The human eye is the oldest known optical instrument, existing long before the formal science of optics was established. Its structure and basic parameters—such as resolution and color sensitivity—are described, together with the most common vision defects, including myopia (nearsightedness) and hyperopia (farsightedness). The structure of elementary optical instruments composed of two and more components is introduced. This includes systems such as Galilean and Keplerian telescopes, telephoto and wide-field lenses, microscopes, viewfinders, rangefinders, and telescopes. Mathematical expressions defining the fundamental parameters of these systems are derived.