How can different patients with different Rxs end up with the same Focus® PROGRESSIVES trial lens?
An example of the above case may be: Patient A has a spectacle Rx of -3.50 D with an add of +1.00 D. Patient B has an Rx of -4.50 D with an add of +2.50 D. Both patients require a Focus® PROGRESSIVES trial lens of -3.00 D. It is important to understand how the formula and chart were derived in order to explain which trial lens power is appropriate for the corresponding distance Rx and BiFocal add. The first step was to record and tabulate clinical results from fitting thousands of patients with this lens design. By analyzing these results, we were able to establish a fitting chart which best predicted the correct trial lens power to be used with every spectacle power and add combination available. In order to simplify fitting even further, we were able to establish a mathematical relationship between the initial trial lenses and the original distance Rx and add power. The relationship can best be described by the equation: Vertexed spherical equivalent distance Rx + 1/2 add power. By a
