Wiktionary
focal plane
n. 1 (context optics English) Any plane perpendicular to the axis of a lens or mirror 2 (context optics English) Such a plane, in a camera, microscope etc, in which the real image is in focus
Usage examples of "focal plane".
But she produced a piece of white paper, moved it so it was in the focal plane of the long lens and projected a bright image of the star.
Letters began resolving in the focal plane of my left eye, forming a message from the tiny ferret-ditto we had sent scaling the mansion walls.
Once I found my focal plane, I saw spiral shaped bacteria darting quickly across the field of view.