Lenses and mirrors
The focus of a lens
or parabolic mirror
is the point onto which collimated light parallel to the axis is focused. The focus of an elliptic
mirror is either of two points such that light from one converges on the other. The focus of a hyperbolic
mirror is either of two points such that light from one is reflected as if it came from the other.
In an ellipse, foci are the two "off-center" points.
See also Focus (band), focus (computing).