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Thread: Face-Forming

  1. #1
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    Confused Face-Forming

    i want to some explanations about the Face-Forming ( Fannin page 283). I've read that we must set 0,37 prism base out, but i dont agree with this theory. I think that we have to calculate just like we do with the Pantoscopic tilt, the same theory.

    What do you think?

    thanks to everybody!!

    Até + ( see you)

    Edgley

  2. #2
    Master OptiBoarder Darryl Meister's Avatar
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    Kansas City, Kansas, United States
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    Hi Edgley,

    Unless the frame has a significant amount of face-form, such as is the case with "wrap-around" sunglasses, it probably won't be an issue with the vast majority of prescriptions. You can decenter the lenses out to prevent the induced astigmatism (caused by the lens tilt), but this will obviously cause some residual prism: base out for plus lenses and base in for minus lenses. This horizontal prism may be a bigger issue than the actual induced astigmatism it was meant to prevent (particularly in minus lenses, since it would require the eyes to diverge to fuse). The good news is, if I remember correctly, that face-form tilt will also induce some base-out prism (with or without decentration), which will offset some of the base in prism for minus lenses.

    Consequently, compenstating for face-form tilt is probably unecessary in most cases. The best approach when it is necessary is to change the Rx to compensate for the induced astigmatism, but -- again -- the amount of compensation is generally quite small for most prescription and frame combinations. However, this will circumvent any prism issues. A conversation with your lab might be appropriate in those rare situations that require the use of high powers and/or wrap frames.

    Best regards,
    Darryl
    Last edited by Darryl Meister; 02-18-2002 at 02:26 AM.

  3. #3
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    FRAME WRAP ANGLES ACCORDING TO THE LENS SHAPE
    please help

    thanks


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