position of wear of frames were driven mostly by changes in frame fashion. Starting in the 1940s, novel frame styles like the harlequin, brow bar and the aviator drove frame and sunglasses fashion, and opticians and their labs had to contend with new challenges in frame fitting and lens fabrication. Remember, this was still the age of glass lenses, with weight and blank size considerations effectively placing real hard stops on lens availability and frame sizes. Fast forward to the 1960s, and ophthalmic frame fashion begins to be ever more influenced by changes in fashion sunglasses. Sunglasses, starting in the 1930s, never stopped evolving stylistically because non-prescription sunglasses did not need nor seek approval from the critical eye of the optometrist or optician. By the early 1960s, it became obvious that the datum system of fitting and measuring frames, formally adopted in the U.K. back in 1935, had lost its effectiveness in both lab fabrication and at the dispensing desk. It was officially replaced in the United States in 1962 by the Boxing System, which redefined lens and bridge sizes to help aid lens layout and reduce costs as labs began to process the newer, fashionable frame styles with their non-symmetrical shapes. The Boxing System was originally suggested by Larry Stull, an employee of Winchester Optical Lab in Elmira, N.Y.

POW AS THE AGE OF FASHION ARRIVES
As fashion and frame designers began to explore greater novelty in frame size, shape and styling, opticians faced the central challenge of managing the cosmetic and weight considerations generated by increased lens thickness from larger frame styles and the resultant lens decentrations. Remember, in the 1970s and 1980s, lens material options were limited to CR-39 monomer by PPG, crown glass and the occasional use of 1.70 flint high index glass, which was heavy and had a very low abbe value. As opticians struggled to produce eyewear that was cosmetically acceptable to a fashion driven buying public, traditional optical and frame fit considerations became less of a priority. Attempts to maintain a close-fitting vertex distance and zero values for pantoscopic tilt and frame wrap were no longer prime considerations. Bottom line: Opticians, who at the time had little-to-no training in the burgeoning science of position of wear, started to abandon most optical considerations and instead, focus mainly on the cosmetics of the finished eyewear. By the late 1960s, very few optical schools in the U.S. were teaching the basics of pantoscopic lens alignment using Martin’s Rule of Tilt, and most eyewear was fabricated and dispensed with the optical center placed at the vertical midpoint of the lens shape. Even today, this remains the way most glasses are fitted and made, with little consideration for eye position and pantoscopic tilt (See Fig. 1). As fashion became the primary driver in choosing eyewear, opticians found themselves newly embroiled in a war between optics and cosmetics considerations. And cosmetics started to win most of its battles with optics, “winning” came to be defined by a ratio between satisfying eyewear consumers and how few complaints were being heard. Today, we refer to this metric as “20/Happy,” and it has found a place of usefulness in other optical areas such as refractive and cataract surgery.

IDEAL SPECTACLE POSITION OF WEAR
With cosmetics driving almost all layout and fabrication decisions, opticians might have been excused for not being aware of a proper starting point for position of wear. Further, the increased demand for cosmetically popular progressive lenses introduced some new considerations regarding spectacle position of wear. Seminars and clinics, sponsored by progressive lens companies and often done over dinner and wine, advised increasing both pantoscopic tilt and frame wrap angle, while decreasing vertex distance, in order to deliver better patient comfort. This was especially important with the excessively large frame styles popular in the 1980s. What these seminars told eyecare professionals at the time was that adjusting the position of eyewear in this way would reduce visual discomfort and complaints during reading and improve clarity in the lens periphery. Unfortunately, these adjustments also increased the tilt of the lenses in front of the eye, particularly in the distance, aka primary gaze, and opticians had to be more mindful about how much wrap angle was being introduced. Wrap angle is not a yoked or similar directional adjustment for both eyes, unlike pantoscopic tilt. The lens companies further went on to state that their progressive lens designs were now formulated expecting these larger frames to be fitted with 10 to 12 degrees of pantoscopic tilt and 6 to 8 degrees of frame wrap angle. Regarding vertex distance, the primary considerations that limited how close frames could be fit were eyelash clearance and brow or cheek contact.

WHAT’S LEFT OUT
What the lens companies left out of these presentations were clear technical explanations of the optical consequences behind their frame fit recommendations. First, tilting glasses in this way helped to better normalize and reduce variations in eye-to-lens distances across the full lens surface, which is important with all eyewear, but especially so with larger frame sizes using progressive lenses. By positioning the lens plane in this way, the residual optical errors of progressive lenses of this time weren’t actually reduced. Rather, there was a welcome reduction in dynamic prism and magnification effects due to a reduction in lens surface obliquity. This normalization of eye-to-lens distance coincidentally brings the alignment of the lens surface with the intent of the lens designers of the period. The goal here is to better align the image shell produced by the lens to the eye’s far point sphere for all rotational gaze angles. Ideal alignment of the spectacle plane remains one of the most important concepts opticians must master and understand about position of wear. It is also important to know that eye-to-lens distances are better normalized when using a best-form base curve for the prescription and material index chosen. But best-form curves tend to be steeper than those chosen for optimal cosmetics or lens-to-frame fitment. With regards to prescription, moderate to stronger corrections will always benefit from the time and attention paid to frame fit and base curve choice for ideal alignment of the lens surface. The central challenge today remains that most novel frame styles, particularly when you factor in unorthodox individual frame-fit preferences, often work against the normalization of eye-to-lens distances.

EYEGLASS PURCHASES: WHO’S DRIVING THE BUS?
The difference between how glasses were fitted and positioned in the 1920s and ’30s, and those fitted as fashion became the primary driver in an eyewear purchase of the 1970s, is best seen by visualizing who is actually “driving the bus” between the two periods. When the datum system ruled, there was a defined fitting approach in place, with the optical professional clearly in charge of the transaction. As fashion and cosmetic considerations came to dominate in the 1980s, the consumer became the driver of the transaction, and eyecare professionals often took a back seat for the ride. Although there were instances of consumer dissatisfaction with the optics of these larger fashion glasses, by and large if the buyer loved the fashion and finished cosmetics, little complaint was heard about the optics or was tolerated, aka 20/Happy.

But the age of living with these oversize monstrosities would soon come to an end as the early 1990s began, as eyewear fashion returned to the smaller, rounder styles introduced anew by designers such as Giorgio Armani via Luxottica and Sanford Hutton via Colors in Optics. There were also stalwart makers of these classic styles, such as Anglo American and Sir Winston Eyewear, who never dropped these classic styles from their catalogue. Now, the accumulated sins from an age of opticians who knowingly let the uneducated consumer lead them from a righteous optical path to the excesses and visual chaos of hugely oversized glasses were about to be forgiven. That was the good news. The bad news was that these smaller frame shapes further forgave opticians who actually never mastered knowledge about proper position of wear, and these people became the mentors of subsequent generations of opticians. If you pointed out how they prioritized cosmetics over optics and proper fitting, most would simply point out they must be doing something right because they were not hearing a lot of complaints. And once again, that means 20/Happy for the win.

ENTER THE GAME CHANGER
With the arrival of freeform lens technology, amazing optical advancements became possible, many for the very first time. Here are just a few:

1. The ability for an astigmatic correction to achieve point-focal correction simultaneously in both major lens meridians, even when used with a single base curve on the front surface.
2. The ability to allow more fully deliver excellent peripheral optics even when there were departures from the traditional best form curves.
3. The ability to allow departures in fitting and maintaining ideal position of wear while continuing to deliver overall excellent optical correction. These departures include variations in vertex distance, pantoscopic tilt, frame wrap angle, best-form base curve… or even a combination of all four!

Freeform technology became an optical game changer because it fundamentally changed the game of ophthalmic optics. It helps optically correct for how most frames introduce a degree of lens tilt to the eye’s primary gaze. These tilts introduce Rx departures in sphere, cylinder, axis and prism that often supersede ANSI standards. And even when a frame is in ideal alignment for position of wear, the lenses remain tilted in front of the eye. With freeform optical compensation, the negative effects of lens tilt can be largely overcome. And when cosmetic or lens-to-frame fit considerations dictate the use of a non-best-form curve, good optics can still be maintained.

TARGETING THE PREMIUM DISPENSING EXPERIENCE
In exchange for its tremendous suite of benefits, all freeform technology asks of the optician is a full understanding of the importance behind taking personalized position of wear measurements for every pair of glasses. But more than a quarter of a century after freeform technology entered the optical marketplace, still less than 10 percent of all lab orders include custom values for vertex distance, pantoscopic tilt and frame wrap angle. The reasons for this remain two-fold: First, there remains a lack of understanding of how every pair of glasses benefits from personalized power compensation. Second, opticians have become too comfortable using 20/Happy as a benchmark for evaluating their own skill sets.

Today, as online eyewear also finds 20/Happy, an ideal metric for re-enforcing the public’s perception of prescription eyewear as a commodity, it is important that the in-person, in-store buying experience be seen as a premium one. A premium experience demands a personalized approach to patient vision and comfort. Proper position of wear is an essential dispensing methodology that opticians must master to convey that the premium eyewear experience is the result of skill rather than serendipity or luck. ■