Impaired vision and blindness are not uncommon. About 3.4 million people in the U.S. are visually impaired. Vision is usually measured by visual acuity charts, and normal vision is expressed as 20/20.

It’s often called the optic that best approximates human vision, but approximation is relative. An Object Lesson. But the concept of “normal vision,” let alone the 50-mm lens’s ability to reproduce it ...