glasses, prescriptions, and depth of field

Discussion in 'Glasses' started by Robin, Jul 25, 2005.

  1. Robin

    Robin Guest

    I have used tri-focals for the past 2 years, but I don't normally read with
    my glasses on. I do need glasses to see the computer and for distance. The
    lines in the tri-focals drive me crazy, especially the reading lens which is
    too low for me to see through.

    I don't drive. My 19" monitor is 5 feet away. My prescription is:

    OD -2.25 Cylinder -1.75 axis 020 Add 1.50

    OS -1.75 Cylinder -1.00 axis 170 Add 1.50

    Here are my questions:

    Can I get single vision glasses, one eye for distance, the other eye
    corrected for the intermediate lens of the tri-focal? How successful do you
    think that would be? Am I crazy or would that really work?

    As an alternative, how feasable are (read, how happy would I be with)
    single vision glasses, with a weak add, say -.3 or - .5?

    The last option I can think of is to get bi-focals with the intermediate
    prescription and distance prescription, which is what I asked the Dr. at
    Lenscrafters to prescribe. When I got my glasses they were for reading and
    distance, not computer and distance, so I returned them, and want to get my
    lenses elsewhere. I was surprised to find you can get glasses online and
    am thinking about ordering them from FramesDirect for less than half what
    Lenscrafters charged. Does anyone have experience with

    Thank you for helping me with my questions!

    Robin, Jul 25, 2005
  2. Robin

    otisbrown Guest

    Dear Rob,

    I am an engineer -- not an OD.

    If you ignore the astigmatism
    "correction" it looks like your
    near vision should have

    don't drive. My 19" monitor is 5 feet away. My prescription is:

    OD -2.25 Cylinder -1.75 axis 020 Add 1.50

    OD -2.25 +1.5 = -0.75 diopters. This
    sugests that you natural "mono-vision"
    and can read the monitor with no
    glasses. You moght check and
    see if "depth of field" will "take out"
    any residual blur.

    OS -1.75 Cylinder -1.00 axis 170 Add 1.50

    OS -1.75 and "add" 1.5 means
    that your near vision is -0.25 diopters,
    or that basically you could just take
    your glasses off for working at
    20 inches or so.

    Whether you wish to wear glasses
    for "close work" is up to you -- a
    personal decision and judgment.

    Your distant vision requires the
    minus lens prescribed as you

    You will receive other suggestions
    on this site about your choice in
    this matter.


    otisbrown, Jul 25, 2005
  3. Robin

    p.clarkii Guest

    Robin, ignor this reply from Otis
    Otis, your reply is incorrect and virtually illiterate.
    p.clarkii, Jul 25, 2005
  4. Robin

    p.clarkii Guest

    My 19" monitor is 5 feet away

    is your computer monitor really 5 feet away? that quite a long
    distance for a 19" monitor.
    i would consider moving it closer (easier to see details and small
    print) and getting a pair of "computer glasses" that are focused at
    that distance. why not put the monitor at 2 feet?
    this is certainly possible. we call these "monovision glasses". you
    would have to try it to see if it would work for you. alternatively
    you could get a special pair of bifocals (not trifocals) where the top
    part of the lens is set for your computer screen distance and the
    bottom part for standard reading distance. they would not be focused
    at all for distance so they become dedicated "computer glasses". also
    you could get distance glasses with a weak bifocal add that is focused
    on the computer-- the problem is that you have to tilt your head back
    to use the bifocal all the time you are working on your computer and
    secondly you don't have enough power for reading with these glasses on.
    This is the approach you apparently tried at Lenscrafters.

    i wouldn't recommend any online suppliers for bifocal glasses. the
    reason is that the segment height and pupillary distance measurements
    are very important for what you are trying to accomplish. this is
    measured manually at the opticians office and can't be reliably
    determined otherwise. also, you might need to return or exchange your
    glasses if things don't work out and doing so from an online supplier
    could be difficult. otherwise i don't know anything about FramesDirect.
    p.clarkii, Jul 25, 2005
  5. I can't remember the last time I prescribed trifocals with that low of
    an add! It doesn't make sense. Get a pair of regular bifocals with say
    a 1 or 1.25 add, and relatively high set segs so you'll be comfy
    looking at your monitor, assuming you're completely happy with the near
    vision without glasses.

    You could, but it usually doesn't work in glasses (fairly successful in
    I'm sure you mean + here, but this is another compromised situation.
    You'd be better off with an ordinary bifocal for general wear and a pair
    of computer glasses (single vision with say a + 1.00 add over your
    distance refraction.
    You were right on line with my 1st suggestion above. Stupid LC doc
    didn't believe you knew what you wanted. What can I say?

    When I got my glasses they were for reading and

    w.stacy, o.d.
    William Stacy, Jul 25, 2005
  6. I missed that part. That IS a far distance, but unlike the above
    advice, I like the idea. The farther the better in my opinion. For
    that distance, you really don't even need an add. You should be able to
    see it just fine with your distance Rx, or a very slight undercorrection
    (your .25 or .5 add idea makes sense here) in a single vision lens.

    w.stacy, o.d.
    William Stacy, Jul 25, 2005
  7. Robin

    Robin Guest

    Yes, it's 5' from the keyboard, on a buffet with hutch. Recently I moved my
    office to the dining room and need to hide the equipment in furniture that
    opens up to reveal the monitor, printer etc. and I sit at the dining room
    table with a wireless keyboard. My excellent old computer glasses won't
    focus to that distance sharply.

    I didn't think of that, sounds like a pain in the neck!

    Am I correct in assuming single vision distance glasses would have greater
    depth of field than the distance part of a bi-focal in the same frame, due
    to the lens being larger?
    I specifically asked for computer / distance bi-focals, and was dissapointed
    when I received
    the reading / distance glasses. I told her I didn't use glasses for
    reading. When I called her back to tell her I couldn't see the monitor at
    all with them, she gave me a hard time. Very unsatisfactory experience.

    The clerk at the Lenscrafter Optomertist took digital pictures of my
    retina - big bright flashes - and then set me up for the Dr to do the eye
    exam. I was surprised that I couldn't read anything on the eye chart - not
    even the two biggest letters - without correction, and I didn't wonder until
    later if being blinded by those flashes would throw off the eye test.

    Thank you for your quick and knowlegable answers.

    Robin, Jul 25, 2005
  8. No. Lens size has no effect, only lens power.

    w.stacy, o.d.
    William Stacy, Jul 25, 2005
  9. Robin

    Robin Guest

    Thank you, Dr. Stacy.

    What is the "add" for 5' .... .25 or .5 or am I on the wrong track?

    Robin, Jul 25, 2005
  10. Well, mathematically, 5' is close to 2 meters, which equates to .50
    diopters. With an expected depth of focus of at least .25 D., the .25 D
    should theoretically be enough (the calculation simply involves adding,
    algebraically, the add of +.25 or +.50 to the first number, the sphere
    power, of the distance Rx, keeping all other numbers the same). However,
    if you are under 50 years of age, you probably don't need any add, since
    you've still got some focusing ability. If you're over 50, welcome, and
    use the .50 add.

    w.stacy, o.d.
    William Stacy, Jul 25, 2005
  11. Robin

    A Lieberman Guest


    Like Otis says, he is not in the medical profession.

    Please disregard his postings as he is not in the medical profession nor is
    he in any position to give medical advice.

    Thank you!

    A Lieberman, Jul 25, 2005
  12. Dont understand why you need trifocals with a low add.

    If the 1.5 add was set to help you read at 40cm. you would need an extra
    1.00 diopter of accommodation to read at that distance.(40cm.) If half of
    your accomodation is kept in reserve (Ideal case) Then through your bifocal
    segment you should have a range of 66.66 cm to 28.57 cm. You will only need
    to use 0 to 1.50 diopters from your available 2.00 Diopters to have a range
    of infinity to 66.66 cm through your distance portion. a bifocal should
    therefore offer you the range that you need. Provided of course when the
    measurements were made half your accommodation at 40 cm was kept in reserve.

    Roland J. Izaac
    Philip D Izaac, Jul 26, 2005
  13. Robin

    Robin Guest

    I don't know why they were prescribed either, since I don't need glasses to
    I only need them to see clearly from 1.6 meters out to infinity. Maybe I
    should stop frequenting LensCrafters? ;)

    I don't follow. Where do you get the "available 2 diopters of
    accomodation", is that a standard for the human eye?

    Robin, Jul 26, 2005
  14. Robin

    otisbrown Guest

    otisbrown, Jul 26, 2005
  15. Where possible, it is a good idea to keep about half of your accomodation in
    reserve when reading at your prefered distance. I asumed your reading
    distance to be 40cm. at 40cm you need 2.5 Diopters of accommodation. 1.5
    Diopters is provided by the lower segment of your bifocal. therefore you
    need to accommodate another 1.00 Diopter. If only half your accommodation
    is used, then you have the other half in reserve, making two diopters.

    Roland J. Izaac
    Philip D Izaac, Jul 27, 2005
  16. Robin

    Robin Guest

    I received my AARP invitation three years ago :)

    So it's a straightforward .5 diopter add for 2 meters, 1 diopter for 1
    meter, 1.5 diopter for 66 cm etc?

    Robin, Jul 27, 2005
  17. exactly

    w.stacy, o.d.
    William Stacy, Jul 27, 2005
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