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Thread: Bifocal prismatic effect

  1. #26
    Bad address email on file RetroRat's Avatar
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    Apologies for the dodgy paint image, gotta leave for work in 7 minutes! (Gotta down this coffee in 6!!!)

  2. #27
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    Quote Originally Posted by gmc View Post
    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.
    Always base down? Not quite accurate.

    If any type of segment has its optical centre on the seg line, then as soon as your eye crosses that line to look near, your eye will experience a Base Up effect from this style of seg. As in an Executive segment or a very large FT with OC on seg line.

    To determine base direction, always consider 'where is the base in relation to the eye', and Not 'where is the eye in relation to the base.

    Remember it's only the eye that moves... up or down (or in or out), while the distance lens (and its bifocal segment) stays stationary, locked in position in the frame.

  3. #28
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    Click image for larger version. 

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    Please forgive the crude drawings.

    The first represents a round seg. The horizontal line represents the near OC. If you are looking through the seg above that point, you have base down prism and the image you see is deviated up. Since the prismatic effect is a function of power and decentration, this seg will have the most jump at the top of the seg. As your eye looks deeper into the seg, the prismatic effect lessens.

    The second image represents a flat top seg with the near OC on the seg line like a FT35. There is no prismatic effect at the seg line. As your eye drops deeper into the seg, you have a base up prismatic effect.

    The third image represents a flat top seg with the near OC below the seg line like a FT28. At the seg line and just below you have a base down prismatic effect. As your eye drops deeper into the seg, you have no prismatic effect at the near OC. If your eye drops below the near OC, you have a base up prismatic effect.

    As Wes has said, the seg can be considered as an individual lens. The same rules for prism apply in the seg as they do in a normal lens.

    tmorse is also correct. If the near OC is on the seg line, no prismatic effect is present there and there is BU prism below the seg line. Thanks for the clarification.

  4. #29
    Optimentor Diane's Avatar
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    Gary,

    I was doing a crude drawing at the same time as you. I won't go into detail like you, just post my drawing.
    Click image for larger version. 

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    Diane
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  5. #30
    Doh! braheem24's Avatar
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    Posters arguing with the ABOMs are assuming the original plus lens was cut in half.

    A FT28 is deeper then 28MM which means it is cut above the OC, Diane's drawing shows it nicely.

    Shallower then the width = BU
    Deeper then the width = BD

  6. #31
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    What no one has mentioned yet is that the base direction of a segment’s prism depends on the ‘reading level’(or how far the eye drops down into the seg from straight ahead distance gaze)
    in order to read. All segments are ‘plus’ powered, and a seg line can be manufactured to cut at the optical centre of seg (Executive) or higher (as in FT28), or even have no seg line at all as in a round top, but the ‘reading level’... the distance that the eye moves downwards to read will indicate the ultimate base direction of the seg’s prism.

    Say a FT28 has a seg OC 4mm below its seg line... and a person wants to read a book. And say the distance OC is also 4mm, but above the seg line. When the patient drops down only 1mm from straight ahead, he can’t read (hasn’t reached the seg yet), and a similar effect exists when he drops down 2 and 3mm from straight ahead. But when patients drops 4mm down, his eye finally hits the FT28 line and he can begin to read. But he is slightly confused by the seeing both distance and near at the same time at this line, so he drops his eye another 1mm further into the seg. Now he experiences a combined prismatic effect of distance (down 5mm) and, at the same time, he is now located 3mm from the FT28 OC of this seg’s plus power. Think of the seg as a separate very small chopped ‘plus’ lens glued to the distance lens to create this FT28 bifocal.

    Now forget about whatever distance prism this 5mm produces, since we are discussing only seg prism direction. So after dropping say 5mm down to read, his eye’s line of sight is now located 3mm above the OC of this FT 28 bifocal, and this produces a BD prism effect.

    But no one says you can read only at 5mm down. Your reading level can be changed at will... you can drop your eyes all the way down to the bottom of the seg lens if you like, and still be able to read. But as you lower your reading level beyond 5mm, your eye’s line of sight is getting closer and closer to the seg’s OC in this FT28. If you drop your eye to a new reading level of 8mm, you eye will be looking exactly through the FT28's OC, and no seg prism effect will be produced at all. And if you keep going down and drop even lower than 8mm reading level, you eye will be looking below the FT 28 OC and now experiencing a BU effect...the base direction has reversed!! As you read lower yet and you will experience a larger and larger BU effect, until your reading level finally reaches the bottom of the FT28 seg.

    But an Executive bifocal with OC on seg line... as soon as your reading level drops to say 5mm, your eye’s line of sight is now 1mm below this Executive OC and you will experience a BU seg prismatic effect. And a larger reading level than 5mm will place your eye even further from the Executive’s OC (located overhead), and produce a corresponding larger BU effect.

    But you are reading quite a distance down from your distance OC, and in a bifocal that will produce a combined distance and near prismatic effect (assuming your have distance power in the 90 meridian) with the amount of seg prism actually produced at your chosen reading level.
    Most textbook authorities suggest a reading level of 8-10mm for bifocals, but that ‘reading level’ distance is not cut in stone. This combined prismatic effect can be left for another thread.

  7. #32
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    my apologies i certainly over reacted. surely i couldn't be that wrong. but sense i'm not sure myself anymore i will humbly leave this one alone. good luck to all!
    sincerely,
    junebug

  8. #33
    Bad address email on file fvc2020's Avatar
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    lovely pic

  9. #34
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    Quote Originally Posted by junebug72 View Post
    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?
    Because the OC of the seg is below the segment line, not at it. So until you hit the seg center, you have about 4-5 mm where the prism is BD.

  10. #35
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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,

    Draw a plus lens (side view) and now a horizontal line straight through its optical center... no deviation seen.

    Now draw another horizontal straight line 10mm above and parallel to this first optical center line and label on it Object “A”. This ray goes through the plus lens 10mm above the optical center and it does deviate towards the base.

    Now draw your eye behind this lens and look at Object “A”... looking through the lens 10mm above the optical center produces this BD prismatic effect.

    But the eye behind this lens at this point sees the image of Object “A” as a light ray angled downwards into the eye, and if you extrapolate this bent line back (to in front of this lens), the original position of Object “A” will seem to you the observer as having been deviated upwards, towards the apex of this prism.

    So the rule is this... Above (or below OC or the right or left of OC) a light ray is indeed bent towards the base of the corresponding prism, but the image of an object as seen by the observer, will indeed be deviated “upwards”, toward the prism’s apex”.

    Unfortunately I don’t have the computer skills to draw this picture for you .

  11. #36
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    Quote Originally Posted by chip anderson View Post
    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. Chip
    This belief that FT in glass or plastic have BU prism is probably the source of your difficulty in understanding the concept of segment prism base direction.

    Not just 'round ones' but all segment styles that have a seg OC below the top edge (or seg line) will produce this BD prismatic effect. But if you drop your eyes to a lower reading level (further downward and past the seg OC) this BD prismatic effect will lessen, become zero (0) at the seg OC and then change to a BU effect at all points below the seg OC.

    At the FT seg line, you will experience BD as the 'plus' segment's contribution to the combined prismatic effect of distance and near, regardless of lens material.

  12. #37
    What's up? drk's Avatar
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    I think a clarifying idea is that lenses are gradient prisms.

    We're thinking in terms of "absolute" prisms, like a loose prism that has the same deviating power no matter where you look through it.

    Not so a lens...it has anywhere from zero prism at the center to a maximum amount at the edge of the lens.

    Not only is a lens a gradient prism, but depending where you look through it (not unlike turning a loose prism) it can be BI, BO, BU or a combination of anything! It's a "concentric" prism. (I made that term up.)

    Add the fact that with a segmented MF we're essentially adding a lens to a lens (distance power) we have a really fun combination of net prismatic effect.

    And, of course, add the fact that segmented MFs have all sorts of shapes possible (old Ultex, round, FT, ribbon, exec) and that makes for a confusing thread.

  13. #38
    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by chip anderson View Post
    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.
    You're right, in the sense that a round seg will tend to counteract the induced, yoked BU prism due to significant plus power found on the vertical meridian in the distance. A round seg will have an additive effect on a minus lens, contributing to the BD prism. A Straight Top is essentially neutral (depending on the reading depth and set placement), with the OC about 4mm below the top of a 28mm wide seg.
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  14. #39
    Optimentor Diane's Avatar
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  15. #40
    Doh! braheem24's Avatar
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    Diane, Linky no worky.

  16. #41
    Optimentor Diane's Avatar
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    Let me try again

    Click image for larger version. 

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    Anything worth doing is worth doing well.

  17. #42
    Optimentor Diane's Avatar
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    They loaded out of order....Oh well.

    Diane
    Anything worth doing is worth doing well.

  18. #43
    ABOM Wes's Avatar
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    Quote Originally Posted by Diane View Post
    They loaded out of order....Oh well.

    Diane
    No worries. Can't win em all.
    Good demo though.
    And Chip's bound to believe it since you posted it.

  19. #44
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    to calculate base in prism, in the seg, do I just move the seg relative to the distance OCs?

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    Hi Laurie, I'm not sure if I stated my question correctly. The Rx calls for 2 diopters of base in prism O.U. using the segs with a +2.50 add I calculate I should move the segs .75 mm closer together for each seg. Does that sound correct?
    Thanks, Randy

  21. #46
    One eye sees, the other feels OptiBoard Silver Supporter
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    Quote Originally Posted by randall c View Post
    to calculate base in prism, in the seg, do I just move the seg relative to the distance OCs?
    Welcome to OB. Nice way to get your feet wet in ophthalmic optics on your first post!

    Use Prentice's Rule- segment power decentered with reference to the near point/pd.

    Here's a sample test question:

    15. Given the following FT-35 bifocal prescription:
    +2.00 DS with a +2.50 D add, O.U. (both eyes)
    PD’s = 66/62 mm
    How could you induce a 1D base in prismatic effect, per eye, at near only?
    a. Use a 56 mm distance PD
    b. Use a 60 mm near PD
    c. Use a 54 mm near PD
    d. Can’t be done

    Here's how I answered it:

    If we ignore the prismatic effect of the eyes converging behind the distance Rx, a near PD of 62mm - 8mm = 54mm (.8cm x 2.50 D = 2∆ BI total) is the correct answer.

    If we include the effect of the distance Rx which is 2.00 D x .4cm = .8∆ BO total, that can be cancelled by decentering the segs an additional 3.2mm (.32cm x 2.50 D = .8∆ BI total). A near PD 50.8 achieves a resultant prismatic effect of 2∆ BI total during the near gaze.

    I'm not going to consider the fact that the eye will rotate towards the prism apex .3mm per prism diopter upsetting the distance calculation in a way that's somewhat like going back in time and killing your grandparents.
    Hope this helps,

    Robert Martellaro
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



  22. #47
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    Quote Originally Posted by Wes View Post

    And again, prisms bend light in the direction of the base, but displace the image in the direction of the apex.
    Took me a long time until that clicked. Sitting at a lensometer really helped get a natural feel for the concept. Until I grabbed lenses to correct mixed astigmatism and confused myself all over again

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