The focal plane is the plane behind the lens at which the image forms sharply, coinciding with the surface of the sensor or film. For any given subject distance and lens, light converges to a single flat plane, and the sensor must sit exactly there to record a sharp image. The focal plane indicator on most camera bodies, a small circle with a horizontal line through it, marks its exact position, usually engraved on the top plate near the hot shoe.
That marking matters most in work where focus distances need to be measured rather than estimated, since lens markings reference subject-to-sensor distance, not subject-to-front-element. In macro and product photography, the difference between measuring from the front of the lens and measuring from the focal plane can be several centimeters, which at 1:1 magnification is the difference between a sharp image and a soft one. Cinematographers measuring with a tape from the lens to the actor’s eye anchor that tape to the focal plane mark for the same reason.
The focal plane is also the geometry behind the focal-plane shutter, the type of mechanical shutter used in virtually every interchangeable-lens camera. Two curtains travel across the sensor immediately in front of the focal plane, exposing each row in sequence. This is what creates rolling shutter distortion when subjects move quickly, and why flash sync speed is limited by how fast both curtains can clear the frame.
Strictly, every lens has many focal planes, one for each subject distance. The plane at which a lens focuses parallel rays from infinity sits one focal length behind the rear principal plane and is called the rear focal point. As the subject moves closer, the image plane moves farther back, which is why internal focusing mechanisms shift element groups to keep the image at the sensor regardless of subject distance.
The focal plane is always flat in a conventional camera, but the projection of a sharp scene is curved, which is why distant corners can soften slightly even at small apertures. View cameras work around this with tilt and shift movements that swing the lens or the back to bring an oblique subject plane into focus, the basis of the Scheimpflug principle. Modern tilt-shift lenses give a controlled version of the same trick on a full-frame body.
Practical use of the focal plane mark comes up in studio work, scientific imaging, and any time you are stacking distances precisely. For everyday shooting, autofocus handles the alignment, but knowing exactly where the focal plane sits, and that it is the plane that defines depth of field on either side, sharpens how you think about focus.