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Thread: Effective Power error in book...

  1. #1
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    Effective Power error in book...

    Hey guys,
    I wanted to put this to you guys. I have done a little bit of research, but I wanted to know what you thought. There is a discrepancy between the effective power formula in a few books:

    In "System for Ophthalmic Dispensing" the formula reads:


    In "Clinical Optics", the formula reads:


    However, in "Optical Formulas Tutorial", it reads:


    note: I've changed all of the variables to the same symbols to avoid confusion

    Now, I know that the Clinical Optics version is the same as System (just a simplified version). But Something is very obviously wrong because even after every derivation I can think of applying to the top two equations, you cannot get the formula from Optical Formulas Tutorial. I sure it's a typo or just an error in simplification. It's interesting considering this is a text that is so highly recommended. Just goes to show you that you can't take everything you read as accurate.

  2. #2
    Master OptiBoarder lensgrinder's Avatar
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    I do not have all texts in front of me, but d is negative or positive depending on the new position. For example, if the vertex distance is 14 and the new position is 16 d is negative.

    I believe in "...Formulas" it has two one for compensated and one for effective, you might be looking at the one for compensated.

  3. #3
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    Quote Originally Posted by lensgrinder View Post
    I do not have all texts in front of me, but d is negative or positive depending on the new position. For example, if the vertex distance is 14 and the new position is 16 d is negative.

    I believe in "...Formulas" it has two one for compensated and one for effective, you might be looking at the one for compensated.
    I went back to the books after reading your reply and found that you were right I hadn't been as thorough in reading the passages. However, I was reading the correct equations. It seems that they are a little misleading with terminology, though. If you look at both System and Clinical, they both list the formulas as I've done above for effective power, but then they define the term. Clinical Optics says: "... it's major use is to indicate the change in lens power required if a lens in moved from one position to another in front of the eye." and System for Ophthalmic Dispensing says: "The vergence power a lens produces at a position other than that occupied by the lens itself is known as the effective power..." Neither lists a formula for compensated (because these formulas are both technically the "compensated power" formulas). Optical Formulas Tutorial never actually defines effective power in a very straight-forward way (ie: in a sentence or two). But, after reading over their "definition", I'm inclined, ironically, to agree more with them. I guess it does make more sense to have two formulas for effective and compensated powers. This is why we should push for more consistent information across the board... By the way, thanks lensgrinder for getting me back to the book to look a bit closer.:shiner:

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    Master OptiBoarder lensgrinder's Avatar
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    In "Optical Formulas Tutorial" it tries to make the wording easier to understand. Where it calls the formula the compensated formula, "Clinical Optics" and "System ..." call it effective power.
    I think the term effective can take on different meanings from text to text.
    The formulas are changing the power of the lens to focus light on the retina.

  5. #5
    Master OptiBoarder Darryl Meister's Avatar
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    As you noted, the formulas are essentially the same. Multiply the top and bottom of the System equation by FV / FV and you will get the Clinical Optics version.

    The difference between these two equations and the bottom Tutorial equation is just the sign convention for the change in distance or displacement d. The movement must be measured from the opposite direction with the Tutorial equation, so that the value of d would be equal to -d in the other equations.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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