Fitting height - guessing in Corona times

Hello,
we are slowly but steadily approaching the fourth wave of the pandemic here in Europe. In the last year and half we had to do quite a few glasses for people in retirement homes who were not able to come in person. I have a question about OC height.

What have we been doing:
1. Reading glasses: 50% height
2. For distance: 2mm above 50% height
3. Multifocals: just guessing

As expected we had some hits and misses with multifocals. Sadly I'm sure we will have to do some more of them because of the times ahead. I'm curious if you have any recommendation on OC height for customers who we can't measure in person and have a complicated prescription. Is there a rough formula that we could guesstimate better for PALs? How do the online retailers do it?

Thanks, Marin

Wow. That's a tough one. I've had to play the guessing game in nursing home emergency situations. It's never fun. I've been doing this long enough that I can get pretty close with an educated guess. Another idea is putting the glasses on and measuring yourself. You can get pretty close that way. I did that back when I was starting out and had no other option but to guess. Good luck and be safe!

I have asked the nursing staff if they could simply mark the lens, just have the patient look directly at them then they mark. For FT drop 7mm for Tri, and PAL drop 1mm. Have yet to have a redo. Knock on wood. Guessing is a different story.

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Originally Posted by mervinek

Wow. That's a tough one. I've had to play the guessing game in nursing home emergency situations. It's never fun. I've been doing this long enough that I can get pretty close with an educated guess. Another idea is putting the glasses on and measuring yourself. You can get pretty close that way. I did that back when I was starting out and had no other option but to guess. Good luck and be safe!

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That is a very good tip, to measure it on ourselves, thanks!

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Originally Posted by Optical Roy

I have asked the nursing staff if they could simply mark the lens, just have the patient look directly at them then they mark. For FT drop 7mm for Tri, and PAL drop 1mm. Have yet to have a redo. Knock on wood. Guessing is a different story.

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We tried that (getting the frames to them first to have them marked), but it didn't work. There is not enough staff to do it and everything is chaotic.

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Originally Posted by mmarin

We tried that (getting the frames to them first to have them marked), but it didn't work. There is not enough staff to do it and everything is chaotic.

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I've had family members measure in the past and that worked out ok, but now during covid, it's not possible. No one can get in if there is a lockdown. It's a hot mess. Measure yourself and you should get pretty close. Another idea is going with metal frame with pad arms so you can lower or raise as needed.

Also... I STRESS when this is done that this is EMERGENCY ONLY and it may not be perfect, but the important thing is that we are going to get them to see. They understand that we are making an educated guess. Done this for hospice patients too. We don't do remakes for something like this unless we feel we were super off and made a bad mistake. Also.. these are patients that we already have PD measurements from old orders.

If it is a normal style frame 3 below center will work the majority of the time. This number was gathered many years ago when we were trying to automate marking of lenses and tracked the bifocal position. The 3 mm drop came up 94 % of the time.

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Originally Posted by Lensman11

If it is a normal style frame 3 below center will work the majority of the time. This number was gathered many years ago when we were trying to automate marking of lenses and tracked the bifocal position. The 3 mm drop came up 94 % of the time.

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Way back in my Lab days we had an OD customer who would frequently forget the seg height. We would call him and he would say wait a minute to check the record. Always came back "Make it 3 below". Always brightened up our day.

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Originally Posted by Lensman11

If it is a normal style frame 3 below center will work the majority of the time. This number was gathered many years ago when we were trying to automate marking of lenses and tracked the bifocal position. The 3 mm drop came up 94 % of the time.

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Thank you. To clarify, 3mm below 50% of the frame height? That sound very low, no?

The data is the data 3 mm below the middle of the frame.

Ft lens blanks were cast using 5 in and 3 down for the reasons lensman stated.

Can no-one take a good picture of the patient wearing the frame and send to you? If you have the frame in hand, you should be able to measure and using wisdom/experience get closer than any other method, I would think. That's how the online retailers do it; a photo and good software.

DV 2 above datum is good
NV 2 below datum is good. Consider down gaze when reading.
ST/FT 3-5 below based on bridge style.
PAL 2-4 above datum.

I'm reading a lot of talk about frames and lenses and none about noses.

If no one has seen the frame on the patient's nose, the measurement is a guess. Not an estimate, not an approximation, not even an educated guess, just a straight-up guess.

We need to do better than guess. Everyone's phone has a camera on it. At the very least, a caregiver must be able to spare the ten seconds to take a picture with the frame on the patient's face. No matter how busy or chaotic the environment, that is a minimum reasonable expectation for us to be able to do our jobs professionally.

Agree and disagree.

For the ultimate in precision and intent: On the nose, face to face.

Close enough (85% percentile confidence factor—aka 20/Happy): Use 3mm to 6mm above Datum.

B

Without having the patient physically present, all we can do is guess. We can try to make an educated guess - like looking at their previous order and what OCs/segment heights were in the past in relation to the B measurements of the frame, or trying the frame on ourselves and measuring that way, or just doing 3mm or so above the datum line, but everyone's nose is different. All we can do really is guess. For me, I have a bridge that starts right at my eye level and doesn't protrude out that high - so plastic frames without nosepads sit very low on me - I am almost looking at the top of the frame when I try them on. A photo as someone else said is ideal.

What do online glasses retailers do? I imagine they just arbitrarily measure segment heights for multifocals at 3mm above the datum line. For bifocals, I would imagine it's 3-6mm below the datum line, allowing at least 10-11mm for the segment. :/

From when I started out in big box stores with zero optical experience... I had a manager who said if the progressive seg was more than half the B +3 she wanted me to have her double check it. That was 15 years ago, but I think that's still not a terrible idea for a smaller B frame, and probably a little more for the higher B frames. Which is why everyone is saying 3-6 above DATUM.

Not sure where the 3 below is coming from??

3 below sounds terrible. I suppose we should clarify what fitting parameter we are talking about. The fitting cross. I assume the people saying 3 below are talking about the 180 layout marks on a 4 drop lens, since placing the fitting cross 3 below would be a disaster. That makes the fitting cross 1 above which is still way to low IMO. The majority of my face to face measurements result in the fitting cross 6 above. Higher in real deep B frames. Sometimes a couple lower for the wide nose. Very seldom(less than 5%) do I see a measurement that puts the fitting cross less than 3 above. Progressive non adapts/fitting issues are virtually non-existent.


I can't see a world with today's lenses where 3 below is viable, unless you are trying to hide the periphery distortion of an old style PAL.

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Originally Posted by mmarin

Hello,
we are slowly but steadily approaching the fourth wave of the pandemic here in Europe. In the last year and half we had to do quite a few glasses for people in retirement homes who were not able to come in person. I have a question about OC height.

What have we been doing:
1. Reading glasses: 50% height
2. For distance: 2mm above 50% height
3. Multifocals: just guessing

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We should aim for a pupil center position about 4mm above center, the center being the 180/datum line, that is, the midpoint between the boxed topmost and bottommost part of the lens, regardless of lens design, SV or multifocal, distance or near. This assures a proper relationship between the optical axis of the lens with the center of rotation of the eye, assuming a typical panto tilt of eight degrees.

The lab will position the distance OC coincident with the pupil height, or on center, depending on the lens manufacturer's recommendations, exceptions primarily due to vertical imbalance concerns.

A reasonable fitting height clinical rule might be 3mm below center for segmented bifocals, on center for trifocals, and 5mm above center for PALs.

Because of the guesswork, it would be best to use the patient's old frame, preferably replacing it with the same when available.

Hope this helps,

Robert Martellaro

Thank you all for the input. This is really a method of last resort. Sadly we had to do it a few times and it's better than nothing.

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Originally Posted by Robert Martellaro

Because of the guesswork, it would be best to use the patient's old frame, preferably replacing it with the same when available.

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Or, if you can get a hold of their old frame, line it up with the new one, nosepads aligned, and match the relative seg height.

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Originally Posted by AngeHamm

Or, if you can get a hold of their old frame, line it up with the new one, nosepads aligned, and match the relative seg height.

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I like to use a little bit of algebra. If I know a patient had a frame with a 35 B measurement and a seg of 23 and their new frame they are looking at has a B of 32 you just solve for X. X/32 = 23/35, then multiply by 32 to solve for X. So X= (23/35)*32 = 21.

So proportionally a 23 seg in a 35 B frame would be the same proportion as a 21 seg in a 32 B frame. And yes I understand different bridges and frames will fit differently, but this is just a place to start.

This just is splitting nose hairs, but,

All things being equal, you’d use distance from datum differences. A 35 has a 17.5 datum and a 32 has a 16mm. Difference of 1.5mm. With a known hgt. of 23 on a 35 B, that’s 5.5mm - the datum difference (1.5mm), would give you a 21.5 hgt. on a 32 B. Works for seg drop too on below datum measurements. We used this back in the cataract lens days when we measured off of samples in stock, only to order another size for final fabrication.

(Those were the days! Frames in 8 eye sizes, bridges available on each, 16 to 24mm! 5 temple lengths, and cables too! All in at least 3 colors…)

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Originally Posted by Kwill212

3 below sounds terrible. I suppose we should clarify what fitting parameter we are talking about. The fitting cross. I assume the people saying 3 below are talking about the 180 layout marks on a 4 drop lens, since placing the fitting cross 3 below would be a disaster. That makes the fitting cross 1 above which is still way to low IMO. The majority of my face to face measurements result in the fitting cross 6 above. Higher in real deep B frames. Sometimes a couple lower for the wide nose. Very seldom(less than 5%) do I see a measurement that puts the fitting cross less than 3 above. Progressive non adapts/fitting issues are virtually non-existent.


I can't see a world with today's lenses where 3 below is viable, unless you are trying to hide the periphery distortion of an old style PAL.

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Hmmm.

But we all know that setting a progressive too high is a recipe for instant failure.

Combine this observation with how distance-prioritized progressives lower the start of the progressive corridor, and

How good FF designs—such as the IOT Alpha series—are able to manage surface astigmatism in shorter corridor designs with far less negative impact than the legacy of molded compact progressive designs, and…

You get the following good rule of thumb:

Set the unknown progressive height to 4mm above datum and spec a shorter than normal MFH corridor—say 14mm to 15mm—and you’ll no doubt arrive at 20/Happy for 85% of these “blind” fits.

B