Curvature of field is a phenomenon which causes the image formation plane to become curved like the inside of a shallow bowl, preventing the lens from producing a flat image of a flat subject. When the center of the image is in focus, the periphery is out of focus, and when the periphery is in focus, the center is out of focus. The degree of curvature of field is largely affected by the method used for correcting astigmatism. Since the image plane falls between the sagittal and meridional image surfaces, good correction of astigmatism results in small curvature of field. Since curvature of field cannot be improved very much by stopping down the lens, lens designers reduce it as much as possible using various methods such as changing the shapes of the various single lens elements making up the lens and changing the position of the aperture. In doing this, one necessary condition that must be satisfied to simultaneously correct astigmatism and curvature of field is Petzvals Condition (1843). Petzvals Condition states that a lens element is good if a result of zero is obtained when the inverse of the product of the index of refraction and focal length of that lens element is added to the total number of lens elements making up the lens. This sum is called Petzvals Sum.canonglossaryeu
.This optical defect causes points on an object plane perpendicular to the lens axisto focus on a curved surface rather than a plane.
One of Seidel s five aberrations. Curvature of Field is a phenomenon in which straight lines are not rendered perfectly straight in the picture. Curvature of field can be improved but not eliminated by stopping down the lens.