in optics, the point where a beam of light rays converges after passage through an optical system when the incidence of the beam on the system is parallel to the optical axis of the system. When parallel rays in a beam diverge after passage through the optical system, the designation “principal focus” applies to the point of intersection of the straight lines that extend from the rays leaving the system. Conversely, a beam of rays originating at the focus becomes parallel to the axis of the optical system after passage through the system. The term “principal front focus” applies to a beam of parallel rays leaving the system and “principal back focus” to a beam of parallel rays entering the system. Both principal foci are located on the optical axis of the system.

In astronomy, the term “principal focus” is often used to describe the surface on which an image of the section of the celestial sphere under observation is constructed (either by the main mirror of the reflector or by the objective of the refractor). To achieve a correction of coma or enlargement of the clear image field, a corrector lens (for example, a Ross lens) is placed in front of the principal focus in the reflector. In the largest reflectors an observation booth is mounted at the principal focus.