Vision is a complex phenomenon that can be addressed from different points of view. Input to the visual system consists of visual stimuli, the final output is visually guided behavior, while visual perceptions are an intermediate product. Clinicians often start by considering the input-related aspects that can be addressed by medical and surgical means. Patients, on the other hand, may be most concerned about the output aspects, that is, the effect on their daily activities. The relations between these 2 points of view are often misunderstood, which may lead to miscommunication. This perspective—based on more than 4 decades devoted to vision rehabilitation—aims at exploring these differences to bridge the communications gap.

Seemingly similar tests may actually assess different aspects. One example is the relationship between letter chart acuity and reading ability, as demonstrated by the difference between Jaeger's and Snellen's tests. Clinical applications require assessment of individuals. Societal applications deal with groups of people; they include research, public health statistics, and eligibility rules for privileges or benefits. Such applications often rely on the application of formulas to input measurements to estimate the consequences on the output side. The implications of such simplifications should be understood.