Accurately measuring dioptre of specs

Discussion in 'Optometry Archives' started by winke, Sep 7, 2007.

  1. winke

    winke Guest

    I have a dozen pairs of ready-made reading glasses.

    Most of them no longer have the strength of the lenses marked on them.
    I do recall that they were originally marked to within 0.25 dioptre.

    How can I work out the dioptre value of each pair using home equipment?
     
    winke, Sep 7, 2007
    #1
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  2. winke

    Mas Plak Guest

    dont need any.

    just hold the lens close and move it slowly away from you till you get a
    blur circle, take that distance and devide it by one meter.
    1 dioptre is one meter. 2 dioptre is at 1/2 meter
    Blur circle is when the image in the lense has blured to a constant value
    across the lens, no image remains.
     
    Mas Plak, Sep 7, 2007
    #2
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  3. winke

    Uncle Al Guest

    By what mechanism do you propose the lenses changed their curvatures
    or refractive indices? Short of cooking in an oven even plastic
    lenses will remain constant over centuries of time. Do you open your
    cupboard to find cheap plastic film drinking cups have deformed?
    Plastic lenses are thicker and stiffer.
     
    Uncle Al, Sep 7, 2007
    #3
  4. winke

    Androcles Guest

    [snip drool]

    --


    'we establish by definition that the "time" required by
    light to travel from A to B equals the "time" it requires
    to travel from B to A' because I SAY SO and you have to
    agree because I'm the great genius, STOOOPID, don't you
    dare question it. -- Rabbi Albert Einstein

    http://www.androcles01.pwp.blueyonder.co.uk/Smart/tAB=tBA.gif

    'we establish by definition that the "time" required by
    light to travel from A to B doesn't equal the "time" it requires
    to travel from B to A in the stationary system, obviously.' --
    Heretic Jan Bielawski, assistant light-bulb changer.

    Ref:

    "SR is GR with G=0." -- Uncle Stooopid.

    The Uncle Stooopid doctrine:
    http://sound.westhost.com/counterfeit.jpg

    "What can be asserted without evidence can also be dismissed without
    evidence." -- Uncle Stooopid.


    "Counterfactual assumptions yield nonsense.
    If such a thing were actually observed, reliably and reproducibly, then
    relativity would immediately need a major overhaul if not a complete
    replacement." -- Humpty Roberts.

    Rabbi Albert Einstein in 1895 failed an examination that would
    have allowed him to study for a diploma as an electrical engineer
    at the Eidgenössische Technische Hochschule in Zurich
    (couldn't even pass the SATs).

    According to Phuckwit Duck it was geography and history that Einstein
    failed on, as if Eidgenössische Technische Hochschule would give a
    damn. That tells you the lengths these lying bastards will go to to
    protect their tin god, but its always a laugh when they slip up.
    Trolls, the lot of them.

    "This is PHYSICS, not math or logic, and "proof" is completely
    irrelevant." -- Humpty Roberts.
     
    Androcles, Sep 7, 2007
    #4
  5. winke

    otisbrown Guest

    Dear Winke,

    Subject: The physics of a plus lens -- optical power

    The power of a lens is given in diopters.

    The relationship is:

    Diopters = 1 / Focal-length

    For instance, a focal length of 0.5 meters
    will have a power of 2 diotpers.

    The easy way, is to just take the lens outdoors,
    and hold it above a paper.

    If the lens forms an image of the sun at
    0.5 meters -- the power is 2 diopters.

    If at 1 meter, the power is 1 diopter.

    Simple as PI.

    Best,

    Otis
     
    otisbrown, Sep 7, 2007
    #5
  6. winke

    lena102938 Guest

    Well, ODs can do it just looking on glasses or putting
    them on with rather good precision.
     
    lena102938, Sep 9, 2007
    #6
  7. winke

    Benj Guest

    CRIPES! Do NOT apologize to Uncle Al. The guy really knows how to
    write and lie with words, but unfortunately hasn't learned how to read
    them yet! The rest of us knew exactly what you were saying. It was
    completely clear.

    By the way if you want an accurate measurement of Diopter values take
    your box of glasses to any place that sells glasses (prescription
    glasses not a drugstore) and they will have a machine to do it. It
    sort of looks like an overgrown microscope. You put the glasses in the
    bottom and there is this large wheel on the side you turn graduated in
    diopters. Works like a champ.

    By the way, in trying to use the "focusing the sun on a piece of
    paper" method, be informed that the paper will likely catch fire!
     
    Benj, Sep 9, 2007
    #7
  8. winke

    Neil Brooks Guest

    X-No-Archive: Yes

    Every NG has its own Otis Brown, eh?

    Sorry....
     
    Neil Brooks, Sep 9, 2007
    #8
  9. winke

    p.clarkii Guest

    the only reasonable way using home equipment is exactly as described
    by Otis' reply to your message a couple of days ago. go outside, hold
    the lens up at the height that results in a sharp point focus of the
    sun, measure that distance in meters, and then calculate the dioptric
    power. dioptric power = reciprocal of the focal distance measured in
    meters.

    opticians and eye doctors use an apparatus called a lensometer to
    precisely measure lens power.

    there are other ways as well, such as trial lens neutralization, etc.
    but these require a little more than "home equipment".
     
    p.clarkii, Sep 9, 2007
    #9
  10. winke

    Androcles Guest

    : >
    : > How can I work out the dioptre value of each pair using home equipment?
    :
    : the only reasonable way using home equipment is exactly as described
    : by Otis' reply to your message a couple of days ago. go outside, hold
    : the lens up at the height that results in a sharp point focus of the
    : sun,


    Doesn't work for concave lenses.
     
    Androcles, Sep 9, 2007
    #10
  11. Reading glasses - which the OP was talking about - are not concave.
    So what is your point?

    --
    Nicolaas.


    .... Some people post not because they have something to say, but because
    the have to say something .... anything.
     
    Nicolaas Hawkins, Sep 9, 2007
    #11
  12. winke

    Androcles Guest

    : On Sun, 09 Sep 2007 16:45:07 GMT, Androcles <>
    : wrote in <:
    : > : >:>
    : >:> How can I work out the dioptre value of each pair using home
    equipment?
    : >:
    : >: the only reasonable way using home equipment is exactly as described
    : >: by Otis' reply to your message a couple of days ago. go outside, hold
    : >: the lens up at the height that results in a sharp point focus of the
    : >: sun,
    : >
    : > Doesn't work for concave lenses.
    :
    : Reading glasses - which the OP was talking about - are not concave.
    : So what is your point?

    Mine are, although I read without them in bed, book 6" from my nose.
    For the computer monitor, concave.
    So what is your point, idiot?
     
    Androcles, Sep 9, 2007
    #12
  13. winke

    Androcles Guest

    : On Sun, 09 Sep 2007 16:45:07 GMT, Androcles <>
    : wrote in <:
    : > : >:>
    : >:> How can I work out the dioptre value of each pair using home
    equipment?
    : >:
    : >: the only reasonable way using home equipment is exactly as described
    : >: by Otis' reply to your message a couple of days ago. go outside, hold
    : >: the lens up at the height that results in a sharp point focus of the
    : >: sun,
    : >
    : > Doesn't work for concave lenses.
    :
    : Reading glasses - which the OP was talking about - are not concave.
    : So what is your point?

    Mine are, although I read without them when in bed, book 6"- 9" from
    my nose. For the computer monitor I wear concave lenses, for driving
    I have varifocals. Doesn't work for concave lenses, imbecile.
    So what is your point, fuckhead?
     
    Androcles, Sep 9, 2007
    #13
  14. winke

    otisbrown Guest

    Subject: PClar is correct.

    The original poster asked only about plus lenses.

    Not a minus lens.

    If you are curious, just hold a minus lens above a sheet
    of paper.

    Now slowly lift the minus lens off the paper. The
    light will "spread" -- and no image can be formed.

    It would be possible to obtain some low-cost
    minus lenses from Zennioptical.com
    for about $5 for each one.

    What Pclar was referring to was the fact that
    a +2 diopter lens can be neutralized by a -2 lens.

    For thin lenses the powers "add" when they are close
    together.

    Just basic science and optical physics.

    Otis
     
    otisbrown, Sep 10, 2007
    #14
  15. winke

    Benj Guest

    And also why people working with lenses use "diopters"

    Note that if you have a positive lens with a known diopter value that
    is stronger than the negative lens, if you put the two together it
    will form an image that focuses and you can measure the diopters of
    the combo. Hence by the above rule you can find the diopters of the
    negative lens.
     
    Benj, Sep 10, 2007
    #15
  16. winke

    lena102938 Guest

    Yes, certainly ,it is the rule how Mystery of minus lens was
    resolved.
     
    lena102938, Sep 10, 2007
    #16
  17. winke

    Dan Abel Guest

    I watched my father do this. It seemed to work. There was little
    chance of a fire. The circles were quite large. I doubt that an OTC
    reading glass could cause a fire.
     
    Dan Abel, Sep 12, 2007
    #17
  18. winke

    lena102938 Guest

    To be precise " reading PLASTIC"
    Regards,
     
    lena102938, Sep 12, 2007
    #18
  19. winke

    lena102938 Guest

    Well. ok
    Lena.
     
    lena102938, Sep 13, 2007
    #19
  20. winke

    otisbrown Guest

    Dear Lena,

    Subject: Using a reading glasses to ignite paper.

    In the prosses of determining focal length, I testes quite
    a few reading-glasses.

    I could not get the paper to ignite.

    Using a 4 inch magnifying glass (with a 20 inch focal length)
    concentrated enough flux on a point to ignite the paper.

    Best,

    Otis
     
    otisbrown, Sep 13, 2007
    #20
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