target refraction

Discussion in 'Optometry Archives' started by Liz, Jun 2, 2010.

  1. Liz

    Liz Guest

    (re: How to choose what focal distance to ask for in cataract

    -0.75. So, if you want to match the first eye exactly, you'd specify

    If it were -1.00, I'd target -0.50 myself.... [because]... I'd be
    trying to choose what is most functional WITHOUT glasses. For me,
    that'd be about -050 in one eye and -100 in the other.

    Ah! Got it. Then you would have a different goal than I do.

    I see the tradeoff.
    For myself, I'd prefer both eyes the same focal distance:

    a) The current focal distance seems to be fine - I can drive and read
    with modest correction, and can see most things around me well enough
    to function without glasses on. If I have to be monofocal, this is a
    good focal distance for me.

    b) I'm eager to have stereo depth perception again. (I still lack
    it after the operation, because the other eye has now become too
    blurry to get good images.)

    c) I'm already used to having both eyes focus together.

    OK. So, if you were to shoot for "identicalness"...
    You'd measure the corneal astigmatism of the second eye, subtract that
    from ...

    -1D ?

    or -0.75D?

    And target the result as the spherical.... right?

    Indianapolis USA
    Liz, Jun 2, 2010
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  2. Liz

    Liz Guest

    I........Most of these calculations are done by averaging the
    astigmatism into a single-number "spherical equivalent" of -0.75.

    Wait, how did you do that?

    Is it
    spherical + cylinder - 1/2[cylinder]?

    0.50 + 0.50 - 1/2[0.5]? Something doesn't look right about that.

    So if someone were -2.50 +0.50,
    then their "spherical equivalent" would be.... oh dear. -2.75,
    because they're
    -3.00 in one axis and -2.50 in the other?

    Liz, Jun 2, 2010
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  3. Liz

    Liz Guest

    So if someone were -2.50 +0.50,

    Is the cylinder always a positive number?

    So that person, who's -2.50 +0.50, would be
    -2.50 in one axis and -2.00 in the other?

    That's the part that's confusing me, is which way the cylinder goes,
    pos or neg.

    Is there ever a scrip such as "-2.50 -0.50"? Or is it always "+0.50"?

    Liz, Jun 2, 2010
  4. Liz

    Dan Abel Guest

    I don't really want to go there, because I always get horribly confused,
    but yes, there is both plus and minus cylinder, but I think not on the
    same machine. ODs use one, MDs use the other. The conversion is not
    Dan Abel, Jun 2, 2010
  5. Liz

    Liz Guest

    Yes, -250-050 can be the same as -300+050.

    COOOOL, Mike, I think I got it now.

    OK, try these examples: :)

    Dick's refraction is -3.00 sph -1.00 cyl (ignore the axis).

    Thus, one of his meridians is -3.00. The other meridian is -4.00.

    His "spherical equivalent" is -3.50.


    Jane's refraction is +2.00 sph +0.50 cyl.

    One of her meridians is +2.00. The other is +2.50.

    Her spherical equivalent is midway between, at +2.25.


    Liz's refraction is -0.50 sph -0.50 cyl.

    Liz's scrip could also be written as

    -1.00 sph +0.50 cyl


    I hope so.

    One thing puzzles me. I tested my spherical equivalent by
    demonstration, by looking at that book cover without glasses, and
    measuring the distance (it was 1 meter).

    So why didn't my scrip turn out to be two numbers that average out to
    -1.00, in agreement with what I saw?
    Why did it instead turn out to be two numbers that average out to

    (I feel SO much better when this all makes sense! Thanks......)

    Liz, Jun 3, 2010
  6. Liz

    Dr Judy Guest

    Mainly because estimating refractive error by measuring distance to
    the far point is not very accurate and does only provide an estimate.
    Getting it within 0.25D is better than one would expect.

    Dr Judy, Jun 3, 2010
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