|Premium lenses designed using free-form technology, like Zeiss Individual from Carl Zeiss Vision, use position-of-wear measurements.|
EACH ISSUE, TECH TIPS WILL EXPLORE SOME INTERESTING ASPECT OF OPTICAL TECHNOLOGY. THIS MONTH LOOKS AT VERTEX DISTANCE.
One of the features of premium version lenses designed and surfaced using free-form technology is their use of position-of-wear (POW) measurements. These premium versions ask you to take measurements like vertex distance (VD), pantoscopic tilt, and face form (wrap) tilt in order to compensate the lens power for induced errors. How important are these measurements? I’ll discuss vertex distance here and cover tilt in an upcoming issue.
WHAT IS POSITIONAL EFFECT?
Positional effect occurs when the VD of the lenses in the refractor (phoropter) is different from the VD of the lenses in the finished eyeglasses. It also occurs when lenses slide closer or farther away from a wearer’s eyes. Positional effect requires power compensation of the original prescription so the wearer receives the power the refractionist intended.
No matter how weak or strong the power is, positional effect occurs in all lenses. But just how significant is it to vision?
A CLOSER LOOK AT POWER ERROR
Fig. 1 illustrates the lens power error induced by VD shift. To use this chart, pick the power of the lens and cross-reference it with the amount of shift you determine is occurring. Since we’re concerned with how much power error might cause blur to a patient, you have to assume a power at which blur might occur. For purposes of this discussion, I’ve chosen 0.25D, which is a reasonable value. Using this value, I’ve circled the powers at which this amount of blur occurs.
FIG. 1: Lens Power Error Induced By VD Shift
Notice that the first time this happens is at the 5.00D level but it takes 10mm of shift to cause that 0.25D (actually 0.26D) error. This is a huge shift for an eyeglass wearer. In fact, 3mm to 5mm is probably the most you’ll see for eyeglasses. For 6.00D it takes 7mm of shift and for 7.00D it takes 5mm of shift. This is the power that textbooks tell you to start compensating for VD shift error (now you know why). From here, the amount of power error created per millimeter of shift becomes more significant. Looking at the lower powers like 1.00D and 2.00D, the power error is minimal; 0.01 to 0.02D for up to 5mm of shift.
So why bother compensating for low power errors due to VD shift? The answer is that today’s premium free-form lenses attempt to correct all the power errors they can. Let’s face it, compensating 0.03D is not worth the effort, but if you compensate that and 0.07D of pantoscopic error and 0.05D of face form error while making the lenses to 0.01D accuracy instead of 0.25D, you’re accomplishing something meaningful.
Remember, if you don’t provide a VD for an Rx when the lens manufacturer or lab asks for one, they will supply one based on average data.
Ed De Gennaro is Director, Professional Content of First Vision Media Group.