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Thread: ABO study question help please!

  1. #1
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    ABO study question help please!

    Happy Saturday fellow Optiboarders!

    I have a question. While studying material for the ABO I have come across a question that has two answers and I am not sure which one is correct. Here is the question:

    When determining the bifocal add you measure the difference between what?

    Opti professor says- The back vertex measurement of distance and near

    The Ophthalmic Dispensing Review Book says - The front vertex measurement of distance and near.

    Can someone please tell me which one is correct?!

    Thank you in advance for your help!

  2. #2
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    Front vertex. The add power will read too high if you use the back vertex measurement on stronger plus powers.

    http://www.optiboard.com/forums/show...ing-add-powers

    The measurement points should be equal in distance from the distance reference point, typically the prism reference point or distance OC (if there is no Rx prism).


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    Last edited by Robert Martellaro; 05-17-2011 at 02:09 PM.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Quote Originally Posted by kpshann View Post
    Happy Saturday fellow Optiboarders!

    I have a question. While studying material for the ABO I have come across a question that has two answers and I am not sure which one is correct. Here is the question:

    When determining the bifocal add you measure the difference between what?

    Opti professor says- The back vertex measurement of distance and near

    The Ophthalmic Dispensing Review Book says - The front vertex measurement of
    distance and near.

    Can someone please tell me which one is correct?!

    Thank you in advance for your help!

    Technically there both right. If you have high plus power lenses you want to read dist. First then flip over to read add but only for high plus powers. In every day operations your going to do front vertex 99.9% of the time you see very few high plus. In the day after cataract surgery it was common to see some one come in with a +9.00 to +13.00 so you would use both to get reading power. But I would agree with Robert because of what I said above with 99.9% of all rx's!

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    Quote Originally Posted by kpshann View Post
    When determining the bifocal add you measure the difference between what?[/B] Opti professor says- The back vertex measurement of distance and near The Ophthalmic Dispensing Review Book says - The front vertex measurement of distance and near. Can someone please tell me which one is correct?!
    While thickness of lens is important in strong pwers, all manufacturers determine their pre-made segment (Add) power based on front vertex power of lens only. So flip glasses (with temples toward you) and determine the distance power first (@mire closest to 090 degrees) and then drop to segment (might have to slightly adjust axis) and determine same mire... difference in power is your 'Add' power... the 'add'itional plus power required to read at about 14-16".
    Some authorities suggest when distance power of bifocal is between +3.00 and -3.00sph, there is no need to flip glasses and you can simply determine 'Add' using back vertex power in both distance and near (temples of glasses away from you) as the numbers in these relatively 'thin' lenses would be the same. However, it only takes a few seconds to flip the glasses for add determination, so just flip them.

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    Technically you measure the bifocal add by flipping the frame to measure front surface powers. Measure the distance front surface power and then the near to work out the add. Try it out and you will find that when you get a high plus lens, measuring the back surface will get a higher add than measuring the front.

    Bifocal lenses are made with the add put into the front surface before the lens power is ground into the back. Therefore the add is only measured through the front during manufacture.

    However, the patient does not see either back surface power or front surface power. They see equivalent power. This means that for example in a high plus BF lens, if the add is +2.25, the patient will see with it as if the add was +2.50. This is something prescribers may not think of.

    So to check accuracy of manufacture measure the front surface. However, the back surface measurement may be closer to the equivalent power and give a closer idea of what the patient actually sees. The equivalent power cannot be measured with a vertometer.

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    Quote Originally Posted by tonyl View Post
    Technically you measure the bifocal add by flipping the frame to measure front surface powers. Measure the distance front surface power and then the near to work out the add. Try it out and you will find that when you get a high plus lens, measuring the back surface will get a higher add than measuring the front.

    Bifocal lenses are made with the add put into the front surface before the lens power is ground into the back. Therefore the add is only measured through the front during manufacture.

    However, the patient does not see either back surface power or front surface power
    . They see equivalent power. This means that for example in a high plus BF lens, if
    the add is +2.25, the patient will see with it as if the add was +2.50. This is
    something prescribers may not think of.

    So to check accuracy of manufacture measure the front surface. However, the back surface measurement may be closer to the equivalent power and give a closer idea of what the patient actually sees. The equivalent power cannot be measured with a vertometer.



    The problem arises when you get a lens of let's say +10 and add of +2.50. You have to realize one the amount of magnification and according to Gaussian thick lens formula you must also take in account the thickness of the lens. With that all said all you do on a high power plus is read front first flip over and read seg and diff. Between the two is your add power. But in answer to your question Robert is right. For the test use that answer. :bbg:

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    Thank you all for your help! I knew my fellow Optiboarders would come through for me!

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    When taking the state boards in Florida in order to pass you are required to flip the frame over and read the add from the back

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    Quote Originally Posted by Kmvhdo View Post
    When taking the state boards in Florida in order to pass you are required to flip the frame over and read the add from the back

    Really! What do they base this upon? Ralph Drew in his classic Ophthalmic Dispensing says to read upper portion of lens then down to read lower portion the add. In another classic by Russell Stimson on ophthalmic dispensing says the same so what gives?

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