Abstract: When a lens system is focussed at a fixed length, what range of an object is clearly photographed is determined by the focal length and F number of the lens. In order that a zoom lens which requires no focussing operation is mounted on the beginners' camera to make the focussing range as wide as possible, the focussing position need be deviated as the focal length f varies as zooming takes place. If F number is selected in the range of F.gtoreq.2.8.times.10.sup.-3 .times.f.sup.2, a performance camera can be realized with a zoom lens including a wide angle and having a variable power ratio of the order of 2.