Nov
2005

Shake-a-Lens




A recently dispensed patient returns waving his new eyewear and complaining, “I can’t see with these darn things.” Wouldn’t it be nice if just a wave of the hand would diagnose that patient’s problem and other post-dispensing issues? Sometimes all you need to do is shake a lens. Shaking lenses is an old, but still useful technique. It’s both a parlor trick and a useful tool. It’s based on the relative motion of objects seen through a moving lens and the image displacement that occurs when viewing “off axis.” Shaking a lens allows you to determine whether a lens is a sphere or a cylinder. It allows easy identification of plus and minus power and permits you to locate optical centers. It makes it possible to differentiate between spheres and cylinders. If the lens in question is a cylindrical lens, it permits you to locate both major meridians and will reveal which is the plus axis and which is the minus. Best of all, shake-a-lens is easy to use and easy to teach. In almost every dispensary there’s a new staff member who needs some “inside info” to build their confidence and introduce them to the optical mysteries surrounding beginners in ophthalmic practices. Knowing a little about shake-a-lens could be exactly the needed “inside info” to build their confidence.

Starting With Spheres
When shaking a lens, the lens is held at about a normal reading distance. The distance to the object being viewed (i.e. object distance) should be fairly long for lower power lenses. Ten to 12 feet works well. A shorter object distance is better for higher power lenses. Your new employee should stick to powers of plus or minus two diopters or so until he or she gains a little experience.

Rule One
Minus power causes objects to appear to move in the same direction as the lens is being moved. This is called “with motion.”

 

Rule Two
Plus power causes objects to appear to move in a direction opposite to the direction in which the lens is being moved. This is called “against motion.”

Rule Three
If light rays from the object you are viewing pass through the optical center of the lens, the image you see will not be displaced If you put a ruler against the lens in this position and draw a line over the image of the edge of the door, that line will pass through the optical center of the lens. If you then rotate the lens by 90 degrees and draw a second line over the image of the door edge (still positioned so that it is not displaced), the point at which the two lines cross is the optical center of the lens. But what if it’s not a sphere? If you rotate the lens a few degrees counterclockwise and if the edge of the doorway as seen through the lens rotates counterclockwise, you have “with-motion rotation,” and the meridian you are testing is the minus axis meridian. If the axis rotates clockwise, you have “against-motion rotation” and the meridian you are testing is the plus axis meridian.

Prism
In higher powers prism is revealed by the optical center’s location, but with lower powers there may be no optical center to locate. It is useful to remember that images through a prism are displaced in the direction of the apex. For example, a lens calling for base down prism will displace objects viewed through the MRP upward.

 

Putting Shake-A-Lens To Use
Of course there are many things that might cause a patient to claim new eyewear is unwearable. Shake-a-lens cannot reveal issues of frame choice, uncomfortable fit, incorrect base curve or many other problems, but it does give you something to focus your attention on as a starting point. Shake-a-lens makes it easy to be sure the cylinder axis is not 090 degrees off—an error that is easy to overlook during final, pre-dispensing verification. It also allows you to estimate the location of the optical center, which can tip you off to an incorrect pd measurement or an “upside down insertion.” Upside down insertions leave the axis unchanged, but any decentration will be in the wrong direction. This is possible only with round and rectangular single-vision lenses, but it does happen. The patient reinserting a loose lens is usually the source of this error. The patient’s line of sight will fall at a distance from the MRP that is twice the amount of the decentration so the “unwearability” becomes exponentially greater with high powers and high decentrations.

 

That New Staff Member
Shaking a lens to unravel problems is best left to someone familiar with ophthalmic optics, however new hires can quickly learn to identify nearsightedness, farsightedness and astigmatism. They should limit their observations to the upper part of lenses and not mention adds, especially to progressive lenses wearers who might be “in-the-closet” presbyopes. Their comments should always be tactful and limited to nearsightedness, farsightedness, and astigmatism. Knowing the rudiments of shaking a lens may seem like a minor thing, but to a beginner in the field it can feel like a privileged peek into a profession they wish to join. It can be the extra lift that carries them through their early difficult and confusing times in your practice. LT
Palmer R. Cook, OD is director of education for Diversified Ophthalmics in Cincinnati.

 

L&T basics
They’ve Gotta Talk the Talk If They’re Gonna Walk the Walk
Understanding the terminology is always a big part of any learning situation. Use the following terms and tips to round out any shake-a-lens training. A good understanding of these will be fundamental to your students’ future optical training.

MRP – The MRP or Major Reference Point is the point in the lens that gives the desired lens power. Except in lenses with prescribed prism, the MRP coincides with the optical center of the lens.

Spherical Lenses – Spherical lenses (i.e. spheres) have the same power in all meridians.

Cylindrical Lenses – Cylindrical lenses (i.e. cylinders) have power that varies from meridian to meridian. Rotating a lens reveals whether it is a sphere or a cylinder.

Major Meridians – Cylinders always have two major meridians. One is the plus major meridian and the other is the minus major meridian. The plus major meridian is the one with the most plus (or least minus) power. If your practice writes lens prescriptions in plus cylinder form (e.g. plano +1.00 x 180), the meridian of most plus power will be 90 degrees from the written axis and the meridian of most minus power will lie on the written axis. If your practice prefers lens writes lens prescriptions in minus cylinder form (e.g. +1.00 -1.00 x 090), the meridian of most minus power will be 90 degrees from the written axis and the meridian of most plus power will lie on the written axis.

Shaking Cylinders – It may be less confusing to turn the lens or eyewear until you are on one major meridian as indicated by a vertical contour in your field of vision and then “shake” the lens horizontally. The image movement will indicate the lens power at 90 degrees from the direction of your movement (i.e. your movement is lateral and the power indicated is in the vertical meridian). When you rotate the lens 90 degrees to evaluate the other major meridian, the power evaluated will again be 90 degrees from your horizontal “shake.” Using this method allows you to always use a vertical contour and eliminates the need to find two contours at right angles to each other when evaluating a lens.


 

 

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