Focal length of a extremely myopic eye?

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

  1. douglas

    douglas Guest

    What's the focal length of my eye (prescription -11.25 -2.50x178)? Is
    that the range between my near point and far point?
    douglas, Aug 17, 2009
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  2. douglas

    Dan Abel Guest

    1. Don't know.
    2. No. The range between your near point and far point varies. Those
    numbers have nothing to do with it.
    Dan Abel, Aug 17, 2009
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  3. douglas

    Otis Guest

    Dear Bill,

    The standard power of the "normal" eye is given as about 60 diopters.

    Otis, Aug 18, 2009
  4. douglas

    Otis Guest

    Dear Bill,

    If memory serves me correctly, the "standard power" of the eye is
    given as about 60 diopters.

    Otis, Aug 18, 2009
  5. douglas

    Dr Judy Guest

    Not sure what you mean by "focal length of my eye". Do you mean the
    axial length? You can't calculate your axial length from your
    prescription, you need to have it measured.

    Dr Judy, Aug 18, 2009
  6. douglas

    douglas Guest

    Focal length, like on a camera. Would a severely myopic eye be like a
    telephoto or a wide-angle, fast or slow lens? What exactly does the
    value in mm mean? The longest distance before an image goes out of
    douglas, Aug 18, 2009
  7. douglas

    Dan Abel Guest

    Focal length (FL) is a pretty common optical term. It should be
    measurable. I suspect it's pretty useless, though. Most people, unlike
    me, have accommodation. You are the expert here, and perhaps there are
    other factors. What difference does it make? If it gets measured at
    one point in time, and you look at something at a different distance, it
    won't be the same.
    I had mine measured. I don't know what it is. The surgeon needed to
    know, for my cataract surgery.
    Dan Abel, Aug 18, 2009
  8. douglas

    douglas Guest

    Focal length is the reciprocal of the lens strength. Minus lens make
    it shorter, plus lenses make it longer. It moves the near/far point of
    your eyes. I think
    douglas, Aug 18, 2009
  9. douglas

    Otis Guest


    There are two "lengths" to consider.

    In the normal adult eye, the power is given as 60 diopters (in some
    texts, 58 diopters.)

    The average phycial length is about 2.4 cm.

    The OPTICAL length is about 1.7 cm.

    You can calculate both these dimensions as a difference from 58

    It took about a -12 dioper lens to "Clear your Snellen", so your total
    power is calculated as:

    58 diopters + 12 diopters = 70 diopters.

    Your requested dimensions can be calculted from 70 diopters. (See my
    book for these equations.)

    The "range of accommodation" depends on age, and for the young can be
    as much as 12 diopters (great variation).

    As you get older that "range" decreases, so that at age 50 it may be
    as little as 1 to 2 diopters.

    You could also determine this by objective testing.

    Otis, Aug 18, 2009
  10. douglas

    Otis Guest


    With the information you provided, you can not calculate the "length"
    of your eye -- if that is what you are asking for.

    But the normal adult eye has a length (phycial) of 2.4 cm, and a power
    of 58 diopters. (Standard value, provided by Gullistrand.)

    With your "nearsighted" eye, you would be able to see clearly at 40
    inches / 12 = 3.3 inches.

    Move the object away to the distance, and the image will form in front
    of the retina.

    This means that, with the value you provided (12 diopters spherical
    equivalent), your focal length is:

    Power = Index-of-Refraction / Distance

    Distance = Index / Power

    Distance = 1.38 / 70

    0.02 meters (approximate)

    Thus, your eyes with a power of 70 diopters will form a image 0.024 -
    0.020 = 0.004 meters in front of your retina.

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

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


    Otis, Aug 18, 2009
  11. douglas

    Dr Judy Guest

    The camera lens analogy doesn't really work for eyes, the properties
    of camera lenses cannot be compared to the properties of an eye.

    The size (mm) of a camera lens refers to the distance from the front
    of the lens to the film plane of the camera which determines how wide
    the field of view will be on the photograph. A 50mm camera lens
    captures a field that is roughly eguivalent to the field of view of
    the human eye. Thats why a 50mm lens is often called "normal", A
    camera lens that captures a wider field is called wide angle, a camera
    lens that captures a smaller field is telephoto. The human eye cannot
    vary its field of view, it does not vary much between individuals and
    does not vary as a function of refractive error.

    "Fast" and "slow" with regard to camera lenses refers to how wide the
    F-stop can be set when using with the lens, this number depends upon
    the sizes and propeties of the various parts of the lens system. The
    human eye "f-stop" is the pupil which varies in size depending upon
    the ambient light level. Pupil size does not depend upon your
    prescription either.

    Anyway, your eye, like the eye of all humans, is roughly like a 50mm
    camera lens.

    With regard to he eye far point is how far something can be from your
    eye and still seen clearly. Near point is how close something can be
    to the eye and still be seen clearly.

    I assume you mean with glasses off. The easiest way to determine far
    and near point is to measure them, use small print (ie back of a
    medicine bottle) and move it away until it just blurs, then move it
    closer until it just blurs. The away distance is far point, the close
    distance is near point.

    If you want to calculate it:
    First of all, you have a large amount of astigmatism which means that
    things will not be really clear at any distance with glasses off.
    Disregarding the astigmatic blur, your "spherical equivalent" myopia
    is -12.50. Far point, from the spectacle plane, then is 1/12.5metres
    or 8cm, that would be about 9cm from your eyes. This is an estimate,
    as far point also depends upon pupil size.

    Near point depends upon how much accommodation you have which depends
    on individual factors like your age, so cannot be calculated and can
    only be measured.

    Dr Judy, Aug 18, 2009
  12. douglas

    Neil Brooks Guest

    Several posts, on this thread, appear to be replies to a post by Otis

    Oddly, though, I cannot find that original Otis post.

    Usually, this happens when Otis realizes (because it was pointed out
    to him, repeatedly, by credible participants) that his post removed
    ANY lingering doubt about his idiocy.

    Otis?? Did you make yet ANOTHER absolutely idiotic post, and then
    realize it ... and then DELETE it????

    Just another day in Insane Uncle Otie-ville.
    Neil Brooks, Aug 18, 2009
  13. douglas

    Otis Guest

    Dear Mike,

    The calculated powers provided are all postive, not "too negative".

    Otis, Aug 18, 2009
  14. douglas

    Otis Guest

    Dear Mike,

    You fail to understand the term, "relative".

    The descriptive words "minus or plus" depends on the coordinate system
    you choose to use.

    Relativity and Science Best,
    Otis, Aug 18, 2009
  15. douglas

    Neil Brooks Guest

    I sure understand it.

    You hold your "relatives" (Keith and Joy Benson) to be examples of the
    use of plus lenses and the avoidance of minus lenses.

    You hold Keith out as a success story, in spite of the fact that
    evidence that YOU presented shows that only 15 in 100 children BECOME

    You also fail to mention Joy ... because ... despite your protocol ...
    she's a myope with a restricted driver's license.

    That's "relative."

    What's absolute -- on the other hand -- is that you are an idiot :)
    Neil Brooks, Aug 18, 2009
  16. douglas

    BD Guest

    To paraphrase Ed, from Shaun of the Dead - Yeeaah, boyeeee!!
    BD, Aug 19, 2009
  17. douglas

    Dr Judy Guest

    Focal length gets shorter with increased lens power, whether plus or

    Focal length of a lens has no effect on the far/near points of the
    eye. The far/near points of the eye are determined by the refractive
    power, axial length and amplitude of accommodation of the eye.

    Dr Judy, Aug 19, 2009
  18. douglas

    Dr Judy Guest

    Yes it is and used with reference to optical lenses, not with
    reference to the eye.

    Dr Judy, Aug 19, 2009
  19. douglas

    douglas Guest

    The eye does not have an optical lens system? I'm positive you can
    apply focal length and f-stop to the eye.
    douglas, Aug 19, 2009
  20. douglas

    Otis Guest


    Douglas> The eye does not have an optical lens system? I'm positive
    you can
    apply focal length and f-stop to the eye.

    Otis> You are correct. The eye DOES have an optical lens system.
    The eye doe have a calculated focal "length".
    Otis, Aug 19, 2009
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