glasses & contact prescription differ in opposite directions for each eye

Discussion in 'Glasses' started by burrokeet, Jun 10, 2005.

  1. burrokeet

    burrokeet Guest

    just had an eye exam and got new contacts and glasses

    glasses prescription is:

    R -4.00 -0.25 110
    L -5.00 -0.25 095

    contact prescription is:

    R -4.25 8.6
    L -4.75 8.6

    the contacts are really comfortable vision wise, but the glasses are
    making me feel slightly strained- i am wondering why they strength
    would differ in the opposite direction on each eye? is this typical.
     
    burrokeet, Jun 10, 2005
    #1
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  2. Just looking at the math, at appears that your right contact lens is
    overcorrected compared to your glasses. If the contact vision is
    comfortable, you might need a bit more power on the right side of the
    glasses.
    There are different ways to balance a patients vision. If this was
    determined by testing you in a phoropter, these numbers might be just
    fine. But, just looking at the math alone, you are right...the right
    contact should be a -4.00 to match the specs.
     
    doctor_my_eye, Jun 10, 2005
    #2
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  3. thanks,

    the contacts are actually quite comfortable vision-wise, i notice my eyes feel a bit "strained"
    when wearing my new glasses. it just seemed odd that the difference between the two prescriptions
    wasn't in the same direction. i am going to try the glasses for a few days and see if it is just
    in my head before going back.

    -
    (the testing was done with a phoropter)
     
    Christopher Albin Edmonds, Jun 10, 2005
    #3
  4. burrokeet

    LarryDoc Guest

    The reply by "doctor-my-eyes" is not correct, or at least we can not
    know if it is correct because we do not know what kind of contact lens
    you are wearing and how the fitting curve relates to your cornea curve.
    Further, a contact lens power on the eye is not mathematically
    equivelent to the spectacle power. For example:


    your glasses prescription is:

    R -4.00 -0.25 110>>>>> this relates to a theoretical contact lens power
    of -4.00 or -4.25, the higher power more likely if it is
    silicone-hydrogel type lens. That .25 difference may or may not matter
    in any event.

    L -5.00 -0.25 095>>>>>this relates to a theoretical contact lens power
    of -4.50 or -4.75, because as mentioned above *and* the fact that a
    spectacle lens power measured at 13mm in front of the eye needs to be
    adjusted to less minus power when measured at the cornea (like a contact
    lens) for the same net effective power.

    your contact prescription is:

    R -4.25 8.6
    L -4.75 8.6

    And therefor both prescriptions are darn near the same and likely
    correct, especially if as you mentioned, the practioner measured the
    powers in the phoroptor or with hand held lenses.

    --L.B., O.D.
     
    LarryDoc, Jun 10, 2005
    #4
  5. yep. I said that. I left out that silicone stuff, though. Nice
    rebound. Wanna have a beer? (burp)
     
    doctor_my_eye, Jun 10, 2005
    #5
  6. Hi- thanks for the reply. The contacts in question are Optix O2.

    My curiosity was really why the variation was in the opposite direction, i.e. the -4 for glass went to -4.25
    for the contacts, but the -5 for glasses went to -4.75 for contacts. I would think they would both adjust in
    the same numerical direction, but at the same time I don't understand the third number in the prescription
    (the 110 and 95)
     
    Christopher Albin Edmonds, Jun 10, 2005
    #6
  7. burrokeet

    LarryDoc Guest

    You're welcome. Silicone hydrogel indeed!
    You'd think, but here's why and why not:

    1. The vertex distance alteration in power occurs in powers above + or -
    4.50 diopters at least in any noticable degree.

    2. The "stiffness" of silicone lenses tends to create a plus lens tear
    layer under the lens as the plastic vaults over the cornes apex. Hence
    the corresponding increase in minus to neutralize the tear lens.

    3. The third number in the rx is the astigmatism component, and it is a
    minor amount and of little or not consequence due to the aspheric optics
    and vaulting design of the lens.

    Hope that answers your query!

    --LB, O.D.
     
    LarryDoc, Jun 11, 2005
    #7
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