Base curves chosen, and base curves measured, in different indices.

[drk]

OK, here's what I need to know: how far off is my lens clock, which is labeled in diopters of curvature for crown glass (n = 1.523 IIRC)?

I know that it's accurate enough for CR39 (n = 1.4985).

Therefore it should be accurate enough for polycarbonate (n = 1.59), right?

I mean, it's off a weensy, but it should be under even a 1/2 D, right?

I NEED TO KNOW!!!!


Why, do you ask?

I have an impatient who was getting replacement poly -1.00 FT 28 lenses for an old frame that we sold him (no major change in Rx).

The old frame is one of those jobbies where it's semi-rimless on the bottom, and the upper metal eyewire is a thin metal that sits in the groove (Emporio Armani...tre chic. Whatevs.)

The original lens base curve measures +3.0

His new lenses measure +3.0 (by our lens clock).

They were dispensed OTC to his wife, just cause.

Weeks later, his life is not worth living and complains of "everything is distorted/I'm in a fishbowl" or whatever. I measure the temporary EVEN OLDER glasses that he's been wearing until the job got done, and it was a +4.50 base. So, I assume that he's adapted to his old, old base curve and isn't patient enough to re-adapt to the original base curve in which he had heretofore been living a happy existence.

Now, I could tell him to suck it up, he must have "de-adapted" to the +3.0 while we was wearing the +4.5, give it time, but again, the world is ending, so I decide to remake the new lenses to match what he's temporarily wearing.

So I send it back to the baL PSV (use a mirror to decode that) and request a +4.50 base curve or anything remotely close. They keep sending me +3.0 base! Again and again! They swear they enter +4.0 base when we call. But it always reads +3.0 to us.

Now I understand that maybe there can be some computer overriding the entry because of the goofy frame front, and possibly flatter is better for glazing?

But why the discrepancy in what the lab says and what we measure?

[Robert Martellaro]

Check that the clock is accurate. I doubt that the lab is physically measuring the lens- isn't this all robotic now at PSV sbaL?

Double check that the lenses on the recent pair are not warped- they might measure +3.00 in the horizontal median but a two diopter warp would measure +5.00 in vertical, and was originally a +4.00 base curve.

Check the moon phase.

It doesn't sound like the base curve is causing the distress if the wearer can switch from a really old pair of lenses with a +4.50 BC, then new lenses a few years ago with a +3.00 BC, then back to the really old lenses again with the +4.50 BC.

However, the symptoms fit a too flat BC, so maybe the old lenses really were a steeper base.

Best regards,

Robert Martellaro

[grudyfan13]

I have an impatient

That made me laugh.

[drk]

I try to laugh at my plight. I enjoy the company.

RM:
1. The tabletop test is +0.00
2. Good point on warpage. I'll check.
3. Good point on prior adaptability.

As to the symptoms that match a steeper- or flatter-than-the-adapted-to base curve: I would like to know what you know. Do you have a general feel for which symptoms tend to go with which change?

[Lensman11]

To my knowledge no manufacturer makes a 3.00 bc poly bifocal It would be 2.5 then 4.25 approximately. The Rx in question would never go on a 2.5 unless you requested it so it looks like you have a 4.25 bc lens and your clock is out of calibration. With the frame you described it sounds like the lenses are warped, do the horizontal and vertical check. Lenses always warp vertically so it will show up horizontally. Take the lenses out of the frame and check the curve you will get a more accurate reading. The lenses might be wavy check the power 15 mm off center and see what it looks like.

[drk]

Good advice and much appreciated.

As to calibrating a lens clock: to check, you simply see if it reads zero on a tabletop, right?

[Robert Martellaro]

As to the symptoms that match a steeper- or flatter-than-the-adapted-to base curve: I would like to know what you know. Do you have a general feel for which symptoms tend to go with which change?

https://www.optiboard.com/forums/showthread.php/27631-The-logic-of-flatter-base-curves

Mostly a problem with low minus through high plus. CC can be distortion, lash crash, reflections when uncoated.

Checking the clock on a glass tabletop or high quality mirror sounds good to me. Make sure the hand or center pin isn't binding, and the pins perpendicular to the surface. Note: true curves and labeled curves can vary by .50 D or more, and are sometimes referenced to the refractive index of the material instead of modern crown glass.

Hope this helps,

Robert Martellaro

[tsmith1315]

But why the discrepancy in what the lab says and what we measure?

Does the lab invoice show 4.00 base?

[Lensman11]

Just checking on a flat surface will not tell you the clock is working correctly. If you have access to a Younger lens gauge it has a master curve of zero +6.00 and -6.00. The clock could read correct on zero but not on curves. The points could be worn or not parallel causing miss reading on curves. If you can’t get access to a gauge ask your lab for a few semi finished lenses with the true curve written on the box. The will always be accurate within a few hundredths of a diop sufficient for your accuracy needs. If the clocks shows bad readings on curves and good on a flat you will have to replace it.

[Kwill212]

OK, here's what I need to know: how far off is my lens clock, which is labeled in diopters of curvature for crown glass (n = 1.523 IIRC)?

I know that it's accurate enough for CR39 (n = 1.4985).

Therefore it should be accurate enough for polycarbonate (n = 1.59), right?

I mean, it's off a weensy, but it should be under even a 1/2 D, right?

I NEED TO KNOW!!!!

If all you are using it for is to measure curves the material doesn't matter. If you are using it to calculate power, then you will need to calculate for the index.

[ChrisBowers]

The unit "diopter" is the measurement of a deviation of light at 1 meter. We know that a single surface can't bend light ( yes I have a fair understand of ray tracing and the mathematics of said). A lens Clock really measure sagitta which is crudely converted to a base curve measure. Lens clocks indexes are a throwback to pre WW2 and poor mathematics of opticians of yesteryear. Old world glass had a refractive index of 1.53 so lens clocks were made to reflect that index. Now we could use for simple math like +6.00 front curve and a -5.00 curve would result in a +1.00 diopter powered lens.

Again, WW2 happens and the European glass of 1.53 is no longer available. Thus the introduction of western crown glass at an index of 1.523. We should have changed the lens clocks and surface charts but chose not to do this. It should be noted that today's lens calculations are made by converting lens surfaces to radius and lens indexes are use along with these for lens making and converted back to dioptric values. Interestingly enough we started compensating the base curves of lenses to allow simple algebraic equations in lens formulas. A glass 6.25 based lens was actually 6.19. When we stay within the corrective curve theory of lens base curve selection this worked well.
Enter CR-39™ and index 1.498... Again rather than teach the math/science of radius we started another bunch of charts and work arounds. One thing also worth talking about are the lap tools we use in lens making, the tools we given dioptric values (aluminum can't bend light.) and we compensated their radius for errors due to pad thickness.

Back to the patient problem. As stated there is no readily available 3.00 base ft28. We now need to have a discussion on Marked base and true base and index base. Marked base is just a nominal number. A 4 base can range from 4.26 to 3.75 in true base measurement. This varies from different manufactures. Marked base and true base are not expressed in a lens' particular index. I've made quite a few pairs of glasses and have seen a proportionate percent of lenses made over due to base curve selection. Seldom does this solve the problem. Mostly the patient just leaves or settles with the frustration. When you describe a low powers spherical lens and non-adaptation I would pursue lens waviness and poor surface quality. (we can hide a host of surface imperfections with lacquer coatings). Also measuring the front spherical curve in two meridians searching for the highest and lowest. this value deviation need s to be less than 0.75 diopter. Try viewing this lens through your projector. This allows you to see the whole lens system. Remember that your lensometer only views a 7mm circle and lensometers can only describe the total base curve and total cross curve everything in between is not measurable. Best yet is to apply lens mapping. Good wholesale manufactures use 100% mapping process for all digital forms even toric surfaces using digital processing.
Also testing you lens clock on a flat surface assures only that reading. Errors can be (rare) non-linear. A Gauge Master should be a must in every lens making and dispensary.

To finish, with today's digital aspherical, atoric and progressive surfaces, a lens clock can be more of a nightmare than help. It should be also noted that our industry a few years back elected to lower base curve selection to offer better comedic fits. Today's frames have standard 3.50 diopter form fit as opposed the thee 6.25 of decades past. I use diopter as an expression that most others reading this will have 3 dimensional understanding. Perhaps shame on me.

[tsmith1315]

Vision Ease still shows a Tegra poly aspheric D28 available as 3.25 BC. I seem to remember a few patient complaints years ago from those Tegra aspherics.

[Oscar]

Speak to someone who's more technically oriented when you call in. Some products only come in strange numbered bc's and will round up or down depending on the Rx without the data entry monkey being aware of what's happening. Example the product is only available in 1.0, 3.25, 5.5, 8.0. Thus when you ***** and scream that you ordered a 4 but still got a 3, it likely could have rounded back down to the 3.25. Depending on how well quiped their lab management software is, sometimes it won't even reflect the true base curve on your invoice only what was entered in the system.

[drk]

I am impressed and grateful for the amazing posts here!

I'm a little embarassed to say that my optician re-dispensed the job with the most recent iteration of +3.00 BC (which I thought was too flat) and he apparently walked out smiling.

So...I misdiagnosed his problem,seemingly, regarding BC sensitivity. Maybe it was indeed a warpage, manufacturing issue, or somesuch.

Your instincts have proven to be right.

Tip of the cap.

[Barry Santini]

Base curve is perceptual, therefore part of comfort.

B