A digital camera typically includes a lens system for projecting and focusing the image of a subject onto the surface of an electronic sensor. Too meet various desires of digital camera uses, such as for enhancing the image quality, for increasing the magnification, and for reducing the size and the power consumption of the cameras, the photographing lens of digital cameras must have a zooming function. A digital camera is generally equipped with a zoom lens which allows a photographer to zoom from a telephoto angle to a wide angle by adjusting the zoom lens. A zoom camera, whether it is a film type camera or a digital camera, generally includes a zoom lens barrel assembly having a zooming function for varying magnification of a photographing lens and a focusing function for varying a focal point in response to the distance to a subject. A lens barrel equipped with such a zoom lens typically includes two lens groups which can be advanced or withdrawn inside the lens barrel. The zoom lens barrel includes front and rear lens barrel assemblies. The rear lens barrel assembly includes a lens whose focal length may be slightly deviated from the optimal by a production tolerance. The two lens groups are separated by a gap. The focal length of the lens barrel is adjusted by changing the gap between the two lens groups, thereby allowing telephoto angle or wide angle photography to be performed according to the desired magnification of a photographic image. In order to adjust the deviated focal length, a plate having a predetermined thickness is inserted between the lens barrel and a film or between the lens barrel and a charging surface of a charge coupled device (CCD) in the course of assembling the lens to the lens barrel. Alternatively, a screw or a cam may be used to adjust the focal length. Zoom action is performed by adjusting the distance between the front lens group and the rear lens group and the distance between the respective front and rear lens groups and the surface of a film installed at a main body of a camera. In most zoom lenses, lens groups of the zoom lens are moved along optical axis by means of a cam mechanism differently between focusing the zoom lens on a subject and adjusting a zoom ratio of the zoom lens. In a zoom lens, a zoom adjustment in which no movement of the position of the focal point of the lens takes place during the zooming operation and a back focus adjustment in which the focal point is identical to a focal plane of a camera must be carried out. In the zoom adjustment, the focal point at a telephoto extremity is made identical to that at a wide angle extremity. In the back focus adjustment, the position of the focal point after completion of the zoom adjustment is made identical to the position of the focal plane (rail surface) of the camera. In recent years, a lens system having a variable focal length, what is known as a variable-focal lens system, has come into wise use in place of ordinary zoom lens system. The variable focal lens system is provided with a zoom lens and a focus lens between an objective lens and an eyepiece, and these zoom lens and focus lens are mutually movable independently from each other. Several techniques have been used to enable the zoom lens to achieve focusing and zooming in different manner. One of the techniques employs separate power drive devices such as electric motors for focusing and zoom ratio adjusting. Another technique employs a single power drive device.

Photographing of digital image is extended not only to a digital camera but also to a portable information terminal unit of portable telephone, and demands of users for digital camera are multilaterally branched. Among them, high image quality and miniaturization are always desired by the users and are given much weight among the various desired items. For this reason, high performance and miniaturization are also required to be compatible in a zoom lens used as a photographic lens. In digital still cameras using a solid-state image sensor such as a CCD or a CMOS devices, since a member such as an optical low-pass filter is disposed between the rearmost part of the lens elements and the solid-state image sensor, a lens system having a comparatively long back focal length is required. The taking optical system of digital cameras is required of an excellent telecentric characteristic in order to avoid shading which is a factor that decreases the quantity of peripheral light on the image surface. There are many kinds of zoom lens systems which may be available for the digital cameras. One type of zoom lens system for a compact digital camera system may include a first optical lens group having a negative focal length, a second optical lens group having a positive focal length, and a third optical lens group having a positive focal length. This zoom lens system further includes an aperture diaphragm which is mounted to the subject side of the second optical lens group in a manner such that the aperture diaphragm and the second optical lens group are integrally movable. Further, when the scaling of the zoom lens system is changed, the second group of lenses monotonously moves from the image side to the object side and the first group of lenses moves so as to correct a displacement of a position of image plane in accordance with the change of magnification. In order to make a good quality lens assembly for a digital camera, a spectral filter has to be incorporated to remove the red and infrared to which the semiconductor type optical sensors which are fitted to such cameras are very sensitive. An absorption gradient filter is also incorporated to homogenise the lighting in the image plane of the lens assembly. Two types of zoom lens devices are used in digital cameras. They are a front-focus zoom lens device and a rear-focus zoom lens device. A rear focus type zoom lens with a focus lens unit placed closer to the image plane than a variator lens unit has an advantage in reducing size and weight of the focus lens unit, and is therefore widely used for an autofocus type zoom lens. In the front-focus zoom lens device, focusing is carried by a focusing lens, which is a combination lens, disposed at an end of a lens barrel, and zooming is carried out by a zooming lens disposed rearward of the focusing lens. In a digital still camera, an image sensor is arranged at the rear of a photographing optical system, thus an object image is formed on the image sensor and is then recorded in a recording medium, such as a memory card. Further, an AF (auto focus) mechanism is provided in the camera. A focused subject image is obtained by the AF mechanism. The autofocus function is realized by detecting the focusing state of an image-taking lens to find a defocus amount, calculating a movement amount of a focus lens corresponding to the defocus amount, and driving the focus lens by the movement amount derived from the calculation.