The resolution capability of photographic films has substantially improved in recent decades. This has resulted in a trend to increasingly smaller film formats and increased requirements on the image-forming optics. The emergence of digital rear units for large-format photography has further accelerated this development. Hence modern objectives for large-format photography are desirable, which enable free selection of positions of the film plane in terms of shift and tilt.
Objective lenses for large-format photography are generally wide-angle lenses which have to be as free from distortion as possible. Reductions in F stop are acceptable with such objective lenses as long as they are within the limits permitted by diffraction parameters which define the smallest resolvable structure. This makes it possible to tilt the film plane with respect to the optical axis of the objective lens and shift it in all directions. These special possibilities allow substantial correction of the image-perspective errors. Manipulations of the focal range are also possible. Known standard objective lenses for large-format photography generally have an aperture ratio of 1:5.6 (F 5.6) or 1:4 (F 4), an apochromatic correction of the longitudinal color aberration in the center of the image and, therefore, a high resolution capability. However, this high resolution capability decreases very quickly with increasing image height so that, in order to achieve a typical image definition of 20 Lp/mm, the lens has to be stopped down to F 22.
However, a 1:22 operating stop (F 22) is no longer acceptable nowadays. The diffraction-dependent resolution capability is so low with this F- stop that the possibilities of present day film material go largely unexploited. Higher apertures are necessary to meet the resolution capability of a modern film. In most cases, this means a working F-number of 1:16 (F 16) or 1:11 (F 11). Sometimes even 1:8 (F 8) is necessary. The resolution capability demanded by the objective lenses in this case lies between 30 and 50 Lp/mm. This can only be achieved in the image center by currently known large-format objectives.
Such a typical standard objective lens is described in DE PS 1 258 134. Although this six-element objective lens has an apochromatically corrected longitudinal color aberration, it is not suitable for higher F stops. The reason for this lies in inadequate field flatness and excessive coma. While with an F stops of 22 a resolution capability of approximately 20 Lp/mm is achieved, this is inadequate from the present viewpoint. This resolution capability even decreases in the field at apertures of F 16 or F 11. The apochromatic correction of the longitudinal color aberration is achieved in this objective lens by means of cemented diverging (negative) lens elements made of glass with a low refractive index and anomalous partial dispersion. Only the short flint types, i.e. glass types with a high antimony content, are feasible for this type of glass. Since-antimony is highly poisonous, these objective lenses must also be classed as questionable from the point of view of environmental compatibility. This results in a further considerable disadvantage of the standard objective lenses which require short flint glass in the production of the negative components of the cemented lens element, as prescribed in DE PS 1 258 134.
USPTO Backgrounds
USPTO Backgrounds