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Thread: Needing help with prisms

  1. #1
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    Needing help with prisms

    Hi guys,

    Im still having trouble with this question, can anyone help?

    6) A prescription reads

    RE 5 base up
    LE 9 base out

    I need to equally divide the prism between the two eyes.
    It seems simple, but I just haven't clicked yet.

  2. #2
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    Also two others!

    14) Calculate the decentration required to obtain the desired amount of prism in the prescription LE +5.50/-1.50x60 4 base down and 3 base out

    15) Calculate the decentration required to obtain the desired amount of prism in the prescription:

    RE +4.00/-1.00x180

    2 base in and 3 base up.


    Thanks so much guys, where would I be without you!!

  3. #3
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Whitwoo View Post
    Hi guys,

    Im still having trouble with this question, can anyone help?

    6) A prescription reads

    RE 5 base up
    LE 9 base out

    I need to equally divide the prism between the two eyes.
    It seems simple, but I just haven't clicked yet.
    OD: 2.5 up 4.5 out
    OS: 2.5 dwn 4.5 out

    -or-

    OD: 5.15 @ 151
    OS: 5.15 @ 029

    Think of it like this 5 base up in the right eye means that the right eye shoudl be up by 5 prism diopters so you could put it all in the right eye 5 up, or all in the left 5 dwn or split between the two 2.5 up in the right and 2.5 dwn in the left. The goal is that the right eye should be up by 5 diopters. The base out prism is different because out for the right eye and out for the left eye compound unlike the vertical prism, you'll also notice that this is the case with the axis, 90 is always up and 270 is always down, but depending on the eye the 0 and 180 can be n different directions so the base out prism would be split in half and both eyes would be out.


    Also two others!

    14) Calculate the decentration required to obtain the desired amount of prism in the prescription LE +5.50/-1.50x60 4 base down and 3 base out

    15) Calculate the decentration required to obtain the desired amount of prism in the prescription:

    RE +4.00/-1.00x180

    2 base in and 3 base up.


    Thanks so much guys, where would I be without you!!
    Decentration for

    OS: +5.50 -1.50 x 060 4 dwn 3 out, we could go into a crazy amount of detail and get a more accurate measure but here's more than likely what your lookign for find the power in both the vertical (090) and horizontal (180) meridians.

    D(090) = +5.50 + (-1.50)sin^2(90-060)
    D(090) = +5.12

    D(180) = +5.50 + (-1.50)sin^2(180-60)
    D(180) = +4.37

    so from here we need to know how much decentration to get 4 dwn

    4 = +5.15 * dec / 10
    dec = 7.8mm

    Base dwn in a (+) lens is created by moving the lens up so decentered 7.8mm up.

    we need 3 out

    3 = +4.37 * dec / 10
    dec = 6.9mm

    to get out in the left eye you need to decenter the lens -6.9mm

    The second question is just like the one outlined here but it's easier since the powers are on the 180 and 90 without need for the oblique power formula so you can skip the first step post your answer if you get stuck.
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  4. #4
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    The biggest problem seems to be the lack of clarity between prism and prism IMBALANCE. Imbalances are what you really need to understand when learning about prism. Always keep in mind that prism displaces light towards its base, which displaces images towards the apex.
    ALWAYS remember that prism displaces images towards the apex.
    If someone is wearing 5D base up in the left eye, the image is displaced down, towards the apex. If the wearer has no prism in the right eye, the imbalance is 5 diopters. If they have 3 base up in the right eye, the image in the left eye is displaced down towards the apex, and the image in the right eye is ALSO displaced down, but a lesser amount. the imbalance is 2 prism diopters. Now make the prism in the right eye 3 diopters base down, the image in the left eye is displaced down 5 diopters, and the image in the right eye is displaced UP 3 diopters... the imbalance is now 8 diopters.
    I hope this helps.

  5. #5
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    Quote Originally Posted by Whitwoo View Post
    Hi guys,

    Im still having trouble with this question, can anyone help?

    6) A prescription reads

    RE 5 base up
    LE 9 base out

    I need to equally divide the prism between the two eyes.
    It seems simple, but I just haven't clicked yet.
    Don't mean to state the obvious, but have you memorized the directions of prism that compound vs cancel? That's what seems to be the key here for me (as anyone past first grade should be able to divide a single digit by two).

  6. #6
    Rising Star
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    seems to me that the axis for the resultant prism would be the same in both eyes @151 instead of 151-R and 29-L as was posted earlier

  7. #7
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    Quote Originally Posted by Kevin Howtopat View Post
    seems to me that the axis for the resultant prism would be the same in both eyes @151 instead of 151-R and 29-L as was posted earlier
    I agree, although you still need to give the vertical base direction (down at 151) or use 331. However, the compounding prism calculator at opticampus http://www.opticampus.com/tools/compounding.php spits out 5.15 at 29. Is this an error in the program? I don't see how base down-and-out for the left eye can be between 0 and 90.
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  8. #8
    ATO Member HarryChiling's Avatar
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    I did use Darryl's calculator to compound it, your right it should read 331 degrees on the 360 scale or 151 down using 180 notation I think the program forgot the negative sign on the -29 degrees which would have made it correct, then some logic could have came up with the correct answer. I am sure Darryl will see this post and correct that.

    Nice catch Kevin.
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  9. #9
    Rising Star
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    Thanks, that just gave me a warm fuzzy feeling

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