By Alex Yoho, ABOM
If there is any certainty in our industry, it is the fact that eyewear styles change constantly, often returning with a few variations as the years go by. Recently we’ve seen a revival of larger frames, some of which can be challenging to dispense. I’ll share a bit of practical history that will help you understand the problems associated with fitting lenses into big frames and teach you some techniques for doing it properly.
Though the concept was ahead of its time, the lab generally produced the routine eyewear of the day, with few exceptions. That meant styles that had been around since the late ’40s through the mid-’60s, with a few trendy frames sprinkled in. Typical eye sizes were 44 to 50 mm, and occasionally a 52 eye for a large person. But frame styles began to change in the ’60s, and we were sometimes asked to put vivid blue or rose glass lenses in a retro frame to emulate the look of a flower child icon. However, this wasn’t any more challenging than the regular eyewear being fabricated, and there was a plethora of glass colors being made at the time.
About the time I started dispensing as well as doing lab work, eyewear fashion exploded. Many new colors and shapes were introduced, and frame sizes grew. In no time, that large 52 eye frame became the smaller frame that nobody wanted. It seemed as though there was no limit to how large frames could become. But in reality, frame size was limited to the largest lens blank size available. In that small lab we made only glass lenses, and most of the blanks from which we surfaced the lenses were 58 mm. You would think with that blank, one could at least edge lenses to a 58 eye. However, that doesn’t consider that the lenses had to be decentered to meet the patient’s PD. If a lens is decentered, the outer edge of the lens will not cut out, and there is a gap between the frame and lens. The first answer was to grind prism on the lens to move the optical center, so we could cut the shape out of the center of the lens blank. This is exactly what happens when a lens is decentered. Edges get thicker on one side, which avoids creating prism for the patient.
Fortunately, lens manufacturers saw an opportunity and began creating larger lens blanks. Glass lens blanks grew to 71 mm, which helped tremendously. But as eyewear grew larger they also became much heavier. Plastic lenses had been around for a while, but they scratched easily and were known for having poor optical quality compared to glass. With the demand for larger fashion frames, plastic lens manufacturers saw an exciting potential and improved their products. With celebrities like Elton John and Jacqueline Kennedy-Onassis sporting larger eyewear, hard resin lenses with a 1.50 index finally came into their own.
With frames approaching the staggering eye size of 62 mm in a few cases, even plastic lenses were quite heavy, not to mention the thickness of lenses had become ridiculous. A -4.00 in hard resin with a 62 PD in a 62-20 frame (lenses decentered 10 mm each) has an edge thickness of about 10 mm! So how do we reduce the thickness of these lenses? You might think, let’s just grind them thinner. The example above was created with a 2 mm center thickness. If we grind it thinner, we are instantly outside of ANSI standards, not to mention the safety aspect and potentially distorted vision as the material gets unstable and wavy. That’s not an option.
One of the most important factors in keeping large lenses thin is selecting a frame that does not require as much decentration. This is the difference between the patient’s PD and the geometric centers of the lens shape. It is calculated monocularly by adding the eye size and the bridge size (62+20), then dividing the result by 2 (41), then subtract the patient’s monocular PD from that (41-31=10).
Each power is different, but with our -4.00 example, every 2-mm drop in eye size will reduce edge thickness by almost 1 mm. This can make quite a difference, and frankly a 58 eye looks very large on most people and could make approximately a 2-mm difference thinner.
Every 2 mm less on bridge size reduces the edge thickness by nearly 0.5 mm. If it’s a frame with nosepads you might be able to use a smaller bridge size and adjust the pads wider.
Since each power is different, you can easily find online lens thickness calculators and plug in variables in eye size and bridge DBL to determine the effect upon thickness.
Even accomplishing a 2-mm reduction in thickness for a patient wanting both larger sizes and thin lenses is often shocking to their sense of how their eyewear should look, so labs developed other options. A rolled and polished edge can reduce and soften the appearance of thick edges by a significant amount. The drawback to this is that the shiny edge, though very clean and smooth, can be seen by others from the front, glaring a bit.
Facets on lenses can reduce edge thickness, perhaps more than anything else. They can cut the actual lens thickness by two thirds, and since they remove quite a bit of stock, they can reduce weight a fair amount too. These decorative beauties are meant to be flashy, so any glint just adds to their fashionable pop.
Efforts were needed to make lenses thinner in the late ’70s to early ’80s, and soon high-index plastic was developed. Higher-index lenses reduce thickness because the material can bend light more than lower indices. This means less curvature is needed to produce the same power and the less the lens curves, the thinner it is. With higher-index lenses, we can also make the entire lens flatter without compromising the optical quality. Flatter lenses are thinner. This is known as the plate thickness of a lens, measured between a flat surface below and on top of a lens.
Early efforts to make high-index produced a 1.54 index. About this time frame sizes were reaching their pinnacle in size. Dispensers would offer larger frames with high-index lenses, and the results were disappointing; often thicker than the patient’s older pair due to the eye size and decentration increase.
This is a valuable lesson to remember today as frame sizes are again on the increase. Just adding a high-index option, though it will help, needs to be augmented with reasonable frame selection. Realistic reductions in thickness began to be noticeable to the patient when 1.60 index lenses were introduced in 1987. Using our original example, the -4.00 in a 62 □ 20 frame, the thickness difference between hard resin and a 1.60 high-index gives a reduction of 2.2 mm. If we could then convince the patient to downsize the eye size and/or the DBL, we could reduce it by about a third of the original thickness.
In 1991, the first 1.67 index lens was introduced. They were an amazing product then and have improved since, making it the go-to lens to make lenses thinner without excessive amounts of chromatic aberration. This material would shave off a full 3 mm compared to our original hard resin example. 1.74 index hit the market in 2001 and is currently the thinnest plastic lens available today. With an Abbe value of 32, it has slightly less chromatic aberration than polycarbonate (28-30). This means in really high powers, it is the thinnest but perhaps not the best optical choice, particularly in large frames (a full 3.6 mm thinner compared to our original hard resin example).
As lens manufacturers were developing various lens materials to try to make lenses thinner, they were also looking into changing the shape of lens surfaces. It has long been known that if you grind a lens with a flatter base curve, you can grind a flatter ocular curve to equal the same power thus producing a lower profile lens. The problem is that particular base curves have been carefully designed to eliminate distortions that naturally occur. Changing that form increases those distortions. Aspheric curves proved to be a good option.
“To stay ahead of the game, it is important to have frames that reflect current styles. And it would appear that really large frames are a part of that, at least for now. My best advice is to embrace them but remember what you’re up against. “Traditionally, lenses have been made with spherical curves, but those are only acceptable in relatively low powers. Aspheric curves had been used in high powered cataract lenses years before implant surgery had started and were quite successful. They were not considered for low- to mid-powered lenses since they were more difficult to make and thus prohibitive in cost. But with the drive to flatten lenses and improvements in manufacturing technology, they soon became the next tool to slim down lenses.
Aspheric lenses are traditionally considered for plus lenses and thin them down beautifully while improving optics as well. Minus aspherics and atorics are also a great option. While they don’t reduce as much thickness in minus as in plus, they do have the ability to improve optics tremendously. Couple an aspheric form with a high-index material, and you have the ultimate in lens design.
As I walked the show floor at Vision Expo East in March, which I have for the past 20 years or so, you could see virtually every frame offered in the world in one place. I have always used this opportunity to keep an eye on what styles are coming soon since manufacturers often release new products just before the show so that they can show off their latest offerings.
It was like I had gone back in time. I saw frames that were reminiscent of the ’50s and ’60s, which feature brow lines and CatEyes for the retro contingent. There were still a good number of rimless frames which have been quite popular for the past eight years or so with the innovation of compression mounting. As usual, smaller, lighter metals and medium-sized plastic frames are always there. All of these gave me great comfort in the knowledge that they are reasonable fit options for the general population.
However, the thing that stood out most to me is the trend to look back to the days of huge eyewear. Now I’ve always been up to a good challenge, but these are the frames that remind me of the great consternation I felt as a young optician. Fortunately, today’s opticians have an arsenal of lens options that we old-timers didn’t have.
To stay ahead of the game, it is important to have frames that reflect current styles. And it would appear that really large frames are a part of that, at least for now. My best advice is to embrace them but remember what you’re up against. Patients will balk at the thickness of large lenses, so arm yourself with some good lens thickness diagrams, or better yet have your lab make some examples to show the difference between a really large frame and a medium frame with a -4.00 or so and an average PD of 62.
Always remember good optical practices such as calculating the decentration by subtracting the patient’s PD from the “frame PD” (eye size + DBL) and try to keep the decentration of each lens to 3 mm (or less for powers higher than 3.00).
You can calculate the lens diameter needed by doubling the decentration per eye and adding it to the frame’s effective diameter (look in that big book or online); the sum is equal to the blank diameter required. You can check with your lab if the blank is available for the power you are working with. Don’t assume that since XYZ Optics makes a 78 mm blank, that it will be available for a -8.00. They don’t make blanks that large for powers that high because they assume we have enough sense not to do that!
Also keep in mind that rounder frames are better than square corners for larger frames. This is particularly true for progressive lenses which will still give you problems. Use the factory cut-out charts to be sure the lens will cut in the frame you have selected. This is as easy as putting a dot at pupil center and placing the dot on the cross of the lens diagram. If the frame is inside the circle representing the lens edge, all is well. If not, it’s time to choose a smaller frame.
Remember that all multifocals are set at a certain place within the blank and can not be moved unless by moving the blank. If you have a large frame you will be ordering a very high seg-height or fitting height with a progressive lens. This can also limit the cut-out of the lens.
Once you have done everything you can using best practices, it’s time to consider high index, aspherics and edge treatments to complete that beautiful eyewear.
Above all, if the lab calls you saying that the lens won’t cut out in the frame you’ve selected, never commit the cardinal sin of saying, “Just do the best you can.” Instead, consider what you should have looked for and learn from mistakes. Then call the patient back and do it right.■
Alex Yoho is an ophthalmic dispensing expert and optical educator.