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Thread: base curve newbie question

  1. #1
    OptiBoard Novice snoopy831's Avatar
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    Confused base curve newbie question

    Hello, I'm not an optician; it would be great if somebody were as
    friendly as to help me with the following question. (Let's assume
    there's no need for any eye correction for simplicity's sake.) I
    wanted to inform myself about how the "base curve" of an eyeglasses
    lens affects the "intensity" in which the lenses are "bent", i.e.,
    you all know there are may sunglasses that are "very bent", i.e.,
    follow the shape of one's head for optimal protection. However
    I found a couple of images where it said "base curve 4" or "6",
    but I couldn't really get an idea which base curve means approx.
    which intensity of "bending". All I was looking for was something
    like:

    [image of almost straight sunglasses lens] -> ~base curve xy
    [image of medium bent lenses] -> ~base curve ..
    [" " strongest bent lenses] -> ~base curve ..

    just to get an idea, so-to-speak.

    Would someone be as kind as to point me to some URL where I
    can find out more, or enlighten me in some other way?

    Thank you very much...

    s.

  2. #2
    Rising Star
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    Dear Snoopy

    As a rough guide I find the most curved sunspecs are usually an 8 base curve
    Compared to say a 2 base which would be quite flat
    hope this helps

    Alan

  3. #3
    OptiBoard Novice snoopy831's Avatar
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    Thank you. I am still utterly confused as to what the
    base curve means. I find *equally* many results in google
    stating "the higher the BC the flatter the lens", and
    "the higher the BC the less flat the lens". My optician
    for intance says the opposite of what you are saying.
    I said I wanted some slightly steeper (more curved) than
    standard lenses, and he said he would then choose 4.5
    instead of 5/6. In the mean time I wanted to check what 4.5
    looks like, but obviously it's total chaos from the
    layman's pov... :-/

    But nevertheless thanks for your help. :-) I don't know
    whom to trust tho...

  4. #4
    Bad address email on file Rich R's Avatar
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    Hi Snoopy

    Base curve or front curve is an optical measurement of the lens where a zero or plano is flat like a table top and the higher the base curve the more curvature. Most glasses with no prescription are usually about a 6 base or about an 8 base on the wrap frames, the important thing for the lens to have no rx on them or plano power, the base or front and back of the lens have the same curve, the front being convex or plus, the back curve concave or minus.

    Rich R.

  5. #5
    Rising Star
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    Dear Snoopy


    I think I see the confusion

    Text book answer to what a base curve is: The lower numerical
    surface power is known as the base curve.

    ie on a +4.00 lens with a
    Front curve of +6.00
    Back curve -2.00 (Both together = +4.00)
    Base curve= -2.00

    On -4.00 lens with a
    front curve of +4.00
    back curve = -8.00
    base curve = +4.00

    However when we order lenses from the lab the front curve is always finished so we call this the base curve
    various front(base)curves from plano to 10.00
    and the back surface is generated to make the correct power

    alan

  6. #6
    OptiBoard Novice snoopy831's Avatar
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    Great, thanks to both of you for your help, I'm now sure that the
    4.5 (for a neutral lens) my optician was about to
    make would have been wrong.

    Since I know exactly what I need (and told him),
    perhaps you are able to solve this quiz...:

    The (neutral) lens is 40mm in width, when lying on the table the
    maximum distance (i.e. in the middle of the lens) from the table
    is 1,45 mm, and the angle at the two points where the lens
    touches the table is 7,2°. Question: what's the approx. base curve?

    (I get some custom-made frames, that's why I need to know, hehe.)

  7. #7
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    The (neutral) lens is 40mm in width, when lying on the table the
    maximum distance (i.e. in the middle of the lens) from the table
    is 1,45 mm, and the angle at the two points where the lens
    touches the table is 7,2°. Question: what's the approx. base curve?
    This would indicate a base of about 4.00, have a look at labcalc, downloadable from this site.

    Bases are front surface powers, if the rear was worked flat then the lens power would be the base (give or take compensation for thickness).

    Rick

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