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Thread: STUPID QUESTION

  1. #1
    Master OptiBoarder LENNY's Avatar
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    Question

    I made a pair of glasses for one of my customers.
    Its a simple pair of FT28 CR-39
    OU +6.00 with 4.00 add
    She could not see out of them so she went back to her MD. MD checked the glasses and told her that the add is wrong.
    I measured the add its 4.00
    I called the dr.
    He measured the add from the front.
    I thought that we check the add from the back side!?
    Who is wright?


  2. #2
    Bad address email on file Rich R's Avatar
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    Hi Lenny,
    I assume the patient is having problems with the reading, if the MD claims the add power is incorrect.
    When you get to a total of +10.00 there are a few other factorsto consider like vertex distance, pantoscopic angle and base curve.
    I would check the old pair of glases to see if any of those are quite a bit different and also how much of a power change there is. I have found on a +4.00 add with that much + in the distance the add power from the front probably checks .25 to .37 strong, you're correct that FT's are checked from the back however. Hope this helps, Rich R

  3. #3
    sub specie aeternitatis Pete Hanlin's Avatar
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    Assuming the lenses are plastic, you are correct- the reading power should be taken from the back of the lens.

    Another way to ascertain the add power in a plastic lens is by using a lens clock to calculate the difference between the base curve of the distance portion of the lens and the base curve of the segment.

    Also, when you say "she couldn't see out of them..." What couldn't she see? Distance, near, both? Does she have cataracts? What is her best corrected VA? What was her last Rx? Does she know she has to hold near objects 25cm (about 10") from her face to use the +4.00 add? The answers to those questions should solve whatever problems she is having.

    Pete

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    Optical Educator
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    Hi Lenny,

    You are definitely on the money. Bifocals should be turned around in the lensometer before reading. Actually, we are reading the front surface, not the back (segs are all on the front...) We turn the glasses so the back surface is facing us, but the optics are being read from the front surface. It is because of the distance 6.00 Diopter power that gives her the problem. (she wouldn't notice the difference in a +1.00 with a +4.00 add). It's a matter of front vertex power Vs. back vertex power. The doc is probably reading them from an auto lensometer which doesn't give front vertex power, or at the least, not flipping them.

    Laurie

  5. #5
    Master OptiBoarder Texas Ranger's Avatar
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    Same old sad song. The pt. sees fine in the environs of the lane where light contrast and you're showing them an add power directly over the OC of the distance rx. but glasses aren't made that way, now are they. you're now dealing with a fair amount of base up prism and, depending on the lens size and PD decentration, lots of thickness. now, vertex distance is another variable; but all the doc knows is that his lensometer doesn't read the right add, and it's the optician's fault. so, what does this approach do for the pt.? and,assuming that the glasses are made to the rx, as well as glasses can be, what are we supposed to remake the lenses to? A little cooperation and understanding on the docs part would help a ton!

  6. #6
    Master OptiBoarder OptiBoard Silver Supporter
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    What did the MD say the power that he read was? Depending on the patients description of the problem will help you decide if an adjustment will help.

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    I was taught with strong lenses and adds: 1) check distance power from back surface. Then 2) check distance power from front, 3) check add power from front. Difference between front distance power and front distance power is the add. The actual distance power is that read from the back. The reason for all this: Vertex differential.

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    the very strength of the add is a tipoff. normally, an advanced presbyope loses 2.50 to 3.00 diopters of "amplitude of accomodation." prescribed adds beyond that are often an effort by the doc to allow the patient to accomodate at shorter distances than usual, which provides "relative distance magnification" ie, the closer the reading material is brought to the eyes, the larger the retinal image, obviously a benefit to someone with poor vision. As mentioned, this patient may have some issues such as cataracts, macular degeneration, or something else.

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    What does it mean when someone can't see. They must have somehow become blind when you put the glasses on. If you have normal vision then take a 6X magnifying glass and hold the object you are viewing close enough you will see it. It sounds like this person has other issues such as cataracts, retinal problems or doesn't like the frame.

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    Master OptiBoarder Alan W's Avatar
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    Idea

    Lenny
    Your MD is running true to form. No optical education!
    On power that high we have to reverse them in the lensometer and read through the lens as the patient does! The true measurement of the add is the difference between the distance front curve and the segment curve (which is on the front.) Use a lens guage. Send your findings to the doctor. Then hand the note to the patient, send him back to the doctor, call the doc and say: . . . "With these high powers, inherent thickness and no refracting vertex distance on your prescription you'll have a hard time getting a spectacle lens to do what you want. Give me your vertex refracting distance and I'll call you back with the compensating calculations. Then I'll remake them to the adjusted Rx, unless you'd like to revisit the prescription first." With 10 diopters at the near, if your patient sneezes, its a changed Rx! This is for the "Duh" files. Your doctor should know better. (So what else is new?)
    Your patient is getting over plused and probably over magnified six ways to Sunday and the Rx isn't telling the whole story. Ten to one says the new Rx will be a quarter to a half less THROUGH the add. Keep your lenses thin, flat, close and hang out the "gone fishin" sign before the patient comes back!

    Bye bye

  11. #11
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    Big Smile

    Hello,

    The best way to check the add is with a lens clock. Clock the lens first then the add.Take away the difference.
    Ex.clock lens 6.25 (base)
    clock add 8.25
    add is 2.00

    Hope it help Duane


  12. #12
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    BUGWIZ,

    Your lens clock is in all probability calibrated to 1.530 index and will not give a truly accurate diopter reading on other materials, especially higher indices. The differential crown glass to CR-39 is 1.06425 diopters and crown glass to 1.670 index is 0.791044 diopters.

  13. #13
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    Not a stupid question, most rx's are measured in the lensometer back vertex power which is the front of the lens facing you when neutralizing the lens. On high plus powers the add will show up to a half dioptre strong if measured this way. Front vertex power should be measured(posterior lens facing you) to achieve the correct add. Also a 4.00 add sounds very strong, usually prescribed for somebody with macular degeneration, I would check her previous specs and go from there. Hope that helps


    Phil

    ------------------

  14. #14
    Master OptiBoarder Texas Ranger's Avatar
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    There are an awful lot of posts relative to which way you read the lens in the lensometer, to what, justify that the glasses were properly made, and perhaps the doc, or their tech. doesn't know how to read them, and that's the problem! What difference does it make? You're still going to be making the lenses over again! The point being, what are they going to be made over to? In the phoropter, ht ept. likes a +6.00 with a +4.00 add. but since thickness, induced prism and vertex make it impossible to emulate the lane, they end up with probably a .50 too much plus, about an 8 inch focal length and the depth of field of the thickness of the lens, so they "can't see". (where they want to see). So, get with the doc and solve the problem, it doesn't do any good to tell them that they don't know how to read the lens in the lensometer. Of course, we all O's have the need to be "right".and that's not the issue at all.

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    Unhappy

    Hey Wes Trayner:


    Your right about the crown glass. But I thing he was asking about cr39. Besides you can't clock a glass ft any how the add is molded into the lens. Thanks for the help
    but next time you might want to read the question first.

  16. #16
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    Bugwiz,

    The lens clock that most opticians use is calibrated in 1.530 index and will not give you true dioptric readings on other than glass. If you read a +6.00 glass lens your lens clock will be correct. If you read a 6.00 diopter 1.670 index lens you will be off by 1.39 diopters. Therefore, if you read a curve on the front of a high index lens and compare your reading of the add power difference you will be off by a factor also. I think that the important factor is that the average lens clock is only accurate on glass and any other material that is close to 1.530 index.

  17. #17
    Master OptiBoarder LENNY's Avatar
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    Why dont they recalibrate the lensclocks?
    Since we using 80% of cr39 material and only 2-5% of crown glass?
    Ahhhhhhh?

  18. #18
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    Lenny,

    That's a real good idea. They should make lens clocks for glass, CR-39, mid index and high index. It would not involve changing the lens clock itself since it is really a sag gauge. The dial on the front is the only change that would have to be made so they could offer lens clocks for different materials.

  19. #19
    Master OptiBoarder Jeff Trail's Avatar
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    Wes,

    atleast since 99.9% are set for glass index then since every one is "technically" wrong then we are alteast all "wrong" the same way.. which makes it "right" in a quirky way :)
    So when I get an RX form with a requested base curve of "6" then I know thier version of old base curve clocked will match my base curve clock.. I would hate to see all my accounts start clocking curves with every indexed allowed for.. my end would be a nightmare

    Jeff" who ever said being wrong is not right" Trail

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    Master OptiBoarder Jeff Trail's Avatar
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    Bugwiz

    There is a way to convert it if you want to go to the trouble, Wes posted that you would have the "wrong" (I'm sure he meant refractive "power") curve because of the index the sag gauge was based on but here is a formula to figure it out :)

    using the example that popped up +6^ curve on crown glass..compared to a sag curve of CR39..

    if the front surface of the lens clocked at +6 then you need to find the radius of curvature to get to the next step...
    (clocked curve)+6= 1.53(index)-1 over r1

    so..1.53-1 would give you .53/+6.00 (clocked curve) which would give you the radius of curvature.. .53 devided by +6 = .0883 m

    now to find the conversion you just run the same formula but plug in the radius as the devider... 1.498 (cr39)-1 over .0883 m(radius from the first formula) so now you end up with .498 devided by .0883 = 5.64^

    So the refractive power of 6^ on the clock on a glass lens is ^.. but if you clocked a CR 39 lens with that same clock, the refractive power of that 6^ is actually 5.64 ...

    I guess if you really had to do it you could go to all the trouble of converting both on the 180 and the 90 and then you could get a precise answer about the seg power.. me personally? I would prefer just popping it into my lensometer :)

    Jeff "can't we make life to complicated at times?"Trail

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    Jeff,

    I have re-read my messages and I don't see where I used the word "wrong". All I am trying to get across is that the lens clock cannot be accurate for measuring segment powers on one piece lenses with indexes that are not close to 1.530 without some mathematical formulas.

    [This message has been edited by Wes Trayner (edited 09-07-2000).]

    [This message has been edited by Wes Trayner (edited 09-07-2000).]

  22. #22
    Master OptiBoarder Jeff Trail's Avatar
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    Originally posted by Wes Trayner:
    Bugwiz,

    The lens clock that most opticians use is calibrated in 1.530 index and will not give you true dioptric readings on other than glass. If you read a +6.00 glass lens your lens clock will be correct. If you read a 6.00 diopter 1.670 index lens you will be off by 1.39 diopters. Therefore, if you read a curve on the front of a high index lens and compare your reading of the add power difference you will be off by a factor also. I think that the important factor is that the average lens clock is only accurate on glass and any other material that is close to 1.530 index.

    This was your posting.. I just figured that boiling it down to simply saying the sag gauge would give you a "wrong" answer :)
    If you prefer I could rephrase and say "due to the calibrated reading based on the index of glass sagging another material, indexed other then glass, it would be a misleading measurement of refractive power"
    I prefer the "wrong" word better, short and sweet :) ... I was just adding the formula to refigure the compensation of calibration.. Just thought it might come in handy to know the way to convert, even if Bugwiz know's the formula possibly someone else, reading the thread, did not and learned something new :)
    Didn't say you was wrong.. just added the math behind what you were saying..

    Jeff "never said I was "politically" correct.." Trail

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    Jeff,

    Fair enough with me.

    Besides, it gives me another post towards getting out of the "Novice" catgory....LOL

  24. #24
    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    Thumbs down

    Originally posted by Jeff Trail:

    Jeff "can't we make life to complicated at times?"Trail
    It could be simpler; just multiply 6 x (498/530).

  25. #25
    OptiWizard
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    Since the patient thinks the glasses were made wrong (ophthalmologist or his tech should have reversed the lenses in the lensometer), the patient will blame the dispenser for the poor vision. Patients with reduced vision usually grasp at straws and are looking for the miracle of "more power" to negate pathology.

    Any add over a +3.00 generally means the patient has some sort of pathology, usually macular degeneration.

    This patient has unrealistic visual expectations and needs to be counseled by the prescriber.

    And in short, they usually side with the opthalmologist, because it means there is a chance for better vision.

    In short, you've got the shaft.


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