Variable amounts of prism in glasses from off centre eye movements.

Discussion in 'Glasses' started by andrewedwardjudd, Apr 10, 2005.

  1. Hi

    Can anybody throw any light on this one?

    I thought that modern glasses were corrected for prism effects but I
    have noticed that I am getting quite noticable amounts of what seems to
    be lateral chromatic abberration in opposite directions when I move my
    head to the left or right.

    These links might give you some idea what i am talking about.

    It is normally hard to notice lateral chromatic abberration but
    longitudinal chromatic abberration is easy to detect with a Cobalt
    filter or mauve coloured filter and is used in sight testing for the
    red green test.

    I was using the cobalt filter and seeing a central red dot and blue
    outer circle and finding that when i move my head the red dot moves to
    the left side or right side of the blue circle.

    Since there is supposedly a 2 diopter difference between red and blue
    in the human eye it seems I am getting a significant prism effect to
    move all of the red dot to the outer edge of the blue circle.

    I get no change at all if i dont wear glasses. The blurred red circle
    stays exactly in the middle of the blurred blue for all head movements.

    Kind of fascinating but I dont like the sound of having built in

    andrewedwardjudd, Apr 10, 2005
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  2. andrewedwardjudd

    Dr. Leukoma Guest

    All lens materials have chromatic aberration. Polycarbonate has the
    most. In general, chromatic aberration seems to increase with the
    index of refraction. Despite the fact that the index of polycarbonate
    isn't all that high, it does seem to be the worst offender.

    Dr. Leukoma, Apr 10, 2005
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  3. andrewedwardjudd

    Dr. Leukoma Guest

    It should also be mentioned that any ophthalmic lens can be thought of
    as two prisms -- either apex-to-apex in the case of minus lenses, or
    base-to-base in the case of plus lenses. This is why decentration
    produces image displacement. Also, any refractive medium causes a
    change in the velocity of light, with shorter wavelengths slowed more
    than longer wavelengths.

    Dr. Leukoma, Apr 10, 2005
  4. andrewedwardjudd

    RM Guest

    I thought that modern glasses were corrected for prism effects but I

    There are prism effects in all glasses. Opthalmic Optics 101. Some
    materials have more problems with this than others, most notably
    polycarbonate. The more off-center the line of site is, the greater the
    problem is.
    Seems a little high to me.
    RM, Apr 10, 2005
  5. As one who uses bichrome often, I think the difference is more on the
    order of .5 D. I ususally find that adding -.25 to the neutral
    (balanced) finding, shifts preference to the green while adding +.25 to
    neutrality shifts preference to the red backgound.

    w.stacy,. o.d.
    William Stacy, Apr 10, 2005
  6. William Stacey Wrote

    order of .5 D.

    Possibly for red to green only that is true.

    Using pure colour diodes red to green difference is 105nm. Red to blue
    is twice that at 230NM. Red to absolute far violet visible light is

    I have never seen a violet diode but i have been told that the light
    refracts so strongly the human eye cannot focus on them clearly.

    When viewing a pure red green blue diode array my tests show that all
    people view blue as a starburst effect at best and often much worse
    than that. Strangle everybody sees the same array with binoculars or
    up close as completely in focus.
    andrewedwardjudd, Apr 10, 2005
  7. RM wrote
    Seems as if the red to blue shift is in the order of .75D

    "If you plot the focal length of a simple lens as a function of
    wavelength, across the visible spectrum, you will find that the
    difference between the minimum focal length (for blue) and the maximum
    (red) is about one and a half percent of the average focal length.
    Thus, a simple lens of nominal focal length 1000 mm might have a focal
    length in the red of 1007.5 mm and in the blue of 992.5 mm."

    so 17mm * 100.75/100 = 17.1275

    17mm * 99.25/100 = 16.875

    With focal length directly proportional to Dioptric power of eye:

    17.1275/.25 = 68.5 so 58D/68.5D difference = .84D
    andrewedwardjudd, Apr 10, 2005
  8. Mike Tyner wrote
    recently and thought it was overstated.

    In fact 2 diopters is correct for the entire spectrum. Refs below.

    If you look thru a cobalt filter at a distant white light you can see
    there is a significant difference for red blue, but subjectively there
    is little difference in red green refraction compared to red blue
    refraction for an array of red green and blue pure color diodes. I
    have heard that Pure violet diodes are impossible to focus upon.
    Presumably its meant that only at reading distances do they become
    clearly focused.

    It seems (from other references) that we see clarity in all this blurr
    because boundaries in colour are detected as interpreted as sharp edge
    by retinal neural processing in the same way that unfocused grids
    appear to be more sharply focused than irregular patterns.

    "an eye correctly refracted for long wavelengths (700nm) will be myopic
    by approximately 2 diopters for short wavelengths (400nm) 3, 4, 5.
    Although this usually goes un-noticed in everyday situations, the eye's
    chromatic difference in refractive error (CDRx) is readily observable
    and exploited in the standard optometric red/green bi-chrome refraction
    test 6 which provides approximately 0.5 diopters difference between the
    typical red and green colors 7."


    4. Bedford RE, Wyszecki G. Axial chromatic aberration of the human eye.
    J. Opt. Soc. Am. 1957; 47: 564-565.

    5. Howarth PA, Bradley A. The longitudinal chromatic aberration of the
    human eye, and its correction. Vision Res. 1986; 26: 361-366.

    andrewedwardjudd, Apr 11, 2005
  9. andrewedwardjudd

    Guest Guest

    For the entire "visible" spectrum this is true.

    When ''white'' light is used to project an object at the retina this
    "bandwith" is also used in adjusting the needed amount of accommodation.

    By focussing the eye to his focal point (no accommodation) the arrays of the
    red light (685 nm) are focussed sharp on the retina, by increasement in
    accommodation, the eye uses his chromatic aberation and use the more shorter
    wavelengts to focus sharp on the retina.

    See the above
    Guest, Apr 11, 2005
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