Curved-field lenses are optically designed to correct for a natural curvature of film in some open-face mounts. KODAK Projection Lenses-Curved-Field Lenses. Improves screen image quality of open-mounted 2 x 2-inch where only one type of mount is used with film original transparencies. . Not for use with glass mounted slides. Not for use in rear projection applications Not for use with some open-faced mounts that have been created using an automated system which flattens the film surface and eliminates pillow curve.