Varifocal lens assembly

A varifocal lens assembly for use with a camera having a focal plane where an image of an object to be focused is projected, which comprises a lens barrel adapted to be coupled to said camera, an objective leans means supported within said lens barrel and including a plurality of lens groups other than a compensator lens group, a focal length varying means coupled to a first one of said lens groups for axially moving said first one of said lens groups along said lens barrel for varying the focal length of the varifocal lens assembly, adjusting means coupled to said varifocal lens assembly for adjusting the position of at least part of the varifocal lens assembly to adjust the position of the image plane formed by said varifocal lens assembly to compensate for the shift of the image plane resulting from the movement of said first lens group, means for detecting the position of the image plane where the object image is formed through said varifocal lens assembly and for subsequently generating an electric output signal in response to displacements between said image plane and the focal plane of the camera, and control means for operating said adjusting means in response to said electric output signal to bring said image plane into alignment with said focal plane.