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Thread: Lens Thickness - Base Curve Consideration

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    Lens Thickness - Base Curve Consideration

    I am using below generally accepted formula for lens thickness calculation:

    Thickness = MinimumThickness + ( ( ((ED+A+DBL-PD)/2)^2 x |Sph+(Cyl x sin(Axis)^2)| ) / (2000*(Index - 1)]) )

    This seems to give decent results BUT we are not considering front Base Curve of lens in this formula.
    Is there a better formula where we can use Front Base Curve input too?

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    Quote Originally Posted by OpticalS978 View Post
    I am using below generally accepted formula for lens thickness calculation:

    Thickness = MinimumThickness + ( ( ((ED+A+DBL-PD)/2)^2 x |Sph+(Cyl x sin(Axis)^2)| ) / (2000*(Index - 1)]) )

    This seems to give decent results BUT we are not considering front Base Curve of lens in this formula.
    Is there a better formula where we can use Front Base Curve input too?
    I was under the impression that only the sagittal depth changes and not the actual thickness. I have no math to prove that so i could be talking out of my ***. even the opticampus calculator doesn't ask for front BC

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    Actual thickness also changes.
    OptiCampus has the option to use manual BC, otherwise it uses appropriate BC with Automatic options.

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    Quote Originally Posted by OpticalS978 View Post
    I am using below generally accepted formula for lens thickness calculation:

    Thickness = MinimumThickness + ( ( ((ED+A+DBL-PD)/2)^2 x |Sph+(Cyl x sin(Axis)^2)| ) / (2000*(Index - 1)]) )

    This seems to give decent results BUT we are not considering front Base Curve of lens in this formula.
    Is there a better formula where we can use Front Base Curve input too?
    This formula is the sagittal approximation formula which uses the power to approximate the thickness. You will also need to add the minimum thickness to get a better approximation.

    To get a more accurate result you will need to find the refractive front and back powers using the refractive power formula.(note to get the power needed on the back surface you will use the back vertex power formula)
    Then find the radius for each surface, then find the sagittal depth for each surface


    Plus lenses, you find the center thickness by:


    Ct = f1_sag - f2_sag + min. Thickness
    ET = Min Thickness


    For Minus lenses:
    CT = Min Thickness
    ET = F2_sag - F1_sag + min thickness.

    I have worked the steps out for you on my website http://brent-mccardle.org

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    Thanks for sharing the formula.
    I will try it and let here know.

    Quote Originally Posted by lensgrinder View Post
    This formula is the sagittal approximation formula which uses the power to approximate the thickness. You will also need to add the minimum thickness to get a better approximation.

    To get a more accurate result you will need to find the refractive front and back powers using the refractive power formula.(note to get the power needed on the back surface you will use the back vertex power formula)
    Then find the radius for each surface, then find the sagittal depth for each surface


    Plus lenses, you find the center thickness by:


    Ct = f1_sag - f2_sag + min. Thickness
    ET = Min Thickness


    For Minus lenses:
    CT = Min Thickness
    ET = F2_sag - F1_sag + min thickness.

    I have worked the steps out for you on my website http://brent-mccardle.org

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