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Thread: bifocal lenses

  1. #1
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    bifocal lenses

    Hi,

    I would like to know the differences among FT28, FT XL, CT25, E-line.. It is obvious that segment is different, but i want to know which segment to use according to prescription.

    Thanks

    Peter

  2. #2
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    Hi Peter
    F.T 28 has a Flat Top segment surface with a 28 diameter ,most popular bifocal in America.In Europe CT25 Curve top is the mostly used bifocal,the top segment line is not flat but rather curved,some profesionals beleive that the small curve on the top smothes down the image jump present on all bifocal lenses .For prescription quality and use for patient is about the same,28 seg has a little more near vision area than the CT25 .E-Line or Executive Lens bifocal ,the entire lens is divided by a long distace and a near viewing area.The bottom half reading ares is extremly big,the whole buttom half,it was recommended for special ocupational porposes for example someone who needs a a very large near viewing area like an Arquitect who wants to see a big architectural drawing,or a fashion designer who needs to work over a big working table,and needs a big viewing area.The E-Line lens is an ugly lens design barely used at all,if a patient needs an extremely large near viewing area you can recomend a F.T. 35 / Flat Top bifocal with a 35 mm seg ,this will help a lot.

  3. #3
    Master OptiBoarder Darryl Meister's Avatar
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    Tito provided a very nice summary.

    Years ago, optometrists were periodically encouraged to choose bifocals that minimized the total prismatic displacement of object at near. For plus lenses, this meant prescribing a bifocal with a low optical center, such as a round seg. For minus lenses, this meant prescribing a bifocal with a higher optical center, such as an Executive-style or flat-top.

    I don't really agree with this philosophy, however. I'm of the opinion that it is hard to beat the prism and optics of a flat-top bifocal segment when a lined multifocal is required or preferred. The widest field of view is high in the segment. Image jump is relatively low. And the prismatic displacement at near isn't much worse than the wearer would experience in similar single vision lenses.

    For flat-top bifocals, the segment optical center is typically around 5 mm below the ledge. For round seg bifocals, the optical center is at the geometric center of the segment. For Executive-style bifocals, the optical center is typically (though, not necessarily) at the bifocal ledge. These distances influence both the image jump and net prismatic effects through a bifocal segment.

    I also agree with Tito regarding Executive-style or E-line bifocals; they can be quite thick, particularly in plus prescriptions with high Add powers. A flat-top 35 or flat-top 45 ("Multi seg") should suffice in the vast majority of situations. Executive-style bifocals are really only ever necessary when a lot of horizontal prism at near only is required. If you do use an Executive-style bifocal in a plus prescription, consider ordering it with prism-thinning.
    Darryl J. Meister, ABOM

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    [QUOTE=Tito;342512]Hi Peter
    .In Europe CT25 Curve top is the mostly used bifocal

    Rather a sweeping statement and definitely not the case in the UK. FT28 is the most popular, although progressives are more popular still.

  5. #5
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    I just came across my first Round Segment Bifocal (RS24) do you use Near or Distant PD when setting up for blocking and edging?

  6. #6
    Rochester Optical WFruit's Avatar
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    Quote Originally Posted by nawsman View Post
    I just came across my first Round Segment Bifocal (RS24) do you use Near or Distant PD when setting up for blocking and edging?
    :drop:

    You use near..... but make sure the lens is on axis....
    There are rules. Knowing those are easy. There are exceptions to the rules. Knowing those are easy. Knowing when to use them is slightly less easy. There are exceptions to the exceptions. Knowing those is a little more tricky, and know when to use those is even more so. Our industry is FULL of all of the above.

  7. #7
    Master OptiBoarder Darryl Meister's Avatar
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    Yeah, use the near PD, but definitely spot the distance optical center and axis. If the lens is a sphere or has a weak cylinder, you may need to rotate the lens blank slightly before blocking to achieve the correct inset and the same seg drop for each eye.
    Darryl J. Meister, ABOM

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    Master OptiBoarder snowmonster's Avatar
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    Reviving a really old thread.

    How do you block a round seg with prism, especially if the lens has no cylinder to simply go by axis and decentration while blocking? I know how to normally block a round seg to get it on axis but the prism is rather variable.

    Thank you in advance.
    -Steve

  9. #9
    Master OptiBoarder optical24/7's Avatar
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    Prism in lenses also have an axis. Spot up your lens on the lensometer marking it with the base and amount ( if vertical and horizontal are required, compound it to a direction and axis need). Then block it up off the three spotting maks at 180. If the lab ground the lenses correctly you should have a 1.5-2 mm inset of the seg in the blocker.


    Nice to see you post here again Snow. At least we still know you’re alive! ( Unless Someone offed you and stole your password here...😂)

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    Master OptiBoarder snowmonster's Avatar
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    Thank you - it's been a while and I learned a lot quickly here. Add in practice growth, teenage kids with activities and I'm just to busy now for a lot of stuff. Pretty good if I can go this long without posting right?
    -Steve

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