GB489381A - Improvements in photographic objectives - Google Patents

Abstract

489,381. Lenses. DJIAN, G. F. Feb. 1, 1937, No. 2955. Convention date, Feb. 13, 1936. [Class 97 (i)] An objective of large aperture comprises a front convergent member consisting of one or more lenses, and in rear of this member a second member consisting of a front convergent lens and a rear divergent lens, the front surface of the convergent lens comprising a central spherical portion and a peripheral aspherical portion the curvature of which increases as the distance from the axis increases, the thickness of the second member being at least 0À4 of the focal length of the objective and the refractive index of the divergent lens of this member being at least 0À15 greater than that of its associated convergent lens. The convergent lens of the second member may be made up, as shown, of a spherical part b and sections m, with aspherical surfaces and of the same refractive index as the part b. Alternatively this lens may consist of a central spherical section surrounded by a number of concentric rings with aspherical surfaces. A rear convergent member may be added, having a focal length less than that of the first member and serving to increase the aperture. When this added member has a plane rear surface a liquid may be interposed between this surface and the film on which the image is formed. Data are given for four examples having apertures greater than F /1, that of the example shown being F/0À6.