Ultrawide angle zoom objective

DETAILED DESCRIPTION The present invention is concerned with a two-component zoom objective, and aims to provide an ultrawide angle zoom objective with the front lens members of reduced diameter, while still permitting good correction of aberrations particularly regarding distortion in the wide angle position without causing this correction of distortion to deteriorate the other aberrations.
For example, the spherical aberration in the telephoto position and the various aberrations in the intermediate zooming region.
According to the present invention, despite the fact that the zoom objective has an ultrawide angular field corresponding to a zoom lens having a range of 20 to 35 mm for the 35 mm single lens reflex camera, the various aberrations beginning with the distortion can be very well corrected by using a reduced number of lens elements.
At the same time, a minimization of the bulk and size of the lens system as a whole can be achieved.
Further since the lens system takes the mechanically-compensated form to maintain a constant image plane, the present invention has the advantage of providing an ultrawide angle zoom objective suitable for use particularly in still cameras, or in cinematographic camera or television-transmitting cameras.
One of the features of the present invention is that the ultrawide angle zoom objective is constructed such that a first lens group I of negative power and a second lens group II of positive power constitute a complete objective and are moved in differential relation to each other to effect a change in the focal length of the entire system, the aforesaid first lens group I comprises four lens elements of which the first L1 counting from the front is a meniscus lens of negative power convex toward the front, the second L2 is a bi-convex lens, the third L3 is a bi-concave lens with its rear surface of strong power, and the fourth L4 is a lens of positive power with its front surface convex towards the front, wherein the absolute value of the radius of curvature of the rear surface R4 of the aforesaid second lens L2 is made to be larger than that of the radius of curvature of the front surface R5 of the aforesaid third lens L3, and good correction of aberrations is facilitated by giving an aspheric surface to the first lens L1