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Thread: Bifocal prismatic effect

  1. #1
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    Bifocal prismatic effect

    Hi everybody!

    I am studying DO and I cannot understant the base directions of the prism due to the segment in bifocals.

    Can somebody please help me?

  2. #2
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    Hi ge2007,

    Welcome to Optiboard!

    Think of a bifocal segment, regardless of the style, as a small plus lens (prisms attached base-to-base) at the bottom of the lens. At the slight second when your eyes roll downward across the segment line, and prior to getting to the near optical center (NOC), they are looking through Base Down Prism. Since the image will be displaced toward the Apex (up), it gives the visual sensation to things 'jumping up', which is why we call it Image Jump.

    The higher the Add power and the lower the NOC placement in relation to the seg-line the greater the image jump. You can calculate it using Prentice's Rule, where the amount of image jump is equal to the add power, times the number of mm the NOC is below the seg line, divided by 10. (others here can post the actual notations in formula style, but I am old.)...

    We do not necessarily assign a base direction (it will always be BD), we simply recognize it, calculate it for the ABO and other state licensing exams, and explain to the patient/client how to learn to ignore it.

    Best Wishes for your opticianry studies, you will find a bunch of helpful 'O's' here.

    : )

    Laurie
    Ophthalmic Optician, Society to Advance Opticianry

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    Thank you Laurie!
    I just hoped there was a specific rule to help me with the exercises... My problem with the prisms is DOWN or UP, In or OUT?!?!?

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    As Laurie said, the prism direction in the seg will always be base down. Different seg types have different OC placements relative to the line. A FT28 normally has a 3mm difference. A FT35 normally has the OC on the seg line. A Round 22 mm seg has an 11 mm difference.

    Apply Prentice's rule: Power x decentration/10. Say you have a +2.50 add in a FT28. You have 2.5 x 3/10. You have 0.75 prism diopters BD.

    Assume you have a +2.50 add in a round seg. You have 2.5 x 11/10 or 2.75 prism diopters BD.

    Hope this helps.

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    I may be disagreeing with others here. But in glass flat tops have base up prism, round segments have base down.
    In plastic straight tops have base up. Probably round ones have base down, at least above center of seg.
    If you ever find one a glass panafocal has no prism.

    Chip

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    Consider a 28mm round lens made of high index material. Cut the lens off a few mm above the widest part of the lens.
    (The seg line on the FT28 lens I checked appeared to be about 4mm above). You have a FT28 mm seg. Since the thickest part of the lens is at the center of the lens and you are looking above that point, you have BD prism.

    I gave it the eyeball test and sure enough, the image was deviated up, indicating BD prism.

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    Still disagreeing: A prism bends light towards it's base. It's base is the thickest part of the prism.

    Chip

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    Master OptiBoarder optical24/7's Avatar
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    Yes Chip, prism bends light towards it's base but displaces images towards it's apex, always.

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    Quote Originally Posted by chip anderson View Post
    Still disagreeing: A prism bends light towards it's base. It's base is the thickest part of the prism.

    Chip
    Okay Chip, picture that 28mm round lens from the side. You have essentially two prisms base to base. Cut off the top of the lens 4mm above the thickest part. As soon as you cross the upper line, you are looking through a base down prism, albeit one with the top part cut off. Hence you have base down prism and the image is deviated up.

    Pull a SF blank with a +3.00 add and check it out for yourself. Look at a straight line such as the edge of a desk. You will see the image clearly deviated upward as you cross the line. You have to drop down several mm to the OC where the line within the seg will align with the line outside the seg.
    Last edited by gmc; 02-17-2012 at 06:02 PM.

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    ABOM Wes's Avatar
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    Chip, straight tops have base down above the center (or oc) of the seg as well. Below the center of the seg is the base up prism. Pretty much like every other plus lens. If the seg line were at the midpoint of the seg, you would be right.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    Master OptiBoarder optical24/7's Avatar
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    Quote Originally Posted by Wes View Post
    Chip, straight tops have base down above the center (or oc) of the seg as well. Below the center of the seg is the base up prism. Pretty much like every other plus lens. If the seg line were at the midpoint of the seg, you would be right.
    Like an exec bifocal.

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    i am going to have to agree with chip the image is deviated up because the seg has base up prism. the base of prism is it's thickest side so unles you turn it upside down how could the prism be base down?

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    i don't think you have to worry about the bifocal so much as the optical center. prism is used to move objects to the oc of the lens. you want your prism out for an eye that goes toward the temple, base in for an eye that goes towards the nasal. up and down prisms are mainly used to avoid vertical imbalances. just remember the bigger the frame the more prism it will take to reach your optical center so i strongly suggest using the smallest frame possible for those needing prism it adds a lot of weight and thickness to their lenses. matter of fact prism is ground in the lens to even have an oc. base in for hypers' base out for myops'. it is calculated by the power, frame measurements and the patients pd. hope this helps.

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    ABOM Wes's Avatar
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    Sorry junebug, but these are two of the most incorrect posts I've ever seen on OB.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    Master OptiBoarder optical24/7's Avatar
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    junebug, read post #8 again.

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    I conceed that many of you seem to have a better understanding of this than I. But I do remember from my early "education" in optics that I was taught to use a straight top to off-set the base down in the lower half of minus lenses and to use round top segs to off-set the base up prism in the lower half of plus lenses.
    What's been said so far in this seems to contradict this.
    Monday I intend to go to the lab and check a bunch of different style distance plano bifocals and git myself straightened out.

    Chip

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    If you think of the seg as a plus lens by itself, it makes more sense. A plus lens can be thought of as two prisms base to base, right? In the portion below the oc of the seg, there is base up prism. In the portion above the oc of the seg, there is base down prism, again, like every other plus lens in existence.

    And again, prisms bend light in the direction of the base, but displace the image in the direction of the apex.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    i don't think u understand the question! the student is having difficulty understanding which direction to place his prism. how can u disagree with the soul purpose of prescribed prism being to place objects to an oc of a lens. the direction depends on where the object is displaced? i think most of these posts has probably confused him more.
    he'd be better off asking someone else.

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    there is no such thing as base to base prism. prism has a base it's thicker side and an apex it's thinnest side without either you do not have prism.

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    Junebug, the confusion being generated on this thread is coming from you. If this is a trolling attempt you succeeded.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    just because u say so where is the facts that prove me wrong. u're arrogance is no substitute for facts. how do you determine which direction to prescribe prism?

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    (Counts to 10 slowly) Grabbing popcorn!

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    Quote Originally Posted by junebug72 View Post
    just because u say so where is the facts that prove me wrong. u're arrogance is no substitute for facts. how do you determine which direction to prescribe prism?
    This attitude is not acceptable on OptiBoard. I suggest you rethink your participation in this thread and possibly the forum.


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    Reminder to self: never argue with idiots; they drag you down to their level and beat you with experience.
    Wesley S. Scott, MBA, MIS, ABOM, NCLE-AC, LDO - SC & GA

    “As our circle of knowledge expands, so does the circumference of darkness surrounding it.” -Albert Einstein

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    Bad address email on file RetroRat's Avatar
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    Quote Originally Posted by ge2007 View Post
    Hi everybody!

    I am studying DO and I cannot understant the base directions of the prism due to the segment in bifocals.

    Can somebody please help me?

    Hopefully this will help in your study. Good luck.

    Click image for larger version. 

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