My novice simulation of lens design parameters..

Discussion in 'Optometry Archives' started by eekthorp, Jan 8, 2007.

  1. eekthorp

    eekthorp Guest

    Hi again guys,

    My last group post a few days ago got the first paragraph cut off,
    sorry about that. Since then I've read some more and played with
    some interesting tools some of you have written, especially Darryl's
    Optical Analysis tools (thanks Darryl and the rest of you for all your
    interesting posts in the groups!). I'm curious what you guys think
    about my (novice but hopefully correct) conclusions/generalizations,
    and sorry for the long post, again..?

    Assuming I'm driving the tools right, for my sph perscription at
    30degrees off-axis most lens designs will have about 0.25D (RMS error)
    of oblique-astig distortion, and this is just a features of having a
    -8D prescription! So, it seems that the Oswalt curve gives best
    off-axis performance, but this isn't necessarily error-free off-axis
    performance. c'est la vie, I guess? I also tried the wollaston
    best-form, but the 'optical analysis' tool gave me a worse rms
    error, shouldn't wollaston give better off-axis performance?
    Hopefully, I'm using the tool right (?!)

    While playing with the tools, optical quality seems to be basically the
    same between the basic design parameters (<0.1D variation over
    materials at 30degrees), so based on this conclusion I might as well go
    with thin, and stick with a good manufacturer..

    I imagine Hoya has a top-notch process(?). So going with Hoya nulux
    (AS) 1.7 w/ a 36 ABBE will get thinner and marginally better CAE than
    poly. Sola Finelite 1.6 offers 42 ABBE, but I'm not sure this small
    ABBE change would be perceptible.. ? So if I go with the 1.7 Hoya, I
    guess the next question is if I should stick with the highly
    recommended Cristal or go with the hoya super-hard AR coat which only
    appears to be avail for a few lenses..

    .........

    And here's my attempt at answering my own questions I asked last
    week..

    * Which lens cut (spheric/aspheric/atoric), if any, improve
    lens-oblique-astigmatism over others?
    Spherics.. at least for people w/o astig

    http://www.opticampus.com/cecourse.php?url=lens_design/
    "[aspherics] Provide both the visual advantages of best form lenses
    and the cosmetic advantages of flat lenses" and "Do not provide
    better optical performance than best form lenses, but simply provide
    comparable performance without the restrictions imposed by best form
    base curve selection".

    I don't see any advantage in a flatter front curve for high myopes,
    so spheric should be 'better'. I'm still not sure why aspherics
    are offered at high myope powers.. I don't see any benefit, unless
    they are rolling off the powers at the edge to further improve
    cosmetics at expense of optics past 30degrees, which might be a good
    reason. (?)

    I assume the local optical optitians, et al, were recommending
    aspherical to improve my vision because they've heard "aspherics
    improve vision over spherics" but not the other half which says
    "when a flater (not-best-form) curve is used with a spherical
    design."

    I'll have to dig more to say anything about what happens when you have
    an astig though. Is spheric-best form still king? I think one of
    Darryl's articles stated that atorics are best, but is that comparing
    atoric to spherical best-form-curves or spherical w/ flatter than best
    form curves?


    * Which types of lens material (high/low index, high/low ABBE) improve
    lens-oblique-astigmatism?
    According to Darryl's Optical analysis tool, index and abbe do not
    impact OAE (again assuming I'm driving the tool right). It appears
    that high-index degrades off-axis error but it shouldn't be
    perceptible.

    I wonder if high-index has a noticeable disadvantage, practically, due
    to physical processing/surface defects having larger impact on lens
    quality? On the other hand, I looked through a poly and hoya 1.67 blank
    off-axis and they were very similar. There's a small chance the
    hoya's marginally better. Not sure what to make of this. Maybe it
    reinforces that error is constant, that manufacturing technique matters
    more than the basic design parameters, or maybe it means looking
    through 60mm blanks at lower -6D 'stock perscription' powers isn't a
    very controlled experiment. :)


    * In addition to ABBE, is high/low index or spheric/aspheric/atoric cut

    better for chromatic aberration?
    CAE is only dependent on ABBE and not cut or index, for a given
    prescription.
    According to Darryl's tools (assuming I'm driving it right, heh).

    So, optimizing ABBE may make some sense, since it's the one quality
    parameter you can have some control over for an ophthalmic lens.


    * Since error should only be about 0.25D, I wonder if my current
    off-axis distortion is due to a lens issue, or maybe the distortion is
    exaggerated since the current pair of lenses are too weak, since it
    sure seems like I'm getting more than 0.25D in my -8D eye..

    Thanks,
    eekthorp
     
    eekthorp, Jan 8, 2007
    #1
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