focal plane

Definition: (1) The plane lying at right angles to the principal axis of a lens or mirror which goes through the focal point. (2) The plane surface which optimally collects the best image or on which the sharpest image lies. * This plane may be inclined to the optical axis e.g. when the Scheimpflug condition is met.
Focus on the compromise
The surface on which an image is sharpest is, for normal photographic lenses, not very flat. Indeed, early cameras curved their film in order to optimise recorded image quality. So by necessity, the focal plane is placed in a compromising position: one which catches as much as possible of the image sharpness over as much of the film as possible. As a result, the fall-off in quality at the corners of a film does not mean the lens is not sharp there, it may simply mean the film was not quite in the right place to catch the sharp image.

Related Terms: Petzval surface

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