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Lenses

How fit are your glasses?

03/12/2018
By Steven Daras
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Some people have no idea how important it is to fit a spectacle frame correctly, but it can massively effect an optical appliance. STEVE DARAS describes his own experience to drive the message home.

For many years, good optometrists and equally good optical dispensers always fitted the spectacle frames to the wearer before taking any facial measurements. Unfortunately, this seems to be a thing-of-the-past – as I experienced myself not too long ago.

After receiving two pairs of complementary spectacles from a large spectacle retailer. I went along to the nearest store and placed my order – even agreeing to pay extra for the photochromic tint.

My wife and the sales assistant/seller (I cannot bring myself to call them an optical dispenser) selected my frames.

The blue frame fitted me like a glove straight off the rack – the first time in more than 20 years that this had occurred. The other black frame while looking OK didn’t fit me as well.

"All frames should be custom fitted to the wearer before taking measurements – no exceptions, no excuses"

Despite this I was sat down and had my monocular PD taken with a pupillometer. Then the seller placed each frame on me and proceeded to measure my pupil heights using a height measuring system on each.

I didn’t feel all that comfortable wearing this bulky device over my frames, and when I voiced my concern about the poor fitting black frame, I was told that it couldn’t be adjusted as it was a sample frame.

However, the seller advised me not to worry as the lab was supplying a brand-new frame for the top of the line HD progressive powered lenses (PPL) and it would be fine.

As you may have guessed, the blue frame that fitted me well was delivered with minimum fuss and I could see very well, and they also felt good. I was quite happy wearing these. However, the other black pair didn’t go as well and I couldn’t see clearly through the distance portion of the lens.

The problem was the lenses were fitted too high and I was looking through more of the intermediate area of my PPL corridor. Everything was blurry, and when I asked them to recheck my frame fitting I was again advised not to worry as most people take a little time to get used to these types of lenses (this despite my happily wearing ‘these types of lenses’ for years).

Visual resilience

Instead of making a fuss I left. I thought to myself what does the public do in this circumstance? Do they trust the advice given and try them for longer?

For a while I tried these at home to see if they improved, which they did, marginally. The sides were still too narrow and pressed against the side of my head, but by doing this the frame then slid forward and down, enabling me to see a little better in the distance.

Regardless of this moderate success, I want to be clear that two wrongs do not make a right.Despite being able to somewhat make do, the frame was still uncomfortable, and the lenses weren’t in great position for other visual tasks.

What this showed to me was that some people can put up with anything to get a better result. I call this visual resilience. Some will put up with it, others won’t and may go back to complain, more still will go elsewhere for a second opinion while others may just give up, believing that these lenses are not suited to them.

The test

Because of this experience, I decided to embark on a small experiment and see what difference custom fitting a frame and not doing so had on the lens positions.

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I selected a new metal frame and had the measurements taken with it unfitted to me i.e. straight-off-the-rack.

Then I had the same frame fitted to my face/head requirements and took the same measurements. Both sets of measurements were taken on a Digital Measuring System (DMS) tablet version (iPad) by the experienced equipment rep:

Measurement

Unfitted frame

Fitted frame

Monocular distance PD

RT 32 mm

LT 31 mm

RT 31.8 mm

LT 31.4 mm

PPL Heights

RT 25.4 mm high

LT 27.6 mm high

RT 22.3 mm high

LT 22.8 mm high

Vertex distance (BVD)

20.5 mm

14.3 mm

Pantoscopic angle (tilt)

15.9°

9.7°

Face form angle (frame wrap)

7.6°

6.8°

The substantial differences between a fitted and unfitted frame

After seeing the difference, it was no wonder then that my previous black pair didn’t allow to me see clearly, as they would have sat at a similar height.

The longer vertex and greater pantoscopic angle would have also affected the field of view along the PPL corridor. Not only were the heights different, but most other measurements also changed. Only the mono PDs were within 0.50 mm tolerance.

This is especially a problem for high tech ‘position of wear’ or ‘as worn’ PPLs as they are reliant on a properly fitted frame, and the measurements taken at this point influence the optimisation of the lenses. Further evidence for this can be found in Clinical assessment on customised PPL (Han et al, 2011), which concluded that correctly fitted frames enhance the visual experience.

The moral of the story is that all frames should be custom fitted to the wearer before taking measurements – no exceptions, no excuses. If the sample frame isn’t fit enough to be adjusted time and again, then it’s probably poor quality. As the title says, glasses must be fit!

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