focusyoudaoicibaDictYouDict[focus 词源字典]
focus: [17] Latin focus meant ‘fireplace’, and in post-classical times it came to be used for ‘fire’ itself – hence French feu, Italian fuoco, Spanish fuego, all meaning ‘fire’, and hence too the English derivatives fuel and fusillade. The first writer known to have used it in its modern sense ‘point of convergence’ was the German astronomer Johannes Kepler, in 1604, but the reason for his choice of word is not clear.

It may have been some metaphorical notion of the ‘hearth’ symbolizing the ‘centre of the home’, but it has also been suggested that it may have been preceded and inspired by the use of focus for the ‘burning point’ of a mirror (not actually recorded until somewhat later). The philosopher Thomas Hobbes appears to have introduced the term into English, in 1656. A medieval Latin derivative of focus was focārius, from which French got foyer ‘hearth, home’, borrowed by English in the 19th century for a public entrance hall or lobby.
=> foyer, fuel, fusillade[focus etymology, focus origin, 英语词源]
focus (n.)youdaoicibaDictYouDict
1640s, "point of convergence," from Latin focus "hearth, fireplace" (also, figuratively, "home, family"), which is of unknown origin. 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 to "center of activity or energy" is first recorded 1796.
focus (v.)youdaoicibaDictYouDict
1775 in optics, "bring into focus" (transitive); 1807 in the figurative sense, from focus (n.). Intransitive use by 1864, originally in photography. Related: Focused; focusing; less commonly focussed; focussing.