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Thread: Which formula to use?

  1. #26
    Master OptiBoarder Darryl Meister's Avatar
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    I fixed you formula with Parenthesis
    The order of operations in mathematics requires that you multiply before adding, so the parentheses aren't necessary. ;)

    Best regards,
    Darryl

  2. #27
    Master OptiBoarder Darryl Meister's Avatar
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    Also Does ABO use only 1995 ANSI STANDARDS?
    Unfortunately, I don't even know that the ABO could tell you for certain which revision of the standard they are using...

    Best regards,
    Darryl

  3. #28
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    Usually, you would compensate it for thickness, refractive index, and the initial front surface power. The formula would look something like this:

    Fc = F1 / (1 + t/n * F1)

    Where Fc is the compensated front surface power, t is the center thickness in meters, and F1 is the nominal -- or initial -- front surface power.
    Hi darryl,
    Am i correct that the above formula is the effective power of the front surface at the back surface?
    If i am trying to determine necessary back surface powers and i am given the front surface could i use:-

    Fc=F1/(1-t/n *F1) and then subtract the powers i need in each meridian from this


    as i asssume the effective power increases with thickness.

    Regards,
    Rick

  4. #29
    Master OptiBoarder Darryl Meister's Avatar
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    Am i correct that the above formula is the effective power of the front surface at the back surface?
    It's actually correcting for (or neutralizing) the effective power at the back surface. The effective power at the back surface is given by your other formula:

    F1 = F1 / (1 - t/n * F1)

    So, given a lens with a +8.00 D front curve, 1.500 refractive index, and 6 mm (0.006 m) center thickness, your effective power at the back surface becomes:

    F1 = 8.00 / (1 - 0.006/1.500 * 8)
    F1 = +8.26 D

    Note that the extra +0.26 D of plus power is due to the gain in power caused by the thickness and form of the lens. You would simply add the back surface power to the effective power of the front surface in order to determine the actual back vertex power of the lens (that is, the power you would measure with a lensometer or focimeter).

    So, if the lens has a back surface of -4.00 D, the back vertex power Fv becomes:

    Fv = 8.26 + (-4.00)
    Fv = +4.26 D

    Now, the compensated front surface power for that nominal +8.00 D front curve is:

    F1 = F1 / (1 + t/n * F1)

    F1 = 8.00 / (1 + 0.006/1.500 * 8)
    F1 = +7.75 D

    This formula basically reduces the front surface power enough to make the actual effective power of the front surface equal to the original uncompensated -- or nominal -- power (i.e., +8.00 D) after that gain in power caused by the thickness and form of the lens.

    What this basically means is that if you made a lens blank with a +7.75 D front curve, but called it a +8.00 D front curve, you could use a simpler formula (i.e., the Lensmaker's or thin lens formula) to determine the back curve instead of the more complicated back vertex power formula:

    F2 = Fv - F1comp

    So, say you needed to produce a +4.00 D sph power. You would just subtract the compensated front curve from this value instead of using a complex formula:

    F2 = 4.00 - 8.00
    F2 = -4.00 D

    Now, assuming that your lens would have roughly a 6-mm center thickness, you can verify that your actual vertex power will equal the desired power with that -4.00 D back curve using the back vertex power formula:

    Fv = F1 / (1 - t/n * F1) + F2

    Fv = 7.75 / (1 - 0.006/1.500 * 7.75) + (-4.00)
    Fv = 8.00 + (-4.00)
    Fv = +4.00 D

    So, in essence, use of a "compensated" front curve was simply to reduce the math involved. By changing the front surface slightly, the manufacturer could save you the trouble of figuring this stuff by hand back before the days of computer software and cheap calculators. This used to be done in the past quite a bit to simplify surfacing calcuations, but it isn't really done anymore. Consequently, you will rarely, if ever, do this sort of thing in practice, but understanding the principle will certainly help you develop a more solid intuition in ophthalmic optics.

    Best regards,
    Darryl

    [Had to edit a positive/negative sign typo in my last formula]
    Last edited by Darryl Meister; 11-11-2003 at 06:43 PM.

  5. #30
    OptiBoardaholic OdTech's Avatar
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    Exclamation Brooks and Borish got the ABO questions

    Hello guys i can't help myself but notice one thing from my review questions and the one that were sent to me by one of the optiboarders for which THANKS SO MUCH!!!
    Well to the point ALL or MOST of questions are in one book which i assume you all have as reference and troubleshooting guide
    "SYSTEM OF OPHTHALMIC DISPENSING" 3rd edition, by BROOKS and BORISH.

    After each chapter there are exercise questions and believe it these are exact questions i've got from my review questions
    at least 3 to 4 questions per topic.

    NOte: To all neophytes and novices taking the ABO or being employed, know the questions as a back of your hand and you won't regret at all.
    :drop: :idea: :cheers:

  6. #31
    Master OptiBoarder Darryl Meister's Avatar
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    Yes, the vast majority of information you will need for the ABO exam is in System for Ophthalmic Dispensing. Many (if not most) dispensing programs at various schools of opticianry and optometry use this book, and every career optician should own a copy, in my opinion. The lastest edition I am aware of is the second edition, and it adds a great deal of information over the first edition. Butterworth-Heinemann publishes this book, but you could also order it from the OAA at one point under the title Professional Dispensing for Opticianry. I have yet to find a more accurate, well-written, or thorough source of information on ophthalmic dispensing.

    Best regards,
    Darryl

  7. #32
    Master OptiBoarder OptiBoard Gold Supporter Judy Canty's Avatar
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    Pure Optics

    Has anyone read fellow OptiBoarder Phernell Walker's newly published Pure Optics ?
    Last edited by Judy Canty; 11-13-2003 at 10:25 AM.

  8. #33
    OptiBoardaholic OdTech's Avatar
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    Hello Darryl
    My questions is Does astigmatism has an axis? for this particualr kind of example

    The refractive anomaly indicated by the Rx +1.75-2.75x90 is

    a. Compound hyperopic astigmatism
    b. Compund myopic
    c. Simple myopic
    d. Mixed

    The answer with out transposing is "D"
    The way i came up with the answer is I know that mixed astg is in Rx "+" "-" signs; but not according to the axis shown.

    Do all of this a - d astgms have any axis?

  9. #34
    Master OptiBoarder Darryl Meister's Avatar
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    My questions is Does astigmatism has an axis
    Yes. Astigmatism is essentially caused by a surface with a curvature in one direction (or meridian) that differs from the curvature in another direction (or meridian). For instance, a basketball would be spherical in shape, since the curvature is the same in every direction. An eye with a "spherical" (at least centrally) cornea would not have astigmatism.

    A football, on the other hand, is an example of an astigmatic (or toric) surface. It is flatter along its length (one meridian) and steeper/rounder along its diameter (the other meridian). A cornea with this kind of shape would have astigmatism, which would be equal to the difference in curvature between these two meridians.

    Now, since a surface with astigmatism has meridians that vary in power, rotating a lens with astigmatism will rotate these different meridians. This means that lenses or surfaces with astigmatism have an orientation. Consequently you need to specify an axis (or angle) in order to describe how far the lens has been -- or needs to be -- rotated. With spectacle prescriptions, the axis corresponds to the sphere meridian of the lens.

    If you rotate a spherical lens, on the other hand, it makes no difference since all of the meridians have the same power. Consequently, a spherical lens does not really have an orientation (at least until you start adding prism and such).

    Back to the football-basketball analogy... A basketball will still look like it is in the same position after you spin (or rotate) it, while a football lying on its side will obviously look like it is in a different position after you spin it, because it has an orientation.

    As far as your problem goes, you really need to understand optical crosses in order to answer this one. The axis of the prescription has nothing to do with the powers or signs of the meridians, just their orientation.

    With this prescription,

    +1.75 -2.75 (the axis is unimportant)

    One meridian (the sphere meridian) is equal to +1.75 (hyperopic), while the other meridian (the cylinder meridian) is equal to +1.75 + (-2.75) = -1.00 (myopic). Since one meridian is positive and the other negative, it indicates mixed astigmatism.

    Had both meridians been positive, you would have compound hyperopic astigmatism, and vice versa for two negative meridians. If one meridian is positive and the other plano (such as +1.00 -1.00, which is also Pl +1.00), you would have simple hyperopic astigmatism, and vice versa for negative and plano meridians.

    Best regards,
    Darryl

  10. #35
    OptiBoardaholic OdTech's Avatar
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    Hello i wonder does moderate to high amounts of cylinder could present such symptoms as dizziness and headache, blurry vision.
    Sphere isn't strong but the cyl is

    Also when the customer comes in with rx
    requesting to wear a bifocal lens
    -3 sph
    -1 sph
    addd +3.75
    St25 lens

    Doesn the optician tell the customer
    A. The benefits and how to adapt to the lens
    B. Tell the customer about the posssibility of vertical imbalance.

    Additionally

    An individual plays Piano wears bifocal lens but only thing is wrong is reading music notes has a little problems

    Do you give

    A. Intermediate single vision
    B. Raise the bifocal
    C. Distance single vision
    D. Near Single Vision

    My theoretical approach woulld say Raise bifocal for sure since he comfortable playing the piano looking through the segs if i raise the segs he will be able to see the music notes.

    Please correctl me if am wrong

    Cosnequently a customer comes in and tell you " Distance objects are large and very round than near"

    I consider the problesm as The Dr gave lots of cylinder.(my guess)

  11. #36
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    Sean:

    Been there done that with a lot of piano/organ players. The "best solution" get patient to sit at his piano, as though he were playing same. Have someone else measure the distance from the eye to the music. Make glasses (probably SV) but possiblly a bifocal, that focus exactly that length.

    Chip

  12. #37
    OptiBoardaholic OdTech's Avatar
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    Hello Does FDA states certain amount of years a practitioner should keep the patients Rx let say

    2, 3,7 years?

  13. #38
    Master OptiBoarder Darryl Meister's Avatar
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    I'd have to double check to be certain, but I leave you are required to keep medical records for seven years by the FDA.

    Best regards,
    Darryl

  14. #39
    OptiBoardaholic OdTech's Avatar
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    Interesting, are you implying that Rx is considered as a medical document of some sort, although its just some numbers and MD reccomendations.

  15. #40
    Master OptiBoarder Darryl Meister's Avatar
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    Interesting, are you implying that Rx is considered as a medical document of some sort, although its just some numbers and MD reccomendations
    Yes, the Rx is a medical document. But it's not really a copy of the Rx that's important, but rather the examination record, itself. The prescription is just one component it. Seven years came to mind, but I am not certain of that figure, and it could very well depend upon individual state requirements, too.

    Also, on a related note, the FDA requires that records be kept by the seller documenting the impact resistance of prescription eyewear for three years, as well.

    Best regards,
    Darryl

  16. #41
    Master OptiBoarder Shwing's Avatar
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    I may be wrong (I've been know to be wrong) but up here (on most maps we're 'up') the record must be kept for 7 years. I also believe this is modelled after the US.

    Reason? Taxes, of all things. You must be able to produce documentation related to your taxes to Revenue Canada/ IRS in case of audit, going back 7 years. Therefore, as many people will write off their specs, you would need to keep the receipts. And of course it goes the other way, in that the prescriber would need to maintain the records for the very same reason.

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