Question

what is principal foci

Answer 1

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge.

Hope this helps ❤

Answer 2

In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle.