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Thread: Sag headache on aspheric lenses

  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 OptiBoard Bronze Supporter 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.

  3. #3
    Master OptiBoarder 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?
    Richard King
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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
    Carl Zeiss Vision OptiBoard Gold Supporter 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.
    Darryl J. Meister, ABOM

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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

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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

  8. #8
    ATO Member OptiBoard Bronze Supporter HarryChiling's Avatar
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    If you post the parameters of the lens and the parameters of your outcome maybe I could help with figuring the problem out. Worse case scenario is everyone on this board will try to solve it and if that doesn't lead to a solution throw the lens away.:bbg:

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    Master OptiBoarder mshimp's Avatar
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    sounds like you need a new program to compute all the latest lens designs.

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    Carl Zeiss Vision OptiBoard Gold Supporter Darryl Meister's Avatar
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    Worse case scenario is everyone on this board will try to solve it and if that doesn't lead to a solution throw the lens away
    Unfortunately, without the aspheric coefficients of each base curve in the range, we would only be guessing.

    He could probably get just as close by sagging the lens blank with a sag gauge at 50 mm, and then using the measured curve for thickness calculations (though the sag provided by the manufacturer should still be used for power calculations).
    Darryl J. Meister, ABOM

  11. #11
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    Quote Originally Posted by mshimp
    sounds like you need a new program to compute all the latest lens designs.
    Or I could just expect the manufacturer to provide the correct front surface power and save 4 grand.

  12. #12
    Objection! shanbaum's Avatar
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    Quote Originally Posted by rsandr
    Or I could just expect the manufacturer to provide the correct front surface power and save 4 grand.
    You'd still have the problem of determining the sag of the lens at the various relevant diameters needed to figure the proper thickness, given the layout geometry and the shape.

    Or, you could run them all thick, work them down to an appropriate thickness by visual inspection, and save four grand.

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    Master OptiBoarder QDO1's Avatar
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    Ask another lab how they process the same blanks - the supplier would be able to give you a list of labs who were doing similar work. Failing that - send the blanks back, and do different work, or insist the supplier provides you with the relevant spec sheets
    Richard King
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    Quote Originally Posted by shanbaum
    You'd still have the problem of determining the sag of the lens at the various relevant diameters needed to figure the proper thickness, given the layout geometry and the shape.

    Or, you could run them all thick, work them down to an appropriate thickness by visual inspection, and save four grand.
    There's the thing, there is normally the need to 'retouch' them on the generator, not a problem however in an outfit of our size/volume.

  15. #15
    ATO Member OptiBoard Bronze Supporter HarryChiling's Avatar
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    I would be curious if the US spec sheet on the same lens if available here would have teh same problems

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    Master OptiBoarder OptiBoard Silver Supporter Jubilee's Avatar
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    Where I used to work at we used RxCalc,then Innovations to generate surfacing data on the Vision Ease Tegras.. We rarely had an off power issue with them. When we did, the issue was normally found to be A) human error, or B) the generator.

    Cassandra
    "Some believe in destiny, and some believe in fate. But I believe that happiness is something we create."-Something More by Sugarland

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    Master OptiBoarder QDO1's Avatar
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    just looked at the website

    http://www.vision-ease.com/

    and they seem to have pretty comprehensive lens specifications. Dont know what lens you are trying to surface, but you might find a PDF there with the info you require
    Richard King
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  18. #18
    Rising Star 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

  19. #19
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    Quote Originally Posted by QDO1
    just looked at the website

    http://www.vision-ease.com/

    and they seem to have pretty comprehensive lens specifications. Dont know what lens you are trying to surface, but you might find a PDF there with the info you require
    Hi,

    I couldn't see it there but this looks more likely.

    http://www.scopus.org.il/asphericsf.html

    They quote the radius, surely this will be the radius. But if a sag isnt a sag then who knows.

    Rick

  20. #20
    ATO Member OptiBoard Bronze Supporter HarryChiling's Avatar
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    Hi,

    I couldn't see it there but this looks more likely.

    http://www.scopus.org.il/asphericsf.html

    They quote the radius, surely this will be the radius. But if a sag isnt a sag then who knows.

    Rick
    If you look at the Actual Curve it has a "*" next to it go to the bottom and you see that the "* Actual Base in Diopters in Reference to Index 1.53" you will need to convert these numbers into the true powers using the former equation. The Radius is the same in these equations they use the index 1.53 take the actual curve given on those specs and run it through the equations and you will see what the true curves and radius are.

    example from data sheet

    Blank Demiensions Mid Index 1.566, Diameter 73, Actual Curve 6.46, Convex Radius 82.08, 60mm Sag 5.44

    True Curve=(1.566-1)/(1.53-1)6.46

    True Curve=(.566/.53)6.46

    True Curve=6.90

    Radius=(1.566-1)/(6.90)

    Radius=82.03mm

    Sag=(82.03)-Squareroot[(82.03)^2-(60/2)^2]

    Sag=(82.03)-Squareroot[(6728.92)-(900)]

    Sag=(82.03)-Squareroot[5828.92]

    Sag=(82.03)-(76.35)

    Sag=5.68

    Compare
    -Before-
    Blank Demiensions Mid Index 1.566, Diameter 73, Actual Curve 6.46, Convex Radius 82.08, 60mm Sag 5.44
    -After-
    Blank Demiensions Mid Index 1.566, Diameter 73, Actual Curve 6.90, Convex Radius 82.03, 60mm Sag 5.68

    I would suggest running all the specs through the formulas and making your own spec sheet for reference in the future if you would need it.

  21. #21
    ATO Member OptiBoard Bronze Supporter HarryChiling's Avatar
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    This calculator is made specific for your problem and should convert the whole sheet for you with little though or effort on your part.:cheers:
    Last edited by HarryChiling; 02-21-2007 at 04:59 AM.

  22. #22
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    Quote Originally Posted by HarryChiling
    This calculator is made specific for your problem and should convert the whole sheet for you with little though or effort on your part.:cheers:
    Many thanks for that Harry, just what the doctor ordered.


    Rick

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