1. Method of rapid and repeated focusing of a lens system in a projection means for projection of volumetric 3D images or in an imaging means for imaging of a 3D object, said lens system including a first lens group and a second lens group, the method including the feature of rapidly and repeatedly changing the optical path length of path So or of path D by a first path length changing means; said path So representing the optical path between said first lens group and the image panel of said projection means or the image sensor of said imaging means; said path D representing the optical path between said first lens group and said second lens group; said first path length changing means including a wedge-shaped optical device and a moving means, said wedge-shaped optical device including a first surface and a second surface, the two surfaces forming a finite wedge angle in between, said moving means rapidly and repeatedly moving said wedge-shaped optical device along the plane of said first surface, this motion causing the intersection of the centerline of the optical path with said second surface to displace along the centerline, this displacement changing the optical path length.