Passive Autofocus (Two-Array Comparison)

Norman Stauffer at Honeywell built a focus detector from two rows of tiny light sensors side by side on one chip. In his FIGURE 2 each row views the subject through its own small lens, a short distance from the other - like the two windows of a rangefinder. How far one row's picture is shifted against the other's gives the subject's distance, and the camera sets the taking lens to match, without trial and error. Compact cameras from 1977 used it; later SLRs moved the same comparison behind the taking lens itself (phase detection). **US 4,002,899**, "Focus detecting apparatus", Stauffer, assigned to Honeywell, filed 31. October 1975, granted 11. January 1977: "radiation responsive elements formed on a single planar substrate and divided into two similar coplanar detector arrays lying immediately adjacent each other". **You see the principle with a card**: a two-hole mask gives a lens two viewpoints, just as Stauffer's two lenses do; out of focus the point splits in two, and which spot vanishes when you cover one hole tells you which way to focus.