Front focal point measurememt.

Discussion in 'Optometry Archives' started by Don W, Sep 25, 2006.

  1. Don W

    Don W Guest

    Appreciate suggestions as to how to measure the distance of the front focal
    point, accomodated or not, off the front surface of the cornea.

    Don w.
    Don W, Sep 25, 2006
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  2. Don W

    otisbrown Guest

    Dear Don,

    I am not certain what you are asking for.

    There are several simple means to determine "blur point" for
    the eye. They are:

    1. Using a trial lens kit, and a Snellen, read the chart
    at say 20/70. Using stronger minus lenses, determine the minus lens
    that clears the 20/20 line (about -1.25 to -1.75 diopters). Thus
    your refractive STATE would be of that value.

    2. Assuming you are at about -2.0 diopters, then simply
    get some printed material, and begin moving it away from
    your face. When the reading starts to blur, at say 20 inches,
    this is an APPROXIMATE measurement of your
    eye's refractive state.

    3. (You need an OD to measure this way). Using "drops" of
    cyclogel, atropine-sulfate, paralize your eyes. Then get
    a retinoscope and make the measurement using that method.

    All of the above will give different values of refractive STATE.

    In items 1 and 2, accommodation is "active". In item
    3, accommodation is frozen.

    Hope this clarifies your question -- approximately.

    otisbrown, Sep 25, 2006
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  3. Don W

    Don W Guest

    Thanks for the link. But I am not sure that is quite that. Is this not
    the case, if a pinhole is at the anterior focal point, of the total optical
    of the eye, then all rays, at whatever angle entering the pinhole will
    exit into the vitreous parallel (and enter the retina perpendicularly). I'm
    assuming that there is not
    a point source on the other side of the pinhole, just say looking at the
    blue sky. I do know, from my own experience, that I can spot floaters doing
    this, where I can't see them without the pinhole. But determining the
    distance to the anterior focal point is still a question. Dr. Folk says
    it's 13mm off the cornea That distance seems to be quite large.

    Don W.
    Don W, Sep 28, 2006
  4. Don W

    Don W Guest

    The anterior focal point is the anterior focal point. Not my definition
    but I had found it used in the Bennett and Rabbetts' book on "Clinical
    Visual Optics", the entoptic chapter and also in Adler's "Physiology of the
    Eye", also the entoptic chapter, describing the same effect, that is of a
    light source emanating at the "anterior focal plane". The effect of rays
    emanating from this point produces parallel rays within the eye. Their
    drawing not mine. Also in references (Smith's book on the eye) are shown
    Gullstrand's eye where the two anterior focal points are shown, for the
    accommodated and (of course) non-accommodated eye.

    The only computation of this distance I found was in Dr. Folk's web site.
    I quoted as 13mm, should have been 17mm (his numbers - sorry). His site:

    and then go down to

    The raytracing would be complicated. A matrix multiplication would be
    easier. But all I was interested in is experimentally verifying the 17mm
    (or whatever) number.
    Don't agree that the smallest pupil acts like a pinhole.

    Don W.
    Don W, Sep 29, 2006
  5. Don W

    Jan Guest

    Don W schreef:
    What you are searching for is the "nodal point"

    In Gullstrands eye about 17mm in front of the retina.

    To be more precise there are two, an anterior and a posterior one
    respectively 7,1 mm and 7,2 mm behind the cornea front.

    Jan (normally Dutch spoken)
    Jan, Sep 29, 2006
  6. Don W

    Don W Guest

    I think this statement is true. In any complex optical system, like what
    we are talking about, the eye, it can be described (for example) by a set of
    matrices. And when these matrices are multipled together, it results is a
    single matrix describing a singular thick lens equivalent. And that thick
    lens equivalent has two nodal points, etc, and _also_ two focal points. I
    am refering to the anterior (left) focal point/plane.


    Don W.
    Don W, Sep 30, 2006
  7. Don W

    Don W Guest

    I don't think nodal points have anything to do with this thread. They are

    I agree that the nodal points do not help with this definition. The
    principal planes/points do. I am not sure of the reverse ray tracing of the
    image from the retina to determine the anterior focal point is without
    confusion. The whole thread was not that this point exists, or how "useful
    it is", but the measurement to it.

    Don W.
    Don W, Sep 30, 2006
  8. Don W

    Jan Guest

    Don W schreef:
    I might say my answer was not what your looking for.

    The trigger for me to mentioning "nodelpoint" is your data "17 mm"

    It is not completely clear what you want.
    Anyhow here is a url of the optics in Gullstrand's eye.

    Take a special look to the posterior and anterior principal points of
    the overall system.

    Where can I find this debate/discussion on pinholes in combination with
    the optical system of the human eye, maybe it enlighten me a bit?

    Jan (normally Dutch spoken)
    Jan, Sep 30, 2006
  9. Don W

    Don W Guest


    Yes, I had seen that Zeiss site before (thank you) but it is the F1
    Anterior Focal Point (far left in this site's drawing) that is under
    discussion here (no debate is ever intended). The above two mentioned
    references are the only two that this specific effect is discussed.
    References to eye and pinhole effects are pretty diffuse throughout the
    literature, far as I know.

    Don W.
    Don W, Oct 1, 2006
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