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Thread: Dioptric power vs. magnification power

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  1. #1
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    Dioptric power vs. magnification power

    Does anyone know how to convert diopters to magnification power? I mean, if I have a 4x magnifying glass, how many diopters is that? Or if I have a 30 diopter plus lens, what magnification power is it?

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    For simplicity's sake, it's times 4.

    4x magnifier = 16 D
    30 D lens = 7.5x

    (If I remember right, it's not exactly 4, but it's the easiest way to calculate it quickly.)
    Last edited by mlm; 02-26-2008 at 01:01 AM. Reason: more explanation

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    Doh! braheem24's Avatar
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    From what I remember there are 2 standards and Asian and a German standard both of which have different conversions but both fall in the 4-5D range.

    Good Luck

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    For Low Vision

    Thanks Guy's I was wondering this myself, excellent question!;)

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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    The Scientific conversion: magnification reference (comparative) distance is 25cm (10 inches). 4D = 1X magnification.

    This means that a lens that permits an object to be in focus at 10 inches away yields an approximately-equivalent retinal image size to an un-aided eye that focuses at the same distance.

    If there reference distance is changed, say to 8 inches (approx 20 cm), then 5D = 1X magnification

    Then, add in marketing by magnifier companies that couldn't explain or find persuasive the concept that a 4D lens only yields 1x mag, and they decided to create the formula:

    Magnication (of a simple maginifier) = Dioptric power of lens/ 4 "+" *1* = magnifying power.

    In this example, a 4D lens = 2X.

    This is all for "linear" magnification. Angular magnification is something else.

    FWIW

    Barry

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    Quote Originally Posted by Barry Santini View Post
    The Scientific conversion: magnification reference (comparative) distance is 25cm (10 inches). 4D = 1X magnification.

    This means that a lens that permits an object to be in focus at 10 inches away yields an approximately-equivalent retinal image size to an un-aided eye that focuses at the same distance.

    If there reference distance is changed, say to 8 inches (approx 20 cm), then 5D = 1X magnification

    Then, add in marketing by magnifier companies that couldn't explain or find persuasive the concept that a 4D lens only yields 1x mag, and they decided to create the formula:

    Magnication (of a simple maginifier) = Dioptric power of lens/ 4 "+" *1* = magnifying power.

    In this example, a 4D lens = 2X.
    Barry
    This IS actually one of the few cases where it´s not (only) marketing, but there is some true optics behind it.;)

    The first formula applies to the case, where the eye itself is focussed to infinity. The "+1" enters for the more realistic case, where the eye is already focused to 25cm and you use of the magnifier, meaning you also need to move the object a bit nearer to the lens (In that latter case, actually the distance eye-magnifier lens enters, the "+1" is an approximation for the limiting case where the magnifier lens is straight in front of the eye).

    小卫

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    Confused Magnifying Power Questions

    I am so glad that I found help help here. There is a study guide out there that says to find the dioptric power of a magnifying glass that one should multiply the "x" power by 2.5, however several sources say to multiply by 4, so thank you all.

    I do have a few questions that I am certain can be answered here.

    1. If an image in a mag lens is clearer at 5cm from the page, what is the mag power of the lens?

    2. If a patient has a 30D hand mag, what distance should they hold it from the page to get the clearest image?

    Thanks once again

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    What's up? drk's Avatar
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    Eliminating accommodation effects and assuming magnifier held at focal plane:
    a. 1/5cm = 20 D
    F/4 +1 = 6X ("conventional magnification") or F/4 = 5x ("relative magnification")

    b. 1/30 D = 3.3 cm
    Last edited by drk; 04-25-2008 at 01:14 PM.

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    Brief comment on the statement by Dr. Santini: This is all for "linear" magnification. Angular magnification is something else."
    Actually the magnification discussed is angular magnification. That is what the patient is interested in.
    Everything I see on this thread correctly points out that the formula assumes a near point (NP) of 25 cm. For an older patient, typically with a larger NP, the generalization is angular magnification = NP x diopter + 1.
    The equations for this, assuming a thin lens, are in many references on the web, e.g. http://www.phy-astr.gsu.edu/cymbalyuk/Lecture4.pdf (I have no association with this university, this was just the first reference that popped up on google.) There is a small typo in the formula for magnification, where the symbol 'q' should be 'f' but everything else is OK. Picture are nice.

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    Master OptiBoarder Darryl Meister's Avatar
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    Quote Originally Posted by heritage972
    Everything I see on this thread correctly points out that the formula assumes a near point (NP) of 25 cm. For an older patient, typically with a larger NP, the generalization is angular magnification = NP x diopter + 1
    Just keep in mind that the formula for conventional or maximum magnification, F / 4 + 1, assumes that the image vergence leaving the magnifier will be equal to the 25 cm reference distance. So I believe that the patient will need to either wear bifocals with close to a +4.00 D add power or make up the difference with ocular accommodation (if available).

    The attached article provides some of the specific mathematics involved:

    Best regards,
    Darryl
    Attached Thumbnails Attached Thumbnails Magnification of Simple Low Vision Aids.pdf  
    Darryl J. Meister, ABOM

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