Astigmatism + Myopia vs Astigmatism alone

Discussion in 'Optometry Archives' started by pershing, Dec 21, 2006.

  1. pershing

    pershing Guest

    Is it true that when calculating the error of an eye, it is ok to say
    that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to
    an astigmatism of -1.00? If not, which option (-075 + -0.25 or
    -1.00)would be considered as "better vision" of the two?
    pershing, Dec 21, 2006
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  2. pershing

    otisbrown Guest

    Dear Pershing,

    An astigmatic value can be converted into a spherical-equivalent.

    What is done is to take 1/2 the astigmatic number and
    add it to the spherical value.

    Is it true that when calculating the error of an eye, it is ok to say
    that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to
    an astigmatism of -1.00?

    Otis> No. That would be like stating that -1.00 diopter spherical
    is the same as -1.00 astigmatism.

    Otis> It would be more correct to say that the spherical
    equivalent would be -0.75 - .25/2 or -0.87 spherical equivalent.

    If not, which option (-075 + -0.25 or
    -1.00)would be considered as "better vision" of the two?

    Otis> The spherical part is determined first, for instance
    if you read the Snellen at 20/70. A Stronger minus lens
    would be used to clear the "best" line possible.
    Once that is done, a cylender lens is used to
    determine if your vision can be made sharper,
    say to 20/15, say a -0.5 diopter lens at 80 degrees.

    That is how your refractive STATE is determined with
    a phoropter.


    otisbrown, Dec 21, 2006
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  3. pershing

    otisbrown Guest


    Is it true that when calculating the error of an eye, it is ok to say
    that an astigmatism of -0.75 + myopia of -0.25 would be equivalent to
    an astigmatism of -1.00?

    A prescription is stated spherical, followed by astigmatism and angle.


    -0.25 diopters, -0.75 at 80 degrees (a typical prescription)

    Spherical equivalent is

    -0.25 - 0.75 / 2 = - 0.62 spherical equivalent.

    Hope this clarifies how you convert spherical/astigmatic
    into spherical equivalent.


    otisbrown, Dec 21, 2006
  4. pershing

    pershing Guest

    If someone has +0.25 myopia and -1.00 astigmatism its spherical
    equivalent is -0.25 right?

    Thanlk you for your help.
    pershing, Dec 21, 2006
  5. pershing

    otisbrown Guest

    That would be correct.

    Perhaps Mike could check your calculation.

    otisbrown, Dec 21, 2006
  6. pershing

    pershing Guest

    Good thanks, In that case I have some more small questions.

    Given In
    section 8 there is a standard for Pilot applicants in regard to Myopia,
    Hyperopia and Astigmatism. I would like to make sure I am interpreting
    the table correctly.

    1. It says that for Astigmatism "Cylinder power not more than 0.75D in
    either eye". If there is no sign in the number, are we talking about
    max being + 0.75 and - 0.75 or there is only a limit on the positive
    side (in which case -1.00 would be acceptable)?

    2. For Myopia "Not more than - 0.25 spherical equivalent in either
    eye". This is a reference to the actual spherical equivalent
    calculation of (Cyl/2) + Sphere as described in above post or is it the
    spherical equivalence of (-0.25/2) + Sphere?

    3. Is the scale allowing for values between 0 and 0.25? I am trying to
    see if (-0.75/2) + 0.175 = -0.25 spherical equivalence is a valid

    Thanks again.
    pershing, Dec 21, 2006
  7. pershing

    pershing Guest

    My current readings from yesterday are:

    1. Right eye: +0.25 (Sphere), -0.25 (Cyl) * 150 , uncorrected vision
    6/6 and no other problems
    2. Left eye: 0 (Sphere), -1.00 (Cyl) * 180 , uncorrected vision 6/7.5
    and no other problems

    I was tested on another occasion about 6 months ago and my reading was
    0 (Sphere), -0.75 (Cyl) in the left eye. Both times the people who
    performed the tests were unaware of the standards I posted, they simply
    made tests.

    You say that a +0.12 (Sphere), -0.75(Cyl) would qualify me. Is there a
    lot of difference between 0 and +0.12? Could this difference be
    associated to measuring instruments? I mean if I try it another time
    could the reading be that different? My eyes have been getting some
    major pounding for the last 3 months (Comp Sc student here) so I little
    bit of brake before the exam may have some influence too.

    The results were not yet sent for official review so I don't know how
    strict they are. I don't know if for example my good right eye could
    compensate for the left eye being slightly worst. I prefer to not take
    any chances and I will try to present my Opt with the standard and see
    if something can be done.

    Thanks for your help
    pershing, Dec 21, 2006
  8. Ok then the R eye is obviously fine. The left is the problem.
    Yesterday's numbers would disqualify on both counts (over .75 cyl and
    sph equiv =-.50), but the other occasion you would pass on the cyl and
    barely fail on the sph. equiv.
    Absolutely. .12 is well within the accuracy of the instrumentation, the
    examiner, and your responses.

    Could help.
    The military doc may well have a bias, and I would tend to think they
    are a bit on the lenient side. Your doc on the other hand if anything
    is biased the other way, esp. if he/she also fits glasses. Sounds to me
    like you'll make it. Avoid as much computer work and heavy reading as
    you can until the official exam.

    w.stacy, o.d.
    William Stacy, Dec 21, 2006
  9. pershing

    pershing Guest

    Other than taking time of books/computer, are there any "eye exercises"
    I can perform to optimize my chances? A quick google query returns some
    results such as this ... but when it looks
    too good to be true it usually is.

    Thank you again
    pershing, Dec 22, 2006
  10. pershing

    otisbrown Guest

    Dear Pershing,

    As suggested by some eye-docs, you could consider
    using a simple reading glass for all close work for
    several weeks prior to an examination. See:



    otisbrown, Dec 22, 2006
  11. pershing

    Neil Brooks Guest


    Otis is our resident 'Net Loon. Please ignore him and you'll be just fine.
    Neil Brooks, Dec 22, 2006
  12. right on. the best thing you could do right now is get as much physical
    activity as you can. Aerobics are definitely in order, and make sure
    your nutrition is good. There may be no discernable effect, but as
    Groucho says, it couldn't hoit.
    William Stacy, Dec 22, 2006
  13. pershing

    pershing Guest

    Roger that. Could passing the exam early in the morning, mid day or
    later at night have some positive/adverse influence? Obviously no
    reading in all cases would apply. Also when you talk about nutrition,
    do you have anything specific in mind? There is the suppl/Vitamin A
    myth (or reality?) going around. Is there any type of food one should

    As usual, thanks again for your input.
    pershing, Dec 23, 2006
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