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Thread: Edge thickness and bifocals

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    Edge thickness and bifocals

    Do bifocal vs. single vision lenses have any difference in edge thickness?

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    Welcome to the forum, esivel. Your question is a good one! The answer will depend on the raw material the lens is made from and power the lens is and whether it is a visible or invisible bifocal. Would you care to share more specific information about the above?

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    Given the same power and same total decentration and same material and same frame size (oh, and same center thickness), a single vision lens will have the exact same edge thickness as a conventional flat top bifocal lens will have.

    Having said that, note I used the word "conventional". Such things as digital surfacing and free form design will have a huge effect on edge thickness. As will a progressive (freeform or not).

    If you compare apples with apples, single vision lenses and conventional flat top bifocals surfaced conventionally will have the exact same edge thickness.

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    Doh! braheem24's Avatar
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    ...oc on SV and mrp placement on bifocals also need to match.

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    Thanks for the reply uncut.
    The material is Medium Index and the bifocal line is visible. The power is around -4.

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    In a 1.6 to 1.67 index plastic the C.T. might be as thin as .8 up to 1.4 mm. for the S.V. lens. In a visible bifocal in the same index the lab usually will surface to specification, but might be reluctant to produce a lens where the segment itself has more thickness than the carrier lens. That would create an unstable lens. If you caliper the C.T. of the bifocal you might find a C.T. that is about 1.8 C.T. which would result in a gain(potential) of 1 mm. to overall lens thickness.

    If you factor in what braheem24 mentioned (oc/mrp placement) and a slight frame area or size shift, thickness may have noticeably increased. Have you contacted your dispenser?

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    Quote Originally Posted by MikeAurelius View Post
    Given the same power and same total decentration and same material and same frame size (oh, and same center thickness), a single vision lens will have the exact same edge thickness as a conventional flat top bifocal lens will have.

    Having said that, note I used the word "conventional". Such things as digital surfacing and free form design will have a huge effect on edge thickness. As will a progressive (freeform or not).

    If you compare apples with apples, single vision lenses and conventional flat top bifocals surfaced conventionally will have the exact same edge thickness.
    Love your "shotgun approach" answer, Mike......now, what would the C.T. be, in a surfaced -1.00 glass lens, +3.50 add, FT-35?
    Last edited by uncut; 08-24-2010 at 05:52 AM. Reason: mind change

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    Usually on bifocals fit over 2 below frame center, a lab will grind the OC to be no less than 2 above the top of the bifocal. In these cases, the higher the bifocal, the thicker the lens. Some times in very low fit bifocals labs have the OC set to a maximum distance from the top of the seg. It takes an extreme case for this to effect the thickness, but it does happen. So the answer is yes, a bifocal can be thicker than SV. There are many ways to fix this. Talk to your lab. If they are ther ones telling you this, with no suggestion as to how to fix it, talk to another lab.

    Edit. And yes as stated above, sometimes in stronger adds GLASS has to be ground thicker to keep from cutting into the seg.
    Last edited by Speed; 08-24-2010 at 06:47 AM. Reason: incomplete

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    Underemployed Genius Jacqui's Avatar
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    Quote Originally Posted by uncut View Post
    Love your "shotgun approach" answer, Mike......now, what would the C.T. be, in a surfaced -1.00 glass lens, +3.50 add, FT-35?
    Here it's 2.2

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    You just broke a lens.
    VE published seg thickness: 6 base is 3.3, 4 base 3.4.

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    Underemployed Genius Jacqui's Avatar
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    Quote Originally Posted by Speed View Post
    You just broke a lens.
    VE published seg thickness: 6 base is 3.3, 4 base 3.4.
    hmm... It worked last week

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    Quote Originally Posted by uncut View Post
    Love your "shotgun approach" answer, Mike......now, what would the C.T. be, in a surfaced -1.00 glass lens, +3.50 add, FT-35?
    Seg thickness (in glass) for a D35 is 3.2 mm. I'd finish at about 3.5 to 3.8 thick. But this is moot, since the lens being talked about is plastic. You will also note that I wrote "same center thickness" (or did you perhaps overlook that part?).

    BTW: I use X-Cel lenses, I won't surface lenses that aren't manufactured in the USA. Vision Ease glass lenses are made in Indonesia. Also, X-Cel uses a higher index lower seg part, which results in thinner segment thicknesses.

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    Mike: The OP hasn't stated whether glass or plastic, just that it's a medium index. Yes, I know you covered your answer to include "same center thickness", but I think the OP needs to be informed that the same C.T. may not be appropriate or possible. I am hoping they will add more specific information to the thread.

    regards

    uncut

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    The question wasn't whether it was appropriate or possible, it was "DO they have any difference". And my answer, given that all things being equal, is that, yes, they should have the same edge thickness.

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    They are plastic, with a flat top segment (+1).

    - esivel

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    Can you provide complete RX including seg ht, pd anf frame a,b, and ed?

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    Hello. I am trying to figure this issue out and I get different answers from different opticians. I need to know if thickness should measured based on total distance power or near power.

    Here is my situation, I have this +0.25 -1.00 lens with a 2.0 add power. What I believe, and what my optician friend tells me is that you only look at the sphere + cylinder in the distance area for total power and meaure by that. So in this case I'd look at the Edge Thickness as it'd become a minus lens. However my boss tells me I should be adding the add power from the seg to that, which makes it a plus lens which means would look at center thickness instead of edge thickness. I don't think that's right, since the add power is contained in that small area of the bifocal, but I need to figure it out for sure. Does anyone here know?

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    Measure center thickness at the optical center, and edge thickness at the thinnest edge. Apply those values to assure you've created the most cosmetically appealing lens that is safe.

    Why is this even in question? I've never heard of anybody measuring thickness in the seg.

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    thickness

    Some of you have forgot the basics, a flat top can be thicker depending on the add power, if you have a 400 add, your not going to be able to grind the same thickness as a 100 add, as if you did you would grind into the bifocal

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    Harry is absolutely correct, when discussing glass lenses, the power within the bifocal(flat top only) will create a requirement to thicken the lens to compensate for the seg, as it is imbedded into the body of the lens from the front surface. The thickness added wouldn't be the thickness of a large sv lens, but a value probably listed on the lens packaging or in tables. This value is needed to generate the lens to a minumum thickness so that the seg isn't invaded during fining and polishing.

    In the case of flat top plastic lenses, the thickness is relative only to the distance power, as long as the lens is stable in it's finished thickness. I would tend to believe that a high powered flat top, especially a 35 mm seg is less likely to crack if the thickness behind it is increased slightly, perhaps by .2 mm.

    jro, welcome to the Forum, your boss is incorrect for a flat top 25/28 mm., slightly correct for a 35 mm. flat top, and correct for an executive bifocal.

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    Lots of good points made. In summary:

    For flat-top bifocal lenses, the biggest difference compared to single vision lenses, if any, will be due to the location of the prism or major reference point (as Braheem noted).

    Single vision lenses typically have the optical center placed along the datum or 180° line of the frame, while bifocal lenses may have the optical center either along the datum line or at some specific distance above the segment ledge (like 3 mm).

    Additionally, any significant difference in base curve will also result in a difference in maximum lens thickness. Flatter base curves are typically thinner than steeper base curves. Your stock or semi-finished single vision lens may have a different base curve than a semi-finished flat-top lens.

    For glass flat-top bifocals, you have the same differences to contend with as well as a potential increase in center thickness, particularly in minus powers, due to the depth of the fused segment (as Harry noted).

    Obviously, if you deliberately change some other constant, like the minimum edge or center thickness, you can expect the thickness to differ accordingly.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Thanks everyone for the great information! My boss and I had a talk and she agreed that the thickness should be checked no differently from a single vision lens. Had it been a progressive lens the add power should be taken into account. Thanks again!

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    Quote Originally Posted by jro View Post
    Thanks everyone for the great information! My boss and I had a talk and she agreed that the thickness should be checked no differently from a single vision lens. Had it been a progressive lens the add power should be taken into account. Thanks again!
    Too bad, this is an incorrect conclusion.

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