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Thread: decentration of progressive lenses

  1. #1
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    decentration of progressive lenses

    Every progressive lenses are decentred about 2.5mm.
    I am wordering why ?
    I am aware that it helps to achieve a bigger diameter, but i am not able to figure out why they are decentred, why was the value of 2.5mm choosen and why it is used by every progressive's producer.

    What about thickness that is affected by decentration? Is eliminated by prism thinning at all?

  2. #2
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    When the fitting point on a lens is decentered, that is done to give the lens a better range of frame sizes. Not every lens is decentered.

    On the other hand most progressives have a near inset of 2.5. Do not confuse the two. As you think some have the fitting point decentered as well. The fitting decentration will not effect the thickness on a properly ground lens. When you chose a frame and the lens is decentered to the proper placement for the patients PD, that is what effects the thickness. Has nothing to do with the semi finished blank placement of the fitting cross.
    Prism thinning has nothing to do with decentration. Prism thinning is to balance the thickness between the top and bottom of the lens. There is more plus power at the bottom(in most cases). If you did not grind base down prism the lens would be thicker on the top. Grinding the prism equalized the thickness difference, top/bottom,allowing you to grind a thinner lens (to a point, you must keep in mind that RXs are often different from eye to eye. The lab should calculate the best balance for the two lenses.)
    It may help you to under stand if you take a close look at an executive type lens. This is easier to see there. (You may also use this tecnique on executive lenes.)

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