focalplane

The focal plane is the plane in an optical instrument where light from a point in the object is brought into focus to form a sharp image. In a simple lens system, this plane lies a finite distance from the lens along the optical axis and is the surface at which the image would be recorded if a detector were placed there. In practical cameras and imaging systems, the focal plane is typically aligned with the sensing surface that records the final image.

In photography and cinematography the focal plane is commonly equated with the sensor or film plane. The term focal-plane shutter refers to a type of shutter located in front of the image sensor at the focal plane; such shutters open and close to expose the recording surface.

Focal plane arrays describe the two-dimensional detectors positioned on the focal plane. They consist of arrays of photosensitive elements, such as CCD or CMOS pixels, that convert light into electrical signals to form images. FPAs are central to digital cameras, telescopes, and infrared imaging, where detectors are often cooled to reduce dark current and noise; infrared FPAs frequently use materials like HgCdTe or InSb.

Optical designers must address field curvature and other aberrations that cause the image to fall out of focus away from the center. Some systems use curved focal planes or specially designed optics to flatten the field, or employ a flat detector combined with corrective optics. Understanding the focal plane is essential for image quality, focusing accuracy, and sensor placement.