Measuring A Lens:
Everyone measures the power of a lens with a Lensmeter (aka Lensometer, Vertometer or Focimeter). Notice how the lens surface is placed perpendicular to the ray beam of the instrument. The Lensmeter is unable to simulate the rotation of the eye and it is not possible to accurately measure the patient's perceived power at any point on the lens. Conventional lenses yield the correct power when measured like this. However the power is perfect only at the Optical Center or PRP. This calculation method is known as nominal power calculation. It assumes the same design is good for every prescription. We call this a "static" design.