Focal length of a extremely myopic eye?

Discussion in 'Optometry Archives' started by douglas, Aug 17, 2009.

  1. douglas

    Jan Guest

    douglas schreef:
    Reread the answers already given by Dr Judy about the possibilities to
    measure the real length of your eyeball.

    Jan (normally Dutch spoken)
    Jan, Aug 20, 2009
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  2. douglas

    Otis Guest

    Dear Bill,

    14 mm for focal length? (I assume that is by exact ultra-sound).

    Then since we know that at 3.3 inches the image is on the retina, then
    at "infinity" the image must be 4 mm in front of the retina.


    "optically" his "eye length" must be 14 mm + 4 mm = 8 mm.


    Otis, Aug 21, 2009
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  3. douglas

    Otis Guest

    Dear Doug,


    14 mm for focal length? (I assume that is by exact physical length by
    ultra-sound. You should check this.)

    Then since we know that at 3.3 inches the image is on your retina,
    at "infinity" the image must be 4 mm in front of the retina.

    Ergo: "optically" your "eye length" must be 14 mm + 4 mm = 18 mm.

    This would be a very short eye. But it is necessary to distinguish
    between Optical length, versus PHYSICAL length.

    Otis, Aug 21, 2009
  4. douglas

    douglas Guest

    Wait. It was the radius. So my eye length would be 28 mm.
    douglas, Aug 21, 2009
  5. douglas

    Otis Guest

    Dear Doug,

    It is not likely that they would say that the radius of your eye was
    14 mm, and you diamter therefore would be 28mm.

    Perhaps you should ask your ophthalmologist to refer you to an optical
    text book. He was probably refering to the "calculated", or "optical"
    focal length. This is not the physical length of your eye. As Judy
    pointed out, you would have to use an ultra-sound measurement to
    establish the physical size of your eye.

    Enjoy the technical analysis,
    Otis, Aug 21, 2009
  6. douglas

    Neil Brooks Guest


    If you haven't figured it out by now ... Otis is an idiot.

    Buyer beware.
    Neil Brooks, Aug 21, 2009
  7. douglas

    douglas Guest

    Oh, I've known that for about three years now. Perhaps we should give
    him a pair of +40D lenses and have him cross the street in Bangkok.

    Judy, have you ever had a patient w/ a prescription so high you ran
    out of lenses on the phoropter?
    douglas, Aug 21, 2009
  8. douglas

    Otis Guest


    Brooks already has his +6 diopter lenses -- so he is very happy.
    Except for his foul mouth.

    Otis, Aug 21, 2009
  9. douglas

    Dr Judy Guest

    Yes. The phoropter goes to -20.00. I have had some patients over
    -20. Running out of cylinder is more common. The phoroptor goes to
    -6.00 cyl.

    Luckily, we have add on lenses.

    Dr Judy, Aug 21, 2009
  10. douglas

    Dr Judy Guest

    That assumes the refractive power of his eye is +70.00. As I pointed
    out earlier, we don't know that. He well could have an eye with
    refractive power of +58 and a very long axial length (ie 28mm) which
    would also result in an image 4mm in front of the retina.

    Dr Judy, Aug 21, 2009
  11. douglas

    Neil Brooks Guest

    That is one thing about Otis: he DOES tend to be VERY polite ... much
    like the old pedophile in the Cadillac, trying to lure kids into his
    car for his sick, twisted, and reprehensible agenda.....
    Neil Brooks, Aug 21, 2009
  12. douglas

    Neil Brooks Guest

    I would also think that trial lenses may be useful in these truly
    exceptional cases, though -- at the thin edge of the bell curve --
    you /really/ need to understand vertex distances, etc., to prescribe
    Neil Brooks, Aug 21, 2009
  13. douglas

    douglas Guest

    LOL. Maybe he is one, literally.
    douglas, Aug 21, 2009
  14. douglas

    Mike Ruskai Guest

    Is this something Otis said? He's in my twit filter, so I didn't see.

    In any case, the statement above is not correct.

    Catadioptric systems use lenses and mirrors to focus light. It has nothing to
    do with the length of the system.

    The most common type of catadioptric telescope - Schmidt-Cassegrain - does
    have a shorter tube length than focal length, which may be the source of the
    confusion. The standard design involves a zero-power full-aperture lens
    element with negative spherical aberration, a concave spherical mirror, and a
    convex spherical secondary mirror. Typically, the mirror has a very low focal
    ratio (on the order of f/2), which the secondary multiplies out to f/10 for
    the system. The tube length is around one fifth the focal length.

    Another catadioptic telescope design is Schmidt-Newtonian, which uses the
    zero-power lens with negative spherical aberration to correct a spherical
    primary mirror. The tube length and focal length are about the same, since
    the primary is the only optical element with power (the secondary is flat, and
    serves only to divert the light cone out the side of the instrument). A
    normal Newtonian uses a paraboloidal primary, which corrects for spherical
    aberration, but adds a significant amount of off-axis coma. The SNT mostly
    avoids coma, and a spherical mirror is easier to make (the corrector plate
    itself is relatively easy to make with heat and vacuum deformation).
    Mike Ruskai, Aug 26, 2009
  15. douglas

    Neil Brooks Guest

    Thank you for making MY point....
    Neil Brooks, Aug 26, 2009
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