I didn't know before, that the burning point of a lens,
which is also its focal point, was derived from Latin "focus" which was the fire on the hearth. The dictionary says Kepler first used the word for optical or mathmatical terms. I'll bet the burning point was one of the first properties of a lens that he learned. I don't know how much he knew about lenses. Gotta check that.
focus
n.,
pl. fo4cus[es or fo$ci# 73sj#8 5ModL, adopted in math. senses by Johannes KEPLER (1604) < L, fireplace, hearth < ? IE base *bhok3, to flame, burn > ? Arm boY, flame6
1 the point where rays of light, heat, etc. or waves of sound come together, or from which they spread or seem to spread; specif., the point where rays of light reflected by a mirror or refracted by a lens meet (called real focus) or the point where they would meet if prolonged backward through the lens or mirror (called virtual focus)
2 FOCAL LENGTH
3 an adjustment of the focal length to make a clear image !to bring a camera into focus"
4 any center of activity, attention, etc.
5 a part of the body where a disease process, as an infection, tumor, etc., is localized or most active
6 the starting point of an earthquake
7 Math. a) either of the two fixed points used in determining an ellipse b) any analogous point for a parabola or hyperbola (see ECCENTRICITY, sense 3)
vt.
3cused or 3cussed, 3cus[ing or 3cus[sing
1 to bring into focus
2 to adjust the focal length of (the eye, a lens, etc.) in order to produce a clear image
3 to fix or settle on one thing; concentrate !to focus one‘s attention on a question"
vi.
1 to meet at a focus
2 to adjust one‘s eye or a lens so as to make a clear image
in focus clear; distinct; sharply defined
out of focus indistinct; blurred; not sharply defined
fo4cus[er
n.
I checked. Galileo developed his astronomical telescope twenty years before Kepler's death, so Kepler could have known about lenses, though I could not find any mention of it.