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Thread: Can this lens be made?

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Can this lens be made?

    MD's rx right -1.00-1.25x100 left -1.75sph add +2.50.

    Doctor wants 1.5 prism diopters OU ---BUT--- only above the FT28 seg line. To my knowledge it's gotta be fresnel or throughout the lens. Any surfacing guru know of a way to do it?

    If it can be done I know this is the place to go ask the experts!
    As always Optiboarders Rule:cheers:

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    My first thought was a franklin type seg. A distance rx with prism, cemented onto a bottom add.

    Seeing that it is ft-28, I am a bit lost. Maybe someone like our own Awtech could give some insight into the freeform capabilities that he can offer? Although, I don't think that he can do anything with a FT.

    Hopefully some of the more knowledgeable members will chime in. Good luck!

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    Master OptiBoarder mshimp's Avatar
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    Yes it can be done. Hope the prism is base down,it makes it alot easier. this is done the very same way a slaboff is done. However if it is base up a different technique is used.
    mike

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    Darryl....

    Where's Darryl, he'll know.

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    Quote Originally Posted by Uncle Fester View Post
    MD's rx right -1.00-1.25x100 left -1.75sph add +2.50.

    Doctor wants 1.5 prism diopters OU ---BUT--- only above the FT28 seg line. To my knowledge it's gotta be fresnel or throughout the lens. Any surfacing guru know of a way to do it?

    If it can be done I know this is the place to go ask the experts!
    As always Optiboarders Rule:cheers:
    Which direction?

  6. #6
    Master OptiBoarder Darryl Meister's Avatar
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    Where's Darryl, he'll know
    I think you guys have it pretty well sorted out. If it's vertical prism, you can do it with a basic slab-off. If it's horizontal prism, you will need to either:

    1) use a Fresnel press-on optic on the top half,
    2) Make a split (Franklin) style bifocal out of two single vision lenses, or
    3) Surface horizontal prism in the distance portion and then decenter each bifocal segment in or out an additional 6 mm to offset the prism at near (this will require a relatively wide segment)
    Darryl J. Meister, ABOM

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    If at all possible do not cement or polish the two halves of a Franlkin bifocal. Reflection will be intolerable. Precisely fit halves for retention by frame and fit of two halves.

    Chip

    Don't see why fresnel paster should be needed, Why not just grind it into the top half of the Franklin?

  8. #8
    Underemployed Genius Jacqui's Avatar
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    I agree with Chip!!!!! I would use the Franklin approach

  9. #9
    Master OptiBoarder Darryl Meister's Avatar
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    Don't see why fresnel paster should be needed, Why not just grind it into the top half of the Franklin?
    Press-on optics are inexpensive, can be applied in office, and allow you to evaluate the effectiveness of the prism correction without necessarily remaking the lenses. Also, while not the best permanent solution, press-on optics are certainly the best temporary solution, especially if the final Rx is still in question.

    That said, this prescription really doesn't require the added cost, complexity, and potential delays of an actual Franklin style bifocal. Unless the patient has selected a really big frame, a standard Executive bifocal lens blank that has been appropriately decentered (+/-6 mm) will work just as well, without the added hassle. Besides, unless the lab making the Franklin bifocal has had a lot of experience in fabricating this type of lens, there is always the chance that the final product won't look that great.

    Of course, this all assumes that the prescribed prism correction is horizontal, which still hasn't been clarified.
    Darryl J. Meister, ABOM

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    Darryl:
    I agree on press-on's for evaluations but the presumably the good doctor will write "fresnel paster" on the Rx. But most patients are happy to get rid of the ventian blind effect as soon as a "real Rx" is prescribed.
    There should be no delay in a Franklin as it's just two pairs of SV lenses cut in half which a good optician can do in his own shop even if he doesn't do his own cutting and edgeing. You just send the frame to the lab with two Rx's and instructions to fit both to the frame exactly. Then the optician does his own cutting, takes about an hour for me and I have done less than a dozen for "experience."

    Chip

  11. #11
    Master OptiBoarder Darryl Meister's Avatar
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    There should be no delay in a Franklin as it's just two pairs of SV lenses cut in half which a good optician can do in his own shop even if he doesn't do his own cutting and edgeing. You just send the frame to the lab with two Rx's and instructions to fit both to the frame exactly.
    I guess I just don't think this practice is nearly as common -- or as simple -- as you do.

    Cutting each lens precisely, at exactly the right height and axis with a nice, straight line seems like something that would require a fair bit of effort and setup (and you have to do it 4 times). And if the lenses were edged in advance, you can't really smooth the edges much after halving the lenses either -- to clean them up or straighten them -- since any material removal will reduce the final size of the bifocal lens (unless they were precisely over-sized in the first place). Finally, if you order finished (edged) single vision lenses from your lab, you have to get this exactly right the first time, since you will have to reorder one or more of the finished lenses if you make any mistakes.
    Darryl J. Meister, ABOM

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    Daryl:
    You use a disc sander after cutting 2mm too tall for A measurment, reduce with disc sander to 1/2mm too tall, finish with a hand file laid on table until top an and bottom exactly match and insert. It's so simple
    it's really mind boggling. Anyone who can file a temple end could do it.
    Cost of disc sander covered by first job done.
    Just for GP the first day I opened my shop, my son's scoutmaster came in with his child who had broken glasses (-6.00, had to have something) I found new frame and cut lenses down and hand edged with a disc sander as my equipment had not arrived yet.

    Chip

    You can make it if you try

  13. #13
    Master OptiBoarder Darryl Meister's Avatar
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    It's so simple it's really mind boggling.
    Nevertheless, ordering an Executive with a special near PD is simpler still. I'm not suggesting that hacking a lens in half with a Dremel tool is difficult... I'm suggesting that to do it right -- that is, in order to make a nice looking and accurate final product -- would take both a great deal of time and care.
    Darryl J. Meister, ABOM

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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Yikes! It's prism out OU!!

    I put a 1 and 2 trial prism diopter out ou respectively over the fresnel os only "upper 3 out only" current glasses and asked if she had any problem reading. She said no so I may be able to put the prism throughout the lenses. I tried to cover my behind by explaining that if a problem occurs after a long period of reading the ultimate solution may have to be 2 pair of glasses. One for reading. One dedicated for distance with prism.

    Lately it seems that if a doc in the area has a prism patient they're sending them to me! Andrew- sure you don't want to move back for the summer at least:)

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    Underemployed Genius Jacqui's Avatar
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    Use a Franklin, it's easier than Daryl says.

  16. #16
    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Darryl- Please help me get this thick Swedish blockhead around your third suggestion:hammer:

    If using a FT35 with a patient PD of 61/58 the distance OC should be made to 73 with 1.5 prism Base out OU but it should be decentered and edged to a 58 near PD
    Last edited by Uncle Fester; 04-23-2007 at 09:43 AM. Reason: spelling

  17. #17
    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by Uncle Fester View Post

    If using a FT35 with a patient PD of 61/58 the distance OC should be made to 73 with 1.5 prism Base out OU but it should be decentered and edged to a 58 near PD
    Your prism is ground from the distance pd of 61, so you will have 1.5^ BO O.U. at 61. You need to inset the seg an additional 6mm for a total of 7.5mm inset. Your distance PD remains at 61 and your Near PD is 53.5. So when you order them from the lab tell them you want a 61/53.5 with 1.5^ BO OU.
    This will create 1.5^ BI OU at near to offset the distance prism.

  18. #18
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Uncle Fester
    MD's rx right -1.00-1.25x100 left -1.75sph add +2.50.
    Quote Originally Posted by Darryl Meister
    3) Surface horizontal prism in the distance portion and then decenter each bifocal segment in or out an additional 6 mm to offset the prism at near (this will require a relatively wide segment)
    I was off look at the post above.
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    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Thank you lensgrinder!:cheers:

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    Master OptiBoarder Darryl Meister's Avatar
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    Lensgrinder is on the right track, although you'd actually have to inset each lens an extra 6 mm if you plan to cancel 1.50 D of base out prism in each eye (that is, 3.0 prism diopters total). Your near PD would therefore be 58 - 2 * 6 = 46 mm. Also, just to clarify, you will measure your base out prism at a distance PD of 61 mm, but keep in mind that the actual optical center distance will be considerably narrower because of the base out prism (probably around 44 mm).
    Darryl J. Meister, ABOM

  21. #21
    ATO Member HarryChiling's Avatar
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    Darryl,

    Would the right eye be 24/30 for the PD?
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    Master OptiBoarder lensgrinder's Avatar
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    Darryl,
    Here is my thinking on this. When the person looks through the PRP they are looking through 1.5^ BO OU. When they look in 1.5mm each eye to read they are looking over the bifocal OC. To cancel the distance prism you need the difference between the Distance PRP and seg oc to be 7.5mm each eye (1.5 for current near inset + 6mm to cancel distance prism). That is why I said to order the lens 61/53.5 with 1.5^BO O.U.. There will be difference of 7.5mm from PRP to center of seg.

  23. #23
    ATO Member HarryChiling's Avatar
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    My thought was that in order to get a 1.50 BO in the distance Rx you need to have the lens decentered about 7mm for the OD with the power above. To offset this prism in the seg you need 6mm decentration the same way since the power is plus. So 7mm + 6mm = 13mm betwwen the Distance OC and the Seg OC minus the inset 13mm - 1.5mm = 11.5mm total difference.

    If the near pd is 58 or 29 mono then the distance PD should be 29 + 11.5 = 40.5 for the right eye.

    so 40.5/29 for the right eye, I think this is correct. I have shifted gears from Lenses to Contacts for the time being, so I will look through my books when I get home to check if it is correct.
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  24. #24
    Compulsive Truthteller OptiBoard Gold Supporter Uncle Fester's Avatar
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    Harry, the distance power is minus.

  25. #25
    ATO Member HarryChiling's Avatar
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    Quote Originally Posted by Uncle Fester
    Harry, the distance power is minus.
    Got it, but to get Base Out in a minus you decenter the lens IN and in the seg to get Base In in a plus you decenter the lens IN. Two INs add together, right; and the prism BO and BI neutralize each other, right?

    Again I will check through my books at home, I am gearing up to take the NCLE-AC so my focus the past month has been contact lenses, unfortunately; to make room for the CL material I have to put the Ophthalmic Lens material on hold.
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