spherical aberration and the human eye

Discussion in 'Optometry Archives' started by Liz, Jul 5, 2009.

  1. Liz

    Liz Guest

    Hi Mike, Dick, Wm Stacy, Calli, everyone....

    Hope you are all enjoying summer.

    I'm reading about lens implants for cataracts.
    I saw the aspheric lens implants, and had to search the web to learn
    what "spherical aberration" is.

    Now I'm wondering....
    If spherical aberration is normal for a lens, including those in our
    eyes, then how the heck does the human eyeball normally ever see
    anything???

    Because it sure looks from the diagrams on this topic as though our
    lens, because of its sorta-spherical shape, could never properly focus
    light on one point. (On the retina, or anywhere else).

    I'm curious.

    thanks,
    Liz
    Indiana USA
     
    Liz, Jul 5, 2009
    #1
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  2. Liz

    Salmon Egg Guest

    Most of the time, spherical aberration is not problem. Only in dim light
    with the pupil wide open would that become a problem, In that situation
    you are not using cone vision anyway.

    Most people know about pinhole glasses. The pinholes allow you to get
    away from most of refractive errors. The tradeoff is that not enough
    light gets to the retina in order for it to function well.

    Bill
     
    Salmon Egg, Jul 5, 2009
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  3. Liz

    Liz Guest

    Hi Mike!
    (but how?)
    By "sorta spherical", I just meant that our lens is pretty much convex
    on both sides, the same as the lenses that exhibited spherical
    aberration in the diagrams I saw. My impression was that their
    convexness causes the s.a.. So our eye would have s.a. too.
    Prolate?

    OK, let me try to simplify this down. The answer to my question is
    that our normal lens does NOT properly focus light onto the retina,
    but that our brain learns to interpret the messy image it receives as
    being sharp anyway. Is that right? I still feel a bit confused.

    cheers,
    Liz
    (it seems so strange)
    Indiana USA
     
    Liz, Jul 6, 2009
    #3
  4. Liz

    Salmon Egg Guest

    While a spherically aberrated image on the retina may be perceived as
    being sharp, that is only a perception. I do not know how vision works
    on this level. Information lost by aberration cannot be recovered by the
    brain. It can only be ignored.

    Bill
     
    Salmon Egg, Jul 6, 2009
    #4
  5. Liz

    Liz Guest

    The human eye DOES focus pretty well, especially at the macula. Else how
    Yes, that's what I couldn't understand.
    I think I found the problem. See this picture:
    amazing-space.stsci.edu/resources/explorations/groundup/lesson/basics/
    g11/

    There are two lenses pictured. When I first visited the site, I saw
    only the top lens pictured. Not knowing exactly what "spherical"
    meant, I assumed that a spherical lens was "any kind of convex lens".
    I could see from the upper picture what a focusing disaster that
    was.
    I didn't realize that not all convex lenses are spherical lenses, and
    that it's apparently possible to have a convex lens like the one
    pictured on the bottom - one that DOES focus well.

    So I assume that our eye lenses more closely resemble the bottom
    picture, surely with plenty of imperfections, but not so bad that they
    normally act like the top picture does. It was that top picture that
    had me wondering how on earth the brain could ever deal with such a
    mess!

    More on track?

    cheers,
    Liz
     
    Liz, Jul 6, 2009
    #5
  6. Liz

    Salmon Egg Guest

    To see clearly, the eye turns so that what you do need to see clearly is
    imaged onto the macula. Can you read off-center text?

    Although I do not understand the process of interpreting visual
    information, my guess is that off-macula vision is mainly used to
    determine where to look.

    It is certainly possible to make an aspherical lens element without
    spherical aberration. That is fine for systems like the eye which turns
    to items of interest. For a camera, such a lens will not be all that
    useful because a single aplanatic element will have greater nonspherical
    aberrations.

    Bill
     
    Salmon Egg, Jul 6, 2009
    #6
  7. Liz

    Liz Guest

    The picture exaggerates spherical aberration by illustrating a huge
    OH. Ah hah.
    Whereas in the eye, we have that iris thing going on. :)
    Right. I see it.
    Got it. I suppose there is also a word for it being more rounded on
    the periphery than a sphere... oh, oblate.
    2/3???!!! It's so thin, that's weird. Oh well.

    [snip]

    Thank you Mike. I was really lost there! I feel better when things
    make sense.

    Onwards to the next stage:
    I notice that many lens implants (for use after cataract surgery) are
    described as being "aspheric".
    If this is a new improvement, does that mean that most previous IOLs
    were spherical?
    Does it mean that the aspheric ones are shaped more like the
    biological lens that is currently in my eye than the spherical ones
    are?
    And does THAT mean that I would see better through an "aspheric" than
    through a "regular" IOL?

    cheers,
    Liz
    Indianapolis (a city soon to be viewed through some kind of IOL)
     
    Liz, Jul 7, 2009
    #7
  8. Liz

    Salmon Egg Guest

    Almost all lens manufacture uses spherical surfaces. That is because
    they are easy to manufacture accurately. Camera lenses are made from
    spherical surfaced lens elements. The design is such that aberrations
    including on-axis, off-axis, and chromatic versions are traded off
    against each other to get satisfactory results. ANY lens element not
    made with spherical surfaces is an aspheric. These aspherics are
    difficult to manufacture using conventional grinding and polishing
    techniques.

    A Newtonian telescope mirror, for example, is parabolic. A considerable
    portion of the cost goes into converting a spherical surface into a
    parabola (figuring) and testing the figure.

    For optometric application, lenses do not have to be made as accurately
    as for some other optical applications. Usually, optometric lenses are
    made in much large numbers than high quality lenses. This makes it
    feasible to spend the investment required to fabricate a mold for
    plastic lenses that are aspheric. Once the mold is made, repetitive cost
    can be made very low.

    One form of aspheric lens is called aplanatic. The surfaces can be
    designed so that an object point can be imaged onto an image point in
    monochromatic light with no aberration whatsoever.

    Bill
     
    Salmon Egg, Jul 7, 2009
    #8
  9. Liz

    Liz Guest

    I notice that many lens implants (for use after cataract surgery) are
    Right.... but the lenses they put inside your eye after cataract
    surgery are made of plastic, so perhaps they are mfr'd more easily. I
    sure hope they're made *well*, at least.

    Spheric vs. aspheric?

    cheers,
    Liz
     
    Liz, Jul 8, 2009
    #9
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