They come together at a point on the principal axis close principal axis An imaginary line which runs through the centre of a lens. called the principal focus F, (or focal point). The centre of ...

Principal focus of a convex lens: It is a point on the principal axis of the convex lens where all the light rays parallel to the principal axis converge after passing through the lens.

We have found that a double focus lens can perform this function very efficiently, and the scheme lends itself to an effective demonstration of holography. Nature - Holography with a Double Focus ...