FIELD OF LENS The imaginary surface at which the sharpest image that can be given is formed. With a theoretically perfect lens, this would be a plane, but in practice the field is usually concave, occasionally convex, and in the case of most anastigmats, plane with an annular depression at a considerable distance from the centre. In the theoretically perfect field all the rays, axial and marginal, come to a focus on a plane which is at right angles to the axis of the lens. This condition is fulfilled by one or two of the modern anastigmats, especially those made for copying. The most ordinary type of field is concave, the concavity being away from the lens. Before the introduction of the special Jena glasses, this was considered normal, in fact, inevitable, for so eminent an optician as the late J. H. Dallmeyer stated that a lens having a perfectly flat field " does not exist, and cannot be made." The amount of curvature of field varies greatly in different types of lenses, being most pronounced in portrait lenses of large aperture, less in single landscape lenses, and least of all in well-constructed rapid rectilinears.
Other things being equal, it will be found that separating the elements of a double combination lens has a tendency to flatten the field, at the risk of increasing the astigmatism present ; while in the case of single lenses the curvature is reduced to a minimum by placing the diaphragm as far as practicable from the lens. The field of a typical anastigmat is flat in the centre for a con siderable distance from the axis, then comes a " dip " and then a recovery to almost the original plane. A field that is convex towards the lens is rarely found in practice, generally occurring in modern anastigmats which have been slightly over-corrected for flatness in the endeavour to attain other qualities.