Often, it is beneficial for the eye care professional to predict the finished thickness of a pair of spectacle lenses. Determining the change in thickness that results from the patient's use of a different frame or lens style is a common example.Patients investing a considerable amount of money into thinner and lighter lenses want to know just how thin their new lenses will be. This is a question that often strikes fear in the hearts of unprepared opticians. Those armed with the knowledge required to provide the answer, however, quickly earn the respect of their patients. Even these opticians face a greater challenge today, since the advent of newer high index lenses.