What do you understand by the terms 'focus' and 'principal focus' in the context of lenses ?

Explanation

Force : The point where image of an obect placed at infinity is formed is called the focus of the lens.
Principal focus : A narrow beam of light incident on a lens in a direction parallel to its principal axis, after refraction through the lens, the rays converge to a point on the principal axis. The point is called principal focus.