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Thread: Aberrations and index of refraction

  1. #1
    One eye sees, the other feels OptiBoard Silver Supporter
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    Aberrations and index of refraction

    Assuming best form spherical curves, with Rxs in the -15.00DS to +8.00DS range, is the level of off-axis oblique astigmatism and power error (ignoring spherical aberration and coma) greater with very high index of refraction ophthalmic lenses compared to lower index materials?

    If true, can this be mitigated or eliminated with asphericity?

    Thanks,
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Master OptiBoarder OptiBoard Silver Supporter Barry Santini's Avatar
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    It depends on whether we agree on what constitutes

    "best form" for the particular *index of refraction* you are dealing with. Typical *best form* has been for 1.523/1.53 index, with CR-39's 1.4905 close enough not to matter in FSV (where curves are less "corrected"...meaning less "one-base-for-many powers" that surfaced work in commonly calculated on).

    Again, typically higher-index = flatter front curves for spherical abberation control. Off axis abbs would have to be qualified with respect to the eye's position, i.e., straight ahead or turning off to the side (one influences the peripheral retina, and gazing off-axis influences central acuity.

    I think the lens designer, when working with essentially only two refracting surfaces, must decide which abberations are to have more "weight" (correction), when they are iterating a design.

    I do believe asphericity could help both off-axis power error [correctly defined in optics as *distortion*, or a variation of magnification (focal-length) with aperture] and oblique astigmatism. Again, the lens designer would have to decide what their goal/degree of correction is, and for what field angle.

    Hope this helps. If my facts are off the mark, I'm hoping Darryl Meister will chime in here.

    Barry Santini, ABOM
    Last edited by Barry Santini; 03-27-2007 at 12:29 PM.

  3. #3
    One eye sees, the other feels OptiBoard Silver Supporter
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    Barry,

    Thanks for your help. There is a discussion at sci.med.vision (cross posted to sci.optics) where a gentleman form sci.optics declared that as the refractive index increases, aberrations decrease (except for chromatic aberration). I know this to be true with spherical aberration and coma, but I can't seem to confirm whether this is true for off-axis oblique astigmatism and power error. Darryl's "Optical Analysis" program seems to show otherwise in high minus, although in moderate plus it close, but only if unrealistically steep base curves are used (+18BC on a +8.00DS 1.9 glass).

    Regards,
    Science is a way of trying not to fool yourself. - Richard P. Feynman

    Experience is the hardest teacher. She gives the test before the lesson.



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    Master OptiBoarder Darryl Meister's Avatar
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    As far as the optical aberrations of concern with spectacle lenses go, most notably oblique astigmatism and curvature of the field, I would say that this statement is not true. In a camera, or some other optical system with a flat image plane, curvature of the field may be reduced as the refractive index increases. However, the ideal image plane of the eye is actually curved, since the eye rotates behind the spectacle lens, which means that there must be significant curvature of the field anyway. You can verify using my Optical Analysis program that neither oblique astigmatism nor curvature of the field (mean power error) is lower when the index increases, as long as a suitable Base (front) curve is utilized. Further, as already noted, chromatic aberration generally increases with refractive index (as the Abbe decreases).
    Darryl J. Meister, ABOM

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