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.
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!