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The Basics of Lensometry


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Lensometry

What is lensometry? A lensometer measures vergence and prism power amongst other aspects of a lens. If a patient presents a pair of spectacles to you and does not know the Rx of the lenses, a lensometer will enable you to determine the sphere, cylinder, axis, and prism present in each lens. It can also be used to determine the ADD in a multi-vision lens (bifocals etc).

The fourth assessment of the year at SUNY Optometry tested our lensometry techniques. We were given 10 minutes to neutralize two pairs of single vision lenses. We were required to determine the sphere, cylinder, prism, and DBC. The DBC signifies the distance between centers.

There is no big secret to using the lensometer. The techniques taught in class are more than sufficient to do well on the assessment. For this assessment, it is more important to remember you have plenty of time to accomplish your task, and you must remember to concentrate on the simple things. Record your numbers correctly, and do not forget to denote plus and minus.

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The only other pieces of advice I can truly offer are:

We generally begin with the right lens on a pair of spectacles; however, if there is a notable difference in the powers of the left and right lenses (look at the thickness of the lenses) choose the lens that is more powerful to neutralize first. This will make prism and DBC measurements more precise.

After you place the lens in the lens stop, always turn the focus wheel to -20D and proceed forward (toward the positive) to focus your triple line first.

The American National Standard says to focus from the negative side as opposed to positive.

You will learn mathematically why this is so, but in simple terms, if you start positively, and move toward the negative, you risk arriving at an incorrect measurement of power. From the positive side, you can theoretically accommodate to clear up any blur and thus arrive at an incorrect reading. Focusing from the negative side will prevent this type of error.

Lensometry is not overly difficult. Once again, this assessment tested single vision lenses. Next semester we will explore multi-vision lenses.

In the mean time, if you have any questions or concerns, please do not hesitate to comment!

Regards,

Antonio Chirumbolo


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Antonio Chirumbolo

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  • OPTOM-STUDENT

    Hi-thanks for all the great tips and advice. I have tried Lensometry and find it much easier than manually checking the prescription using single lenses. However, we haven’t covered yet how to determine DBC. Could you explain that? thanks so much

  • ANTONIO CHIRUMBOLO

    Okay, so DBC as you may know stands for the distance between centers. In order to determine DBC:

    Like you normally would in Lensometry, neutralize the right lens. Find out where the middle triple crosses the single, and assuming you can, align that central cross point in the center of the focusing graticle. Then using the ink stamp, mark the lens. (The ink stamp will make 3 dots across the lens).

    Note: If you have ground prism in the lens, you may not be able to center your crossing point in the center of the focusing graticle. Try to bring the crossover as close as possible to the center.

    Next: Keeping the stage perfectly still, slide the glasses over to neutralize the left lens. Once again, attempt to align your single and triple line crossover point in the center of the focusing graticle (without moving the stage!!!) If it isn’t possible to exactly center the crossover point, bring it as close as you can to the center, and once again, use the ink stamp to mark you lens.

    To measure DBC:
    Use a ruler and pick two corresponding points on the lenses to measure distance between. In other words, measure from the center dot of the left lens to the center dot of the right lens, or the nasal dot of the right lens to the temporal dot of the left lens.

    The distance in mm is your DBC.

    I hope that somehow gives you an idea about measuring DBC. It is actually extremely difficult to explain without showing you exactly what I am attempting to describe.

    Be completely honest, if you did not understand a word of my explanation, I would be more than happy to try to be a bit more clear. If you have any questions at all, let me know. I will think about DBC and then perhaps will try to re-explain it in a more clear and concise manner. Let me know.

    -AC

    You can also reach me at antonio.optometrystudents@gmail.com

  • OPTOM-STUDENT

    Hi-i understand perfectly. thanks for that great explanation.

  • THAI NGUYEN

    Great tips on lensometry! At WesternU, we had 3 ophthalmic optics lab proficiencies that required us to determine the unknown Rxs (including add power, prism power and direction) of both bifocal and PALs lenses using the lensometer. We were also tested on how to properly fit both a bifocal and a PAL. Finally, we were assessed on our understanding on how to properly fit and make necessary frame adjustments to both a plastic and metal frames on fellow classmates.

    To determine the ADD power on a bifocal:

    1) Flip the bifocal over so that the lens stop of the lensometer touches the front surface of the lens.

    NOTE: Remember the Base Curve and ADD segment are both located on the FRONT surface of the lens.

    2) Adjust the lens so it touches the distance portion of the lens. I usually start with the left lens.
    3) Record the power reading using the lensometer as you would in a single vision lens. The power reading is known as the Front Vertex Power for the DISTANCE/carrier Rx.
    4) Adjust the lens so that the lens stop is now touching the ADD/NEAR SEGMENT portion of the bifocal lens.
    5) Record the power of this segment using the same procedure as before for the Distance Rx portion of the lens. This power is known as the TOTAL NEAR Power.
    6) To determine the ADD power, use the following equation:

    TOTAL NEAR Power = Distance Rx + ADD power, thus

    ADD power = Total Near Power – Distance Rx

    NOTE: If the Distance Rx contains a cylinder, just take the difference between the spherical portion of the Rx from the Total Near Power.

    7) Repeat for the other lens.

    For example, if the Distance Rx was determined to be +2.00-1.00 x 180, and the Total Near Power was +4.00 from lensometry, then the ADD power would be:

    ADD power = Total Near Power – Distance Rx = +4.00 – (+2.00) = +2.00 D.

    Hope that helps.

    • JAMES MIYASAKA

      @Thai Nguyen, What great tips you gave! These tips helped me pass my proficiency! Thanks again Thai!

    • CHRYSTAL Y

      @Thai Nguyen, Thanx a million, i did great in my practical exam with your help!!!!

  • CONFUSED FIRST YEAR

    we recently learned lensometry, and we learned to start at high plus power and come down…it seems to me that if you started with the minus, you could accommodate and could get an erroneous reading, so starting from plus makes more sense. can someone please clarify? thanks!
    -confused first year

    • http://OptometryStudents.com MATT GELLER

      @confused first year, That is right, you need to come in from the plus side and not the minus side.

      This is simple optics and how + and – lenses work. Your eye can only go from a zero accommodation state and increase accommodation. That is why you can clear a minus lens (by accommodating the exact amount to the lens). For example, when focusing on a target at infinity: put a -5.00 lens in front of your eye then your accommodative system will put in +5.00 therefore a net result of zero… Yet if you put a +5.00 in front of the eye, the eye cannot relax accommodation past zero. It cannot dial in -5.00, it can only relax to zero…. This is a very basic principle and one you should thoroughly understand for clinic and for understanding the basics of refractive error.

      So with lenso, it is an infinite target: if you came from the minus side you could easily accommodate, cancel out the minus and end up with the wrong finding. Coming from the plus side is different, you cannot just clear the plus with your eye.

      Hope it makes sense.
      -Matt

      • ANONYMOUS

        @Matt Geller, Thanks, that’s exactly what I thought. But, Antonio’s article (and the quote from the American National Standard) seemed to be saying the opposite, to start from negative and go towards positive?

  • ANNE

    tanx y’all 4 ur explanations. But pls i have problem readin spherocylindrical and cylindrical lenses and prisms. Can somebody pls explain

  • ANTONIO

    @confused first year,

    You have a sound theory thinking that by coming from plus you cannot accommodate in theory and get an erroneous reading; however, you must remember that a lensometer is a multilens system and therefore, optically, it is not a simple plus or minus lens.

    You need to consider the equation: x = Ft/Fs^2, I am sure you have been taught this equation in learning lensometry.

    If you were measuring the power of a lens and you came in from the plus side, you could actually get divergent rays leaving the lensometer which you could successfully accommodate to and get an erroneous result. Conversely, by coming from the minus side, this would not occur.

    Best way to convince yourself is to play with the optics of the lensometer. Imagine that you had a lensometer with a standard lens of +25. Now you introduced a test lens of +11. See for yourself how the rays would change from divergent to convergent if you came from the plus, ie: +14 or from the minus +8.

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