From the latin, focus, domestic hearth, fireplace.
The goddess Hestia, ‘chief of the goddesses’, rules the domestic fire.
Focus (v) meaning
Pay particular attention to. Concentrate. Fix. Pivot. Zero in.
Adapt to the prevailing level of light and become able to see clearly.
Cause (one’s eyes) to focus.
Adjust the focus of (a telescope, camera, or other instrument).
Focus (n) etymology
1640s, “point of convergence,” from Latin focus ‘hearth, fireplace’, also, figuratively, ‘home, family’.
Used in post-classical times for “fire” itself; taken by Kepler (1604) in a mathematical sense for “point of convergence,” perhaps on analogy of the burning point of a lens (the purely optical sense of the word may have existed before Kepler, but it is not recorded). Introduced into English 1650s by Hobbes. Sense transfer of meaning from ‘hearth’ to ‘centre of activity or energy’ is first recorded 1796.