Focal length of a extremely myopic eye?

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

  1. douglas

    Dr Judy Guest

    The eye does have a focal system consisting of several refracting
    surfaces, but measuring its focal length would require ultrasound
    measure of the surfaces, anterior chamber depth, lens thickness,
    posterior chamber depth and doing some calculations. It is not as
    simple as the focal length of an optical lens, and is not a meaningful

    Dr Judy, Aug 19, 2009
    1. Advertisements

  2. douglas

    Otis Guest

    Mike> Nominally, the focal length ranges from 17mm to 13mm.

    Otis> And the eye's physical length is 24 mm. Please explain this to
    Mr. Doug.

    Otis> Also, the eye and camera form an image at the focal-length
    (object at infinity). So let us call this an analogy.
    Otis, Aug 19, 2009
    1. Advertisements

  3. douglas

    Neil Brooks Guest

    At least Fred Deakins recognized the HUGE amount of information that
    he didn't know.

    You, on the other hand, continue to proffer posts that scream "I'M

    I like to quote them ... so that ... when you delete them (to save
    future embarrassment), they'll be memorialized for all to see.

    Do you know ANYTHING about optics?

    A "catadioptric lens --" to cite but one example -- uses lenses and
    mirrors to achieve a focal length FAR greater than its physical

    Neil Brooks, Aug 19, 2009
  4. douglas

    Jan Guest

    Otis schreef:
    You can't don't you layman Otis?

    Never heard about focal length in air?
    Never heard either, I suppose, of the corpus vitreum which isn't air?

    Try some basic knowledge in optics layman Otis before you try to give
    answers on a topic you know nothing about.

    BTW the index of the corpus vitreum is 1,336.

    Another hint, the corpus vitreum is not a lens, it just lays the second
    focal point of the optical system a factor 1,336 further than it does in

    Does the above ring a bell layman?

    Jan (normally Dutch spoken)
    Jan, Aug 19, 2009
  5. douglas

    Dr Judy Guest

    Some eyes are 24 mm long, some are longer, some are shorter. And the
    focal length of an eye is not the same as its axial length.

    Douglas asked what about his eye's actual focal length as compared to
    the labelled focal lengths of camera lenses and whether it was "fast"
    or "slow", assuming that his refractive error determined those

    Eyes are not easily compared nor equivalent to camera lenses. Focal
    length as compared to a camera lens is meaningless and focal length of
    an eye cannot be calculated from its refractive error. "Fast" or
    "slow" which refer to the range of F-stop, shutter speed and exposure
    times required when taking photos is also meaningless as eyes do not
    have shutter speeds or exposure times and the F-stop (if pupil size is
    considered F-stop) does not vary with refractive error.

    Dr Judy, Aug 19, 2009
  6. douglas

    vistrainer Guest

    Neil Brooks
    Indeed, Neil, the first sign of intelligence. I wish that I could say
    the same for you!

    Care to elaborate on your "theory" of breaks?

    Care to tell everyone here why you have such a command on the "facts"?

    Didn't think so.
    vistrainer, Aug 19, 2009
  7. douglas

    vistrainer Guest

    Jan schreef
    I believe that Otis was the first to point this out in the
    thread....something the OD's (including yourself) failed to do.

    On Aug 18th

    Otis>It is important to understand that the optical length is
    than the physical length. This is because the speed of light is
    slower in water than in air.

    Otis>The length that MIke refers to (17 mm) is the length in a medium
    a refractive index of 1.38.

    Just a suggestion, but you might try reading all the posts before
    launching into a diatribe. By the way, we're talking about basic
    optical properties....a fundamental subject that ALL engineers are
    exposed to in school. Otis spent a lifetime designing optical systems
    for NASA instrumentation. I think he is well qualified to discuss this
    vistrainer, Aug 19, 2009
  8. douglas

    Jan Guest

    vistrainer schreef:
    Than he/you didn't understand either.
    It shows......
    Jan, Aug 19, 2009
  9. douglas

    Otis Guest

    Dear Reader,


    Fred> Otis spent a lifetime designing optical systems
    for NASA instrumentation. I think he is well qualified to discuss

    Otis> I was employed as a professional engineering consultant,
    designing and evaluating
    electronic systems. The optical analysis was part of the study and
    preparation of my
    analytical book on the dynamic properties of the fundamental eye.

    Otis> Additional analysis of the Gullstrand's static model, and the
    natural eye's dynamic behavior can be found at this link:

    Otis, Aug 19, 2009
  10. douglas

    Jan Guest

    vistrainer schreef:
    Wrong and laymen Otis must agree if it's true what you stated below.
    Jan (normally Dutch spoken)
    Jan, Aug 19, 2009
  11. douglas

    Jan Guest

    Otis schreef:
    Bye Fred..........and for Otis indeed it must have been a "part", more
    precisely a tiny part.

    Jan (normally Dutch spoken)
    Jan, Aug 19, 2009
  12. douglas

    Dr Judy Guest

    Assumes one knows how to use matrices to do calculations.

    The other catch here is that three of the refractive surfaces are
    inside the eye so you will need expensive, complex scanning devices to
    measure the curvature of those surfaces. The index of refraction of
    the lens is variable, both throughout an individual lens and from
    individual to individual.

    Dr Judy, Aug 19, 2009
  13. douglas

    Neil Brooks Guest

    Ouch. More insults.

    I'm reeling, Fred.
    No theory. It's the result of testing. The macaques that were forced
    to wear unnecessary minus lenses did NOT shift myopic when they were
    given breaks FROM the unnecessary lenses.

    Do some research. You'll find the tests.
    Care to discuss your wife's bra size ... or ... would you argue that
    it's irrelevant??
    Drop the word "so," and you'll be headed in the right direction.

    One day, you'll drop the ad hominem attacks.

    Or ... not.
    Neil Brooks, Aug 19, 2009
  14. douglas

    Neil Brooks Guest

    More ad hominem crap, huh?

    He's either RIGHT or he's WRONG, regardless of his CV.

    More often than not, he's wrong.
    Neil Brooks, Aug 19, 2009
  15. douglas

    Dr Judy Guest

    That assumes he has refractive, not axial myopia and his eye is the
    average length. If he has an average power (58D) eye but axial length
    longer than 24mm, then the eye focal length wjill be 24mm, with the
    image in front of the retina.

    We cannot calculate his eye focal length without measuring his axial

    Dr Judy, Aug 20, 2009
  16. douglas

    Otis Guest

    Dear Judy,

    With the information Doug provided, I made it clear that you could
    only calculate a RELATIVE distance, for a 24 mm eye.

    At 3.3 inches the image will form on the retina.

    At "infinity" the image will form approximagely 4 mm in front of the

    Thank you for your clarification.

    Otis, Aug 20, 2009
  17. douglas

    douglas Guest

    My axial length has been estimated at 14 mm.
    douglas, Aug 20, 2009
  18. douglas

    Jan Guest

    Otis schreef:
    Jan, Aug 20, 2009
  19. douglas

    Jan Guest

    douglas schreef:
    Who says?
    Has it been measured?

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

    douglas Guest

    No, but my ophthalmologist's colleague estimated it based on my
    refractive error.
    douglas, Aug 20, 2009
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.