ABSTRACT
A simple lens, i.e. a one element type with spherical surfaces, shows all of the primary aberrations. Chromatic aberrations, spherical aberration, coma, astigmatism and curvature of field all combine to give poor image quality, while distortion leads to misshapen images. Image quality is decidedly poor. But aberrations can be partially reduced by choice of suitable curvatures for the two surfaces and by location of an aperture stop. If, in addition, the subject field covered is restricted, it is possible to obtain image quality that is acceptable for some purposes, especially for simple cameras and single use cameras. However, a simple lens is still limited in image quality, maximum aperture and covering power. To best use the photographic process, improved performance is needed in all three categories. Given the limited sensitivity of early photographic materials, a primary requirement was an increase in relative aperture, to allow pictures, particularly portraits, to be taken without inconveniently long exposures for the subject.