Über photographische Objektive mit sehr großem Bildwinkel

Photographic lenses with extreme wide angle, about 180° or more, are usually subjected to a certain extent of "distortion", this expression meaning in this case faults to the law of central perspective. It is, however, possible to set other laws concerning the construction of pictures, i. e. the relation between the inclination of the light beam coming from the object and its distance from the optical axis in the image plane, this having been done occasionally as far as former systems of this kind were concerned. It is proposed here to base upon the stereographic projection when it is intended to take pictures of the whole sky. Datas are given for the main parts of such a system, their essential aberrations being discussed. Details regarding the connection between the distortion and the decrease of illumination at the margin of the image are added. Finally, it is proposed to combine the essential properties of suoh a wide-angle system with a telescope which may be put in front of any photographic lens of usual construction. An example taken with such a system is demonstrated.