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Thread: SV Finished Lens Numbers: Ex "68/70" ? What do they really mean?

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    SV Finished Lens Numbers: Ex "68/70" ? What do they really mean?

    Over the years I have heard many different answers to why SV finished lenses come with two numbers [ "68/70" being one example ] which one is true?


    1) The blanks often have edge defects, the lower number is the lowest size, the higher, the max (if there is no edge defects)

    2) The lower number represents V bevel lens uncut size, the higher, grooved or drilled rimless

    3) the blank is decentered or elliptical, and the size is different depending on how its laid out.


    What have you heard? There may be other mythical reasons. Which is actually correct?

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    Master OptiBoarder mdeimler's Avatar
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    Hell, I just figured it meant, since they are mass produced, they are somewhere in that diameter range...give or take a few mm's.

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    Master OptiBoarder MakeOptics's Avatar
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    Quote Originally Posted by sharpstick777 View Post
    Over the years I have heard many different answers to why SV finished lenses come with two numbers [ "68/70" being one example ] which one is true?


    1) The blanks often have edge defects, the lower number is the lowest size, the higher, the max (if there is no edge defects)

    2) The lower number represents V bevel lens uncut size, the higher, grooved or drilled rimless

    3) the blank is decentered or elliptical, and the size is different depending on how its laid out.


    What have you heard? There may be other mythical reasons. Which is actually correct?
    The blanks size is 70mm but the cutout is going to be at least 68. That means that the lens could have engravings or molded identification numbers encroaching 1mm in from the edge.

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    A lens labeled 68/70 usually arrives with a flat vertical and horizontal at 90 and 180 axis, in other words the round 70 mm round blank is measured as 68 at 90 and at 180. This designates the uncut useful area.
    Eyes wide open

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    Quote Originally Posted by uncut View Post
    A lens labeled 68/70 usually arrives with a flat vertical and horizontal at 90 and 180 axis, in other words the round 70 mm round blank is measured as 68 at 90 and at 180. This designates the uncut useful area.
    So its slightly elliptical?

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    Master OptiBoarder LENNY's Avatar
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    My understanding of 65/70 mm was some powers usually plus and high minus are 65mm and some usual minus are 70mm....

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    Quote Originally Posted by LENNY View Post
    My understanding of 65/70 mm was some powers usually plus and high minus are 65mm and some usual minus are 70mm....
    Yes, Lenny, but I talking about having 65/70 on the same lens envelope, sorry for not being clear. Some will have 2 numbers, not just one single diameter.

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    Master OptiBoarder optical24/7's Avatar
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    If you were running 100's of thousands of lenses, how many different packaging markings would you prefer? Could also relate to shrinkage/differing mold sizes.

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    Master OptiBoarder LENNY's Avatar
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    I understand!
    I was referring to that also.
    I will give you example!
    Poly lenses will have a same envelope for plus and minus lenses and would be marked 65/70
    65 plus
    70 minus

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    Quote Originally Posted by sharpstick777 View Post
    So its slightly elliptical?

    Naa....dey are cribbed, and/or molded in that that square dimension!
    Eyes wide open

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    Objection! OptiBoard Gold Supporter shanbaum's Avatar
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    65/70 would usually indicate a 65mm lens with the PRP decentered 2.5mm, giving you the same cutout, under certain circumstances, as a 70mm lens. Europeans market in this manner with great enthusiasm.

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    Master OptiBoarder Darryl Meister's Avatar
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    If a semi-finished lens blank is decentered, you will typically see the "effective" diameter marked after the physical lens diameter, as Robert pointed out. In this case, the effective diameter is equal to the sum of the physical diameter and twice the decentration. And 2.5 mm is a very common decentration amount.

    However, this only applies to semi-finished lenses. And the small difference that you are referring to here, 68/70 mm, is probably not due to decentration. As PhiTrace pointed out, the physically usable area of a lens blank may be slightly smaller than the "nominal" or marked size for various reasons, particularly for semi-finished lens blanks.

    There are also different tolerances on the nominal size versus the usable size. It is also worth noting that there is a looser tolerance on the lens diameter being larger than specified, and a tighter tolerance on being smaller than specified.

    That said, while a lens manufacturer is likely to make these different specifications available to laboratories and LMS companies, I don't know how common it is to supply the usable blank size on the package of finished lenses. As far as I know, it is not required by ISO. So, rather than guess, I would recommend contacting the lens manufacturer for clarification.

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by LENNY View Post
    I understand!
    I was referring to that also.
    I will give you example!
    Poly lenses will have a same envelope for plus and minus lenses and would be marked 65/70
    65 plus
    70 minus
    Going to second this. Most of our stock lenses come labled 65/70 for the +/- Powers. Seems like there's more to it than I thought though!

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    Master OptiBoarder Darryl Meister's Avatar
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    Most of our stock lenses come labled 65/70 for the +/- Powers.
    Is somebody trying to save money on printing labels or something?

    Best regards,
    Darryl
    Darryl J. Meister, ABOM

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    Quote Originally Posted by Darryl Meister View Post
    Is somebody trying to save money on printing labels or something?

    Best regards,
    Darryl
    I would guess so as well.

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    OptiBoard Apprentice gsmahesh's Avatar
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    if the lens envelope shows 65/70; it means, geometrically the lens will be 65mm dia. But with 2.5mm decentration. So the effective diameter will be 70mm. (purpose: less material conception for the manufacturing, saving cost)

    Above eg: it the lens must me geometrically 68mm with 1mm decentration.

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