L&T: RxPertise

Apr
2014

The 411 Behind Single Vision Free-Form


Photographed BY Stephen Mark Sullivan; Model: Alexis Rios/Q; Frame: MYLON Luxon from Mykita

By Barry Santini

Today, no digital lens design is probably more underappreciated and misunderstood than single vision free-form (SV FF) lenses. Eyecare professionals confuse the higher levels of Rx precision possible in a fully-compensated Rx with the eye’s inability to see such small differences. And patients often have difficulty understanding why they are being asked to pay more for an unknown technology, with an unclear vision benefit, when their old, standard single vision lenses have always proved to be “good enough.” Further, many ECPs aren’t fully attuned to many of today’s fitting situations, where a tailored, free-form SV lens could deliver better overall acuity, reduced sensitivity to frame adjustment and superior cosmetics. In order to better understand all the manifold benefits of SV FF, we’ll begin by taking a quick flashback at the optical evolution of single vision lenses.

Can the Human Eye Really See a Hundredth of a Diopter?

In a word... no. Our eyes have evolved to be exquisitely sensitive to optimizing image focus and clarity. But even individuals who have demonstrated super-human acuity—down to 20/10 on the traditional Snellen eye chart—can only distinguish, at best, a 0.06D difference. So many eyecare professionals remain puzzled on why fully compensated, free-form single vision jobs come back from the lab with verification slips specified to a precision of 0.01D.

The answer lies in understanding that no local ECP possesses the necessary instrumentation which could verify the entire lens surface is made in accordance with the manufacturer's calculated cutting file, and therefore will deliver all of the promised visual benefits. In recognition of this, lens companies help the ECP by providing a single area of verification where the Rx can be checked for basic compliance. Of course, no manual lens meter is capable of reading this level of precision, so an accurate, automated lens meter is strongly recommended for verification purposes. But even with an automated lens meter, trying to strictly apply current ANSI standards for power and axis to a fully compensated SV FF lens is often a frustrating endeavor.

Although the human eye can't discern 0.01D differences in refraction, it can and does sample the entire lens surface continually, both through the eye's micro saccades and longer eye excursions. The integrated perception within the brain of the global free-form lens surface is what generates the "wow" at the delivery of a patient's first pair of single vision free-form glasses.

—BS

IT’S ALL ABOUT THE PERIPHERY
Over the last 200 years, optical innovators like Wollaston, Ostwald, Tyllier and Rayton focused on trying to expand the sweet spot of sharp vision from a small 5-mm area positioned around the optical center to one that offered excellent edge to edge clarity. In the early 1900s, Dr. von Rohr of Zeiss achieved this goal. His Punktal lenses provided outstanding correction for both power and astigmatic errors across a 60-degree field of view (FOV). But these lenses required highly-precise and custom curves be calculated and manufactured for each individual lens prescription. Although recognized for their excellence at the time, the market decided Punktal lenses were just too expensive. With Punktal lenses costing about 10 times more than other SV lenses, research continued and arrived at a more cost-effective solution which we know today as corrected curve lenses. Without the need to manufacture every lens to an individually calculated set of curves, these “best form” lenses delivered excellent clarity across a more modest 30-degree FOV as long as both the base curve and recommended fitting protocol were followed.

Over time, as frame fashion evolved, concern for reducing thickness and improving lens-to-frame fit led ECPs to depart from using the recommended best form base curves and adhering to proper pantoscopic fitting recommendations. The result was that consumers encountered both poor peripheral clarity and increased sensitivity to frame fit and tilt.

ENTER FREE-FORM SINGLE VISION
With the broad arrival of free-form lens processing technology in the first decade of the 21st century, single vision lenses were finally able to equal and even exceed the clarity of the 100-year-old, made-to-order Punktal lenses. Besides providing terrific acuity, single vision free-form lenses also deliver the following essential enhancements:

  1. The ability to deviate, up to 2 diopters and more, from the best form base curve for a specific Rx and material index. This benefit provides the flexibility to retain excellent acuity while also allowing:
    1. A more optimal match of lens curve to frame curve. This ensures the selected frame style will retain its desired off-the-shelf, “authentic” fit.
    2. The choice of a steeper base curve to provide increased clearance for eyelashes.
    3. The choice of using a flatter base curve to help diminish the effective wrap angle. By selecting a flatter curve in minus prescriptions, it is possible to rotate the temporal lens edge forward, which effectively decreases the wrap angle. This is accomplished in tandem with customized bevel placement.
  2. The ability to depart from best form fitting protocol. In today’s trendy, large “B” dimension frames, the pupil of the eye can often be found as much as 10 mm above the shape’s mechanical center (MC). If the optical center is placed at the MC—the default processing method for most labs—the pantoscopic tilt required would be as much as 20 degrees... an amount which certainly would compromise the fit of these larger frame styles.
  3. The ability to optimize cylindrical prescriptions. One underappreciated component of corrected curve lens theory that is little appreciated is that best form lenses, even when fitted in compliance with the rule for pantoscopic tilt and optical center placement, are optimized for sphere prescriptions only and not astigmatism. Therefore, as cylindrical corrections exceed 1 diopter, off-axis astigmatism and power errors multiply. The complex, aspheric surfaces possible with SV FF technology allow moderate to greater astigmatism prescriptions to also enjoy wide, sharp fields of view.


BUT WAIT, THERE’S MORE
Single vision free-form lenses can also deliver the following host of advanced performance enhancements:

  1. The ability to deliver excellent acuity in moderate-to-strongly wrapped ophthalmic and sport eyewear. The sophisticated, global optimization of the entire lens surface that is only possible through SV FF lens technology is perfect for today’s larger frame fashions. As frame sizes expand in A dimension, designers often specify facial wrap angles of 10 degrees and more, even in everyday ophthalmic styles. No type of traditional best form or aspheric lens can overcome the acuity compromises that accompany the non-yoked tilting of the lens planes in wrap eyewear styles.
  2. The ability to compensate for extreme pantoscopic tilt. Especially within sport eyewear, where frame styles are designed to hug the face to keep out damaging UV and High Energy Visible Light (HEVL)—aka blue light in the range of 400-440 nm—pantoscopic lens tilts of 15 degrees and more are common. In addition to compensating for the wrap angle of these styles, free-form single vision lenses are perfect for optimizing this yoked-tilt effect while maintaining exceptional clarity.
  3. For higher prescriptions, SV FF lenses can allow edge thickness reduction through lenticularization. It is a common rule of thumb that for a given frame contour, the larger the A dimension of the lenses, the greater the frame’s wrap angle. This is because the longer the chord length—defined by the tangent to the nasal and temporal limits of the lens’ back surface—the greater the wrap angle. Even in moderate Rxs, lens thickness, cosmetics and weight are obstacles that are often difficult to overcome. But by shortening the lens’ prescription area chord length through digital lenticularization, SV FF lenses can be optimized for all three considerations. In addition, lenticularized single vision free-form lenses can calculate the reduction in effective wrap angle in advance, which further enhances cosmetics, thickness, vision clarity. Further, reducing the wrap angle reduces the degree of fishbowl perspective common in wrap sunwear.


WHEN TO USE SINGLE VISION FREE-FORM
The following is a simple checklist to use for evaluating when to use single vision free-form. Keep it near your dispensing area or where you place electronic lens orders:

  1. Any cylinder power of 1.00D.
  2. Any desired deviation in base curve, especially for prescriptions over 3.00 diopters.
  3. Anytime adhering to the standard protocol for fitting best form lenses—dropping the optical center 0.5 mm for every degree of pantoscopic tilt—is impossible to follow in moderate to stronger Rxs.
  4. Any frame, ophthalmic or sun, whose wrap angle exceeds 8 degrees.
  5. Anytime you desire to change the base curve to maintain authentic frame fit or clear eyelashes.

THE PINNACLE OF CLARITY
During most of the last decade, as free-form lens processing technology evolved to become a major player in the marketplace, the phrase “free-form” became almost exclusively associated with progressive lenses. Why? Because while single vision lens problems are generally few and far between, progressive lens problems are, well… a little more common. Interestingly, the close association of free-form technology with progressive problems is also a reason that ECPs hesitate to use more SV FF: “If it ain’t broke, don’t fix it.” So while progressives have clearly benefited from free-form technology’s ability to expand fields of view by lowering distortion, perhaps ECPs should ask themselves this question: What kind of visual performance is possible if you simply took the progressive design out of your favorite free-form lens? The answer is that you’d reach the pinnacle of optical clarity only possible through 21st century single vision free-form lenses.■

The Problem with Understanding Pantoscopic Tilt

Probably no position-of-wear (POW) measurement confounds the average dispensing professional more than determining the pantoscopic tilt of a pair of eyewear. Often confused with pantoscopic angle, which is the angle the temples make with the frame front, today we define pantoscopic tilt as the angle between the lens plane and the line representing the eye’s primary gaze. Some ECPs assume this is only achieved when the wearer’s facial planeーdefined as a line connecting the forehead, orbital brow, lower cheekbone process to the plane of the teethーis perpendicular to the floor. But this would be wrong.

The pantoscoptic value that lens designers are seeking for their optimization software, however, is the angle between the lens plane and the primary gaze line. The distinction between pantoscopic angle and tilt is dependent on the person’s posture. So for the person whose habitual head posture places their facial plane perpendicular to the floor, there actually is no difference. But for those many individuals whose habitual posture departs from perpendicular to some degree—either head tilted forward or head tilted back—their pantoscopic tilt represents the combined sum of the frame’s pantoscopic angle and the angle of their facial plane. Dispensers must be alert to and observe the general posture of a wearer’s head when they are looking straight ahead. Certainly, at times this can be difficult to determine.

But in many ways, evaluating a wearer’s head posture is a dilemma not unlike the Heisenberg Uncertainty Principle in physics: The sheer act of asking a wearer to relax and look “straight ahead” often unnaturally influences the final result. Fortunately, the interactive effects of head, neck, shoulder and gaze angle are dynamic in nature, so we allow ourselves working tolerance of plus or minus 2 degrees for calculating the efficacy of this value.

Measuring Panto in a Special Situation

help illustrate how to approach measuring pantoscopic tilt are reading and distance “TV” glasses. In the latter case, the progressive wearer is often conscious of having to adjust and either lower or tilt their eyewear to reduce the encroachment of the progressive reading area on the clarity of the TV. TV glasses are therefore single vision glasses designed to watch TV while reclining on the sofa or laying in bed. In this fitting scenario, the glasses are not pulled down the nose or tilted abnormally. With the glasses in place normally and the head tilted back significantly,ーa posture common to reclining on a bed or sofa,ーthe angle between the eye’s primary gaze and the plane of the lens is often a zero or even a negative value (see below).

The important thing to keep in mind is that the POW pantoscopic value that the FF lens optimization program is looking for is the angle of the primary gaze with the lens plane which is greatly influenced by the reclining posture of this use.

Negative Pantoscopic Tilt
Of course there can be situations where the angle of the lens plane results in a negative value. Certain frame fits, wearer preferences and posture can turn up a negative number for Panto. Here’s the conundrum: Most FF POW optimizations may allow a negative value to be submitted, but it is important to check in advance if your favorite lab’s SV FF lens design actually uses negative values. Hopefully future software releases may employ negative values. Go figure.

—BS


A quick note regarding measuring vertex distance (VD) in these special situations: The value you are looking for is also to be measured where the eye’s primary gaze intersects the lens plane. This point may be above or below the VD measurement for general-use glasses.




L&T contributing editor Barry Santini is a New York State-licensed optician based in Seaford, N.Y.

 

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