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Thread: Taking Abo in A couple days in san diego need little help with the prentice formula

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    Exclamation Taking Abo in A couple days in san diego need little help with the prentice formula

    im taking the abo on wensday was looking for any help with understandeing the prentice formula (P = hcm * D), if i can get assistants or a breif explination-- i would be greatful

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    Welcome, M.L....to the forum, and best of luck for the Abo...

    Prentice's Rule: Prism Diopters= c times F

    The amount of displacement(expressed in diopters of displacement)= (c)distance from the optical center in centimeters (x)times (F) the dioptric power of the lens.


    Need an example?
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    yes...in the test they give you a simple rx and two pds. we are supposed to compute and give the answer im totall prism and in what direction
    Last edited by M.L91911; 05-20-2013 at 03:33 PM.

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    Ok...lets start: First, direction: If the Rx is plus, the base of the prism is towards the centre of the lens, and conversely, if the lens is a minus lens, the base direction wil be......that's right towards the margin of the lens. Never forget to state direction.

    If anyone is in this forum, and wants to add, or correct, please. jump in!
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    With me so far, ML?

    So for simplicity sake....let's say that the right lens is a +2.00 diopter lens, (ignore the left Rx for now) working on a lens at a time.......

    If the example given is a IPD(PD) of 70mm in the glasses, and the patient's IPD binocular is actually 58 mm, or a error of 6 mm for the right lens(too wide).... then:

    the induced prism will be: Prism = 0.6(in cm) x 2.00 = 1.2 D for the right lens. Follow the same method for the left lens power given........and add the results of both calculations.
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    What direction is the prism????
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    OK...a hint. a plus lens is also considered to be a group of prisms placed base to base, and in a decentred lens ^ will correspond to direction of lens decentration. A minus lens can also be considered to be a series of prisms where the apex(s) or apices are together at the centre.
    Last edited by uncut; 05-20-2013 at 04:38 PM.
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    im with you..just need a lil more explination on the math on how is the best way to write out to remeber..

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    I usually remember by the phrase...PRISM CONFOUNDS, ^ = cF
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    ok so how would u work the problem with both eyes

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    Quote Originally Posted by M.L91911 View Post
    ok so how would u work the problem with both eyes
    You would usually calculate the result for the individual eye's position........and add the results together, for a combined effect. The questions will typically give you the measured, or resultant IPD, and also the patient's actual IPD, or where the IPD was supposed to be.
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    im understanding alot more thanks very much uncut im taking my test later this afternoon, i appreciate all your help

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    You are welcome, M.L91911.......and the very best of wishes for you this afternoon!

    Let us in on the results, please!
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    i will ...they will b givin me more information than i need on the questions on the prentice formulas..they give me a cyl, axis, fpd, pd,all the extra info confuses me..whATS THE quickest way to eliminate what i dont need

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    Quote Originally Posted by M.L91911 View Post
    i will ...they will b givin me more information than i need on the questions on the prentice formulas..they give me a cyl, axis, fpd, pd,all the extra info confuses me..whATS THE quickest way to eliminate what i dont need
    Visualize the glasses. Calculate the power along the meridian in question(vertical, horzontal, or axial). WRITE it down, too
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