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Lenses

Fitting heights for aspherics

13/04/2018
By Jade Cusworth
Aspheric lenses are popular and versatile, but not everyone knows it’s just as important to take heights for these lenses as other more complicated offerings. JADE CUSWORTH explains why.

One story I always tell students in my Certificate IV in Optical Dispensing class is of the first time I took heights for an aspheric lens.

The reason I recount this story so often is because I believe it demonstrates the profound impact we as dispensers can have on our patients’ vision, and consequently the success of our practice as well.

But first, what is an aspheric lens?

Aspheric lenses are not spherical and have a changing surface power across the lens.

Its surface shape changes so that the true power of the lens is positioned in central part of the lens with a reduction in the surface power more peripherally.

Sometimes the dispensing of aspheric lenses is based only on the ‘thinner and lighter’ benefits of the lenses, as the high refractive index of the available lens materials means they are often only available in aspheric design.

However, while thinner and lighter are distinct benefits for the wearer there are other benefits of this lens design too.

"The flatness of aspheric lens design contributes to a reduction in spectacle magnification, eliminating the ‘bulbous’ look of a plus lens"

For example, proper use of aspheric lenses provides a reduction in peripheral aberrations so that when the wearer looks away from the centre of the lens, there is a gradual change of lens power and vision remains clear in the periphery zones of the lens.

The flatness of aspheric lens design also contributes to a reduction in spectacle magnification, eliminating the ‘bulbous’ look of a plus lens.

Additionally, a flatter lens will fit in most frames a lot better too.

However, if the lenses are not positioned correctly, aspheric lenses can have an adverse effect for the wearer who will not really like their glasses but also will not be able to identify why.

It’s for this reason that taking heights for aspheric lenses are so important, which brings me back to the aforementioned patient.

I can still clearly hear his voice in my head: “I don’t want progressives, no one has ever put dots on my lenses before, are you sure you know what you are doing?”

At the time, I was a young, enthusiastic dispenser who had just returned from a block of learning in Melbourne while studying for my Cert IV.

We had learnt that to achieve the best vision for our patients we should be taking heights for aspheric lenses, not only progressive lenses.

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This was a new concept to me as I had only been in the optical industry a few years at this point, and I didn’t understand how aspheric lenses worked and why heights would make a difference.

I was quite keen to test out my new-found skills and knowledge, and took the first opportunity when a gentleman came in for an eye test and needed SVD aspheric lenses.

After asking the patient if everything was ok, he began to question me for taking heights. I explained to him that this was correct way to fit aspheric lenses to get the best visual outcome, but he was still suspicious.

Lenses not positioned correctly can cause problems
Lenses not positioned correctly can cause problems

Eventually, he reluctantly agreed, but about a week after he had left with his new specs I saw the patient approaching the store and my stomach dropped – I felt certain that he was unhappy. I shouldn’t have been worried.

Instead of asking for a replacement pair, the patient told me they were the best pair he had ever had, and that he had not realised his old specs had felt uncomfortable until he had worn the new ones.

What he did next took me by even greater surprise. He took out two other frames (one a sunglass) and asked to have the lenses swapped over to be the same as his new ones!

To say that I had made a loyal life-long patient would be an understatement. Not only did he continue to come and ask only for me, he also recommended me to his family and friends.

The experience had a real impact on me, as I had made a difference to someone’s vision in a positive way and he was thankful. So, the obvious next question is – how do you take heights correctly for an aspheric lens?

A plus lens will generally have the aspheric surface on the front of the lens but this can be both.

Meanwhile, the front surface of a minus lens will be steepened slightly to reduce the edge thickness and will also have its back curve flattened too.

So, you need to drop the regular pupil height by one millimetre for every two degrees of pantoscopic tilt.

It’s also important to get the same result from having the patient lift their chin until the lenses are 90 degrees to the floor and marking the pupil position while they are looking straight ahead.

If you do this, I can guarantee you will have more satisfied customers, despite the slightly more involved fitting process. Happy height taking!

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