The openings, objects and images that gave us words for seeing.
focus (n.)
A focus is a point where light rays meet, or a place at which we direct our attention. In Latin, focus meant a hearth.12
Johannes Kepler gave the word to mathematics in 1604, in a book on the optical part of astronomy. He used it for the two points of an ellipse that are still called its foci.3
The reason for a hearth is given as probable rather than certain. A lens or a curved mirror can gather sunlight onto one point and start a fire there.12
From the point where rays meet, the word reached photography and ordinary speech. We can focus a camera, and we can focus our minds.12
She adjusted the focus until the lettering was clear.