text: 1. Field of the Invention
  The present invention relates to a telephoto lens system, and more particularly to an improved relatively compact telephoto lens system having an aperture ratio of about 1/2.8 or higher, and a field angle of about 18 degrees.
  2. Description of the Prior Art
  A typical lens system in the field of telephoto lens systems is the Ernostar type lens system which consists from the object to the image side of a first positive, a second positive, a third negative and a fourth positive lens. These lens systems are designed to provide a relatively large aperture with a long focal length.
  It has been recently disclosed in Japanese patent applications laid open to public inspection under No. 49-53421 and No. 51-62037, that a telephoto lens system having an aperture ratio of about 1/2.8 and a field angle of about 18 degrees is obtainable by forming the lenses of an Ernostar type lens system into a plurality of meniscus single lenses of relatively large thickness. However a 4 group, 4 component Ernostar type lens system according to the above publications, however, will have problems with field curvature, astigmatism and coma that will be difficult to correct if the lens system is also designed to be of a compact size. Thus, the telephoto ratio of such lens systems can only reach 0.97 at minimum.
  U.S. Pat. No. 3,749,478 is cited for the disclosure of a telephoto lens system having five lenses. This patent discloses a modified Ernostar type lens system having a doublet or composite lens as its second lens and actually formed by separating the doublet into two lenses and by modifying the powers of the two lenses so that the two lenses each have a positive power. Thus, the modified lens system consists of a first positive, a second positive, a third positive, a fourth negative and a fifth positive lense.
  The prior art is still seeking to provide compact lens systems having improved performance characteristics.