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Thread: Dispensing problem- 1.74n

  1. #1
    OptiBoardaholic sarahr's Avatar
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    Confused Dispensing problem- 1.74n

    One of my colleagues has just recieved her new 1.74 plastics. She finds that her eyes are aching after only about 10 mins wear. Her old 1.71 are fine. I've checked BVD, Centration horiz and vert, pantoscopic tilt, bow of frame and base curves. The only difference I can find is the front surface of the new lenses being 1 dioptre flatter and the eye size is a little smaller. She is not noticing excessive chromatic abberration. What else could this be. Do you feel that plastics have reached their limit? I don't know what success rate is in general as I have only been dispensing these lenses for a few weeks. Am I overlooking something?Any suggestions will be appreciated, I don't like to give up on a lens!

    Thanks:cheers:

  2. #2
    OptiBoard Apprentice classicz68's Avatar
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    Is this the same RX as her old lenses?

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    OptiBoardaholic sarahr's Avatar
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    Sorry!:hammer: Yes, exactly the same rx checks out OK and it's not a BVD problem. Very recent Rx. Off the top of my head I think -4.50 -0.50 ish R + L. I don't think there is a lot of thickness difference to be gained between the 1.71 and 1.74 in this Rx in the small eye frame.

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    Bad address email on file John R's Avatar
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    sarahr said:
    I don't think there is a lot of thickness difference to be gained between the 1.71 and 1.74 in this Rx in the small eye frame.
    There wont be much diffrence in a big frame either Sarah.

  5. #5
    sub specie aeternitatis Pete Hanlin's Avatar
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    Sounds as if you've investigated all the usual suspects, Sarah. Your colleague mentions discomfort after about 10 minutes? Is this 10 minutes of distance viewing, near viewing, or a combination? If she wears them while using a computer (which involves near point viewing and little use of the periphery) are the symptoms different than, say, after driving (distance viewing with lots of periphery).

    The problem may be related to magnification (or, in this case, minification) through the new lenses. Or, it may have something to do with the flatter base curve you noted (the effect of which is increased because of the index change).

    Good luck!
    Pete Hanlin, ABOM
    Vice President Professional Services
    Essilor of America

    http://linkedin.com/in/pete-hanlin-72a3a74

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    Master OptiBoarder Jeff Trail's Avatar
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    Not to toss a wrench into Pete's points but most often it would be the ocular curve that is the problem more so than the base curve mixed with the index...especially in the "molded" finished lens choices. One of the bigger problems I see most often is that you can actually OVER index a person and cause multiple problems.
    You have to take into account ray deviation and how it bends images just as much as the other points...and when you increase index of refraction in material you lose ocular curve to get the same amount of corrective power. The result is you get a smaller and smaller "sweet" spot of optimum optics, compounded by the fact that you want to try to have a certain degree of curve to allow for natural image transmission.
    The same theory applies to over compensating curve for things like lash crash, etc., etc.... just because they make it in the power you wanted does that mean it does the best job optically speaking? NOPE... I know I have run into this a lot with opticians selling say 1.67 to someone in a low myopic RX just because they wanted the high end sell than cannot figure out why that -2 RX is not working properly.. it is a weird thing but you can use to high of an index in relation to the power and have problems because of inadequate amount of focal curve.
    The one about base curve is not really a problem as much as the lack of focal curve and to "flat" of an ocular curve, the eye begins to strain as you leave the sweet spot which was reduced in size because of the index change.
    Goes against the grain in most peoples idea of lens choices but you can actually give someone to much index and lose optical quality in places other than the usual song and dance of abbe value etc., etc...
    Just something to consider :-)

    Jeff Trail

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    OptiBoardaholic sarahr's Avatar
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    Thanks for all your opinions and advice, I sort of felt that this high an index was probably overcooking it a little for the Rx in question.I have suggested that we drop back to a 1.67 in this smaller frame and my colleague is quite happy to do so. We have other staff and patients wearing the lens happily but they do haave higher Rx.

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    Master OptiBoarder LENNY's Avatar
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    Jeff!!!
    It is been a long time i did not see your posts!
    Reading your post i want to ask you a question.
    When using the high index material (i have an Rx with a prism) i decenter the lenses the patient will newer see well because he is not looking thru the sweet circle?
    What about ground in prism? The same effect?

  9. #9
    Master OptiBoarder Jeff Trail's Avatar
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    Lenny,

    I take you are talking aspherical surfaces? or atoric? .. than you can NOT use a finished lens and get your degree of induced prism through decentering the lens:) I know a few opticians who always try to take this short cut, it never works properly.

    You need to remember that the PD does not change BUT the angle of the image tranmission (by the amount of power). So that "sweet" spot (apex of the aspherical surface) still needs to be located in the correct place, but you need to "grind" the amount of corrective prism.... this is the ONLY time you can ever, well I should say "when" you can use prism rings while grinding an aspherical surface.

    You still have to remember that that "sweet" spot is where the eye will be (less the lowering for panto tilt and axis of rotation)

    The problem you have to watch out for is that the lab doing the processing understands this as well, I know some labs that do not understand it any further than some opticians or OD's (not all optical people are as "bright" as us on this site;) )

    If they do it correctly than you should have no optical problems at all.. they block the lens on center and grind in the prescribed prism and go about the rest (blocking and edging) as you would normally.


    Is that the answer you were looking for? I can go into more detail if you like but that is the general information that gives you the answer.

    Jeff Trail

  10. #10
    Master OptiBoarder LENNY's Avatar
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    I think this is what i was talking about.
    But is there any way to check how the lens was made.
    Lets say i need a -4.00 SV lens with 4Diopters of prism.
    How do i check if all 4 diopters were ground in?

    BTW i was not discussing aspherics.
    Not to desenter aspherics i learned the hard way!:D

  11. #11
    Master OptiBoarder Jeff Trail's Avatar
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    Lenny,

    If you were not talking about aspherical lens than it really does not matter how they did it, as long as you are able move it around on the lensometer to the correct amount of prism wanted..you than dot the lens...do it to the other side and that center dot "should" be the correct pupilary distance... if they are NOT aspherical or atoric than you can decenter a finished lens.. just don't go to crazy..
    Since an spherical lens is more forgiving (optically speaking) you can pretty well do it how ever you want ... you have a small frame than decenter away to get it.. really only need to grind it if you really need the lens.. of course a ground lens is always going to be thinner than a stock lens because we are grinding it for the exact measurements of the frame and PD while a stock lens is on center and depending on the blank size will always be thicker

    Jeff

  12. #12
    Master OptiBoarder LENNY's Avatar
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    Thanks Jeff!

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