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Vertex shift key consideration when fitting spectacles
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HOUSTON - Technicians must compensate for vertex shift when fitting glasses, because the refracting distance and the fitting distance almost never coincide.
"Vertex distance is the distance from the back of the spectacle lens to the front of the cornea. When a doctor does an examination, he or she places the phoropter at a particular distance from the eye. That's referred to as refracting distance. The distance at which the technician fits a pair of glasses is the fitting distance. Often, these distances do not coincide," said John C. Soper, FNAO, president of Soper Enterprises, an optical consulting and management firm, located here.
A significant shift
Mr. Soper recently conducted a survey to determine the difference between doctors' refracting distance and technicians' fitting distance. The average refracting distance was 10.5 mm. He then measured 100 patients using a distometer and found that the average fitting distance was approximately 19.5 mm. So, the average shift was 9 mm.
"In lighter prescriptions, that's not a huge amount. However, in stronger prescriptions, it is a considerable amount. For example, a 5-D lens will be approximately 0.25 D off power. That is not within government standards for acceptability. A 7-D lens will be almost 0.5 D off, and a 10-D lens will be 1 D off," he said.
"Most optometrists will calculate the vertex shift for contact lenses, meaning that they will compensate for the 0 fitting distance of a contact lens. However, they don't concern themselves with it in eyeglasses, assuming that the fitting distance will be relatively close," he explained.
Plastic frames vs. metal frames
Mr. Soper believes that the vertex shift has increased because most patients now wear metal frames rather than the plastic frames worn years ago. "Plastic frames fit on the crotch of the bridge, right up at the very top of the nose, so that the lenses fit closer to the eye. Now, with metal frames, if you fit them very close to the top of the bridge, the nose pads come very close to the canthus of the eye, and this is uncomfortable. So, it's natural for us to fit the glasses further down on the bridge, so we have arbitrarily moved the lenses further from the eye," he said.
When the lens is moved further from the eye, more plus power is induced. Therefore, a plus lens becomes stronger as it is moved away from the eye, and a minus lens becomes weaker.
"If you work with one doctor all the time, you should know his or her refracting distance. I recommend that if you are going to calculate for vertex shift, you should call the doctor and let him or her know that you're going to do this. Tell the doctor what the change is going to be, and put that in the file," he added.
The accompanying formula will allow you to estimate the vertex shift. "If you don't want to calculate the vertex shift yourself, charts can be used, or you can tell the laboratory what the doctor prescribed at a particular refracting and fitting distance. They have the formula on a computer where they can calculate the change, compensate the Rx and provide exactly what lens is needed," Mr. Soper explained.
Formula for Estimating Vertex Shift |
|---|
| D2/1,000 x vertex shift in millimeters |
| For example, if a 6-D lens has been prescribed and the doctor refracted at 10 mm and you are fitting at 20 mm (which would make the vertex shift 10 mm), the formula would be: 62/1,000 x 10 mm. Therefore, the prescription would need to be adjusted by 0.36 D. |
For Your Information:
- John C. Soper, FNAO, can be reached at Soper Enterprises, 3103 Sackett St., Houston, TX 77098; (713) 524-4661; fax: (713) 524-4663. Mr. Soper has no direct financial interest in the products mentioned in this article, nor is he a paid consultant for any companies mentioned.