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  1. #1
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    Sag headache on aspheric lenses

    I have yet to run a Vision Ease lens without a load of hassle.
    I cant sag them with the clock as they are aspheric, the sag they provide throws up power problems. The people on the phone at VE here in the UK are little help.

    Thus far I have run all their lenses too thick (so I can rework them) with a known back surface, calculated the front surface after foci then calculated a sag, which I use for future orders.

    I am informed the sags are compensated for 1.53 index, this seems daft to me as a sag is a measurement of distance.

    So can somebody confirm the above, and if so remind me of the formula to convert..

    Thanks

  2. #2
    ATO Member HarryChiling's Avatar
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    I would assume that VE means you will have to get the true power of the curve first

    True power= (n-1)/(1.53-1)*Marked Power

    The front curve being the marked power in the equation.

    then you would find out what the sag is

    Sagitta=R- squareroot(R^2 - (d/2)^2)

    d= diameter of the lens or 50mm
    R= radius of the curve (n-1)/D

    I hope this helps.
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    Bad address email on file QDO1's Avatar
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    I assume you need a sag formula for an aspheric surface, not a spherical one.. will think about that one... is there a quoted asphericity for the lens?

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    Quote Originally Posted by QDO1
    I assume you need a sag formula for an aspheric surface, not a spherical one.. will think about that one... is there a quoted asphericity for the lens?
    Aspheric yes. Quoted asphericity no. Index 1.5.
    The lenses are supplied with a printed sag. (I have this problem with only one supplier, all the others I can use the data straight off their sheets.)
    They always seem to come out around 0.50D off.

    Ie a 6 base is described with a sag of 3.91
    If I run this through the computer I get a front power of 5.94, if I then calc the rear surface I will get an off power lens.

    If i run the sag of 3.91 with an index of 1.52 front surface is 6.2.

    If then I calc the sag of a 6.20D/1.5, I get 3.88 which if I then run with yields good results.

    What I cannot decide is whether I am barking totally up the wrong tree with regard to the manufacturers data.
    Or if I am correct would it be just as easy to tell the software that the lens is glass in the first place.
    And how would this work with other index of lenses?

    Im getting a headache.

    Rick

  5. #5
    Master OptiBoarder Darryl Meister's Avatar
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    Generally, the sag value provided by the manufacturer for an aspheric lens is meant for power calculations, only. That is to say, it is not the physical sag of the lens blank, but rather the sag of a sphere comparable in curvature to the central zone of the lens surface.

    These sag values are usually the sag at a fixed diameter (e.g., 50 mm), and would need to be entered along with the actual refractive index of the material. Of course, this assumes that you're making the lenses with the correct center thickness, that the lens blank has the correct surface power from Vision Ease, and that your process is accurate (i.e., your tools aren't off or anything).

    For accurate thickness calculations with an aspheric lens blank, your software needs to "know" how to describe the aspheric surface. Some software companies sag each lens blank at various diameters to figure this out. Others use aspheric surface data from the actual lens manufacturer. Vision Ease may also make the software to do these calculations available to you.
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  6. #6
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    Quote Originally Posted by Darryl Meister
    Generally, the sag value provided by the manufacturer for an aspheric lens is meant for power calculations, only. That is to say, it is not the physical sag of the lens blank, but rather the sag of a sphere comparable in curvature to the central zone of the lens surface.

    These sag values are usually the sag at a fixed diameter (e.g., 50 mm), and would need to be entered along with the actual refractive index of the material. Of course, this assumes that you're making the lenses with the correct center thickness, that the lens blank has the correct surface power from Vision Ease, and that your process is accurate (i.e., your tools aren't off or anything).

    For accurate thickness calculations with an aspheric lens blank, your software needs to "know" how to describe the aspheric surface. Some software companies sag each lens blank at various diameters to figure this out. Others use aspheric surface data from the actual lens manufacturer. Vision Ease may also make the software to do these calculations available to you.
    Thanks for the info.

    The problem is that the software I use does not have data specific to these lenses.
    It isnt a problem as when I have one in I can run it with a certain back surface/thickness and use the findings to correctly run future lenses.
    It is a little laborious however and I would rather understand the data I am being given.
    I will try to contact VE in the States as here in the UK any technical questions tend to draw a blank.
    It is Scopus lenses where I have the issue, are they a VE brand or are they manufactured elsewhere, I note the packaging states they are made in Israel. It may be better to contact the manufacturer direct if this is the case.

    Rick

  7. #7
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    Quote Originally Posted by HarryChiling
    I would assume that VE means you will have to get the true power of the curve first

    True power= (n-1)/(1.53-1)*Marked Power

    The front curve being the marked power in the equation.

    then you would find out what the sag is

    Sagitta=R- squareroot(R^2 - (d/2)^2)

    d= diameter of the lens or 50mm
    R= radius of the curve (n-1)/D

    I hope this helps.
    Thanks Harry, I will run some numbers tomorrow.

    Rick

  8. #8
    Master OptiBoarder lensgrinder's Avatar
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    Quote Originally Posted by rsandr
    I am informed the sags are compensated for 1.53 index, this seems daft to me as a sag is a measurement of distance.
    Greetings,
    The Sag gauge is based on an the tooling index of 1.53 so when a Sag is given and converted to a true curve that curve is based on an index of 1.53 also. You will notice on the Vison-Ease web site that it says true curve(1.53) When you get your true curve convert it to the true power for the given index you are working with using the formula that HarryChiling gave.


    The Sag of 3.91 should equal 6.47 front curve if you are using a Bell Sag gauge with a 50mm diameter.

    F=(S X 2000 X (n-1))/S^2 X Y^2
    F=Front Curve
    S=Sag
    n=refractive index (1.53 for a Sag Gauge)
    Y=1/2 of the Sag gauge diameter (usually 50mm)

    I hope this helps

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