This invention relates to a wide angle objective for a single lens reflex camera.
Objectives for single lens reflex cameras are required to have a back focal length sufficiently long to permit for the quick return operation of the mirror mechanism. In single lens reflex cameras of 35mm format type, it is necessary that this distance be at least 36mm. Wide angle objectives for such single lens reflex cameras are constructed in the form of retrofocus lens in which concave lenses are arranged in the object-side of the lens system for the purpose of meeting the aforementioned requirement. These concave lenses contribute an increase in the back focal length to the lens system, but they have adverse effects on the correction of aberrations, because of the asymmetry of lens configuration of the resulting lens system with respect to the center thereof. Particularly barrel distortion and negative coma which are produced by the object-side concave lenses are more difficult to be corrected in high degree over the entire field. As the field angle of the objective increases, or as the overall focal length is decreased, it is necessary to increase the retro ratio, or the ratio of the back focal length to the overall focal length. As a result, the asymmetry of the lens configuration is intensified which complicates the correction of the aforementioned aberrations. Particularly with wide angle objectives which operate with a retro ratio of more than two, it is impossible to preserve a high standard imagery with respect to the aforementioned aberrations so long as all of the refracting surfaces in the lens system are spherical.