By Tina Lahti, ABOC
In Part 1, we discussed Position of Wear measurements and how they affect compensated power. Now you’ll see why.
Are all these measurements necessary? The answer is yes and no. To create compensated lenses, we must use values for one or more of the key POWs. That doesn’t mean an optician must measure them. And, truth be told, some values are more important than others. Wrap angle is very important, but it isn’t measured relative to a patient’s face. Wrap angle can be pre-measured and recorded. It may even be available from frame manufacturers. Pantoscopic tilt is important, but rarely varies much from wearer to wearer. Vertex distance is only important for a small number of wearers with very high prescriptions. For the most part, what opticians think of as a “proper” frame fit includes consistent ideas about how the frame should wrap around the face, how much it tilts down from top to bottom and how far the lenses are from the eye. This consistency of frame fit allows the option for default position of wear values.
Default position of wear measurements can greatly improve the visual performance of a digital lens. A recent analysis of position of wear values conducted by IOT showed that, with their proprietary default position of wear values, 61% of RXs showed a noticeable improvement in vision as compared to 78% for accurately measured POWs. Even default POWs are a vast improvement over non-compensated lenses. Furthermore, POWs are not all or nothing. It is acceptable to measure some and use defaults for others. The most important POW variables are the easiest to measure and fancy tools are not required. Wrap angle is very important. In many cases it is supplied by the frame manufacturer. It can also be measured in seconds utilizing a paper protractor you can print for free. Pantoscopic tilt is also important. This can be measured manually, in seconds, using a swing arm protractor. This tool is available for $5 or less at any general merchandise or office supply store. Vertex distance is important for high prescriptions, poor frame fits or sport wrap frames. Vertex distance can be measured manually with a PD ruler. Digital measuring devices are fine, if they have been verified as accurate. A well-practiced optician can be just as accurate and fast with a few inexpensive tools.
Here’s what we always know about compensation. Regardless of how complex the digital calculation is, the rules of physics still apply. Designers can have different philosophies and approaches, but their lens calculations are based on math, not magic. That means minus powered lenses will always minify more when they are closer to the eye. A -9 with a very short vertex distance will likely end up with a reduced distance power after compensation. Plus powered lenses will always magnify more when they are further from the eye. A +3 with a +3 add and very flat pantoscopic tilt will likely end up with a reduced add power after compensation. The curve, angle and power of a lens will always affect how light enters the eye resulting in astigmatic power and prism. Higher wrap angles will likely result in more cylinder power and prism in the finished lens.
Compensated lenses can only be inspected when the laboratory provides you with a compensated Rx. The Rx the lab provides will often not be in quarter diopter increments. You will see powers like -4.16. The distance power may be decreased or increased depending on the exact calculation. The add power will as well. Cylinder power and sometimes prism can be introduced even when not on the original Rx. If cylinder was already present the axis will likely change. An example of a compensated Rx is as follows.
Don’t be alarmed by these changes in power. Remember lensometer power is not the same as power to a wearer.