Zoom lens systems, which include a zoom lens and a zoom lens barrel, have recently come into common use in lens-shutter-type cameras. The zoom lens barrel, which controls the zoom lens, alters the focal length of the zoom lens by moving movable lens groups along the zoom lens optical axis with a guiding means, such as a cam, provided at the interior of the zoom lens barrel. In addition, short-distance focusing is achieved by moving a focusing lens group along the optical axis by a focusing drive system, based on an output from a range measurement unit, which detects the location of a photographic subject.
There are three conventional short-distance focusing methods, these being (1) the unison method, wherein the entire lens system moves together, (2) the single-floating-group method, wherein only one of the lens groups is made to move, and (3) the multiple-group method, wherein multiple lens groups move by different amounts. In general, it is common for the unison method and the multiple-group method to be used with unifocal lenses, and for the single-floating-group method to be used for zoom lenses.
A particular short-distance multiple-group focusing method is disclosed in U.S. Pat. No. 2,537,912, and in Japanese Examined Patent Application (Kokoku) No. S45-39875. The method disclosed therein corrects for aberrations produced during short-distance focusing as a result of causing multiple lens groups to move by different amounts.
Also, the zoom lens barrel disclosed at Japanese Unexamined Patent Application (Kokai) No. S60-102437 permits movable lens groups to stop only at prescribed focal length states between the extreme wide-angle positional state and the extreme telephoto positional state. Here, the cam trajectory is set to shorten the distance to the location at which the photographic subject is imaged onto the film surface when the movable lens groups are moved toward the extreme telephoto positional state. This simplifies the control mechanism through elimination of the focusing drive system. However, during short-distance focusing in the extreme telephoto positional state, the total lens length increases when focusing on a nearby object, making this lens barrel unsuitably large for compact cameras.
FIG. 12 shows a schematic representation of the prior art cam trajectory in a zoom lens barrel 320 for a two-group zoom lens (not shown) disclosed at Japanese Patent Application (Kokai) No. S60-102437. In FIG. 12, line A indicates the direction of the motion of the first lens group and line B indicates the direction of the motion of the second lens group. Parameters "a"-"g" indicate the extreme wide angle and extreme telephoto positional states, respectively, of the lens when focused on an object at an "infinite distance," as the term is understood in the art of optics. The parameter "a'"-"g'" indicate the positional states of the lens when carrying out short-distance focusing (SD) from the aforesaid states a through g. Note that in the extreme telephoto positional state (g) there is a tendency for the total lens length to increase quite markedly when changing focus from an infinitely distant object to a nearby object. Accordingly, the total lens length becomes very large when attempting to obtain a closeup shot of a nearby photographic subject (i.e., object).
There are numerous types of zoom lenses suitable for use in combination with zoom lens barrels that result in a low-cost, compact zoom system for lens-shutter type cameras. Lens-shutter-type cameras are superior to single-lens reflex cameras from the standpoint of portability. Also, because the photographic lens is housed within the camera body with lens-shutter-type cameras, reducing the size of the photographic lens will lead to reduction in the size and weight of the camera overall.
Suitable zoom lenses for compact zoom systems are, for example, positive-negative two-group zoom lenses, positive-positive-negative three-group zoom lenses, and so forth. With most zoom lenses, it is easy to achieve a high zoom ratio if the field angle in the extreme wide-angle positional state is narrow. However, this causes the total lens length in the extreme telephoto positional state to be large relative to the image field size (i.e., the length of a diagonal drawn across the image plane). To achieve a zoom lens that is both small in size and possesses a high zoom ratio, it is preferred that the wide-angle state have a wide angular field.
With conventional positive-negative two-group zoom lenses, the first lens group typically comprises a negative subgroup together with a positive subgroup arranged imagewise thereof. Also, the aperture stop is arranged at the image side of the first lens group, and a lens element with a highly concave objectwise surface is located within the negative subgroup. However, it is difficult to satisfactorily correct off-axis aberrations produced by this concave surface, thereby preventing the zoom lens from having a large aperture.
Japanese Unexamined Patent Application (Kokai) No. H5-150161 discloses a zoom lens wherein the aperture stop is located between a negative subgroup and a positive subgroup which make up the second lens group. Furthermore, to correct off-axis aberrations in the extreme wide-angle positional state, either a negative meniscus lens element having an objectwise convex surface or a biconcave lens having a mild objectwise concave surfaces is used as a negative lens element of the second lens group, located nearest the object. However, the refractive power of this lens element is too weak to achieve increased wide-angle performance, and the back-focus distance is inadequate.
Another example of two-group zoom lens for a lens-shutter-type camera is disclosed in Japanese Unexamined Patent Application (Kokai) No. S61-15115. The zoom lens disclosed therein comprises a lens group having positive refractive power, and a lens group having negative refractive power. The image of the photographic subject formed by the first lens group is enlarged by the second lens group. In going from the extreme wide-angle positional state to the extreme telephoto positional state, the lens groups move objectwise, with the distance between the two groups decreasing. An aperture stop is located between the lens groups, and moves together with the first lens group when the positional state of the lens changes.
Multiple-group zoom lenses having three or more movable lens groups (such as positive-positive-negative three-group zoom lenses), allow more freedom in selecting the zooming locus than positive-negative two-group zoom lenses. Thus, they are better suited for achieving a high zoom ratio. Japanese Unexamined Patent Application (Kokai) No. H6-265787 discloses an embodiment of a three-group zoom lens having a reduced number of lens constituents but which still has a high zoom ratio. However, in the zoom lens disclosed therein, the lens thickness of the second lens group is very large, and off-axis light rays passing through the first and third lens groups are far removed from the optical axis. Consequently, it is difficult to achieve the desired reduction in lens diameter.
In addition, zoom lenses such as the positive-positive-negative three-group zoom lens disclosed at Japanese Unexamined Patent Application (Kokai) No. H2-135312, and the positive-negative-positive-negative four-group zoom lens disclosed at Japanese Unexamined Patent Application (Kokai) No. H3-39920, for example, are also known. In these zoom lenses, the first lens group has negative refractive power and the aperture stop is located adjacent the first lens group on the object side. The aperture stop and the first lens group move objectwise such that the distance between the aperture stop and the first lens group decreases when going from the extreme wide- angle positional state to the extreme telephoto positional state. Also, shortening of the total lens length is achieved as a result of the distribution of positive and negative refractive power throughout the lens system. Furthermore, by shortening the back-focal distance in the extreme wide-angle positional state and causing off-axis light rays passing through the negative lens group(s) to recede from the optical axis, the exit pupil is brought closer to the image plane. This allows the lens diameter of the negative lens group(s) to be enlarged. Also, there is independent correction of on-axis and off-axis aberrations. Thus, increasing the size of the change in back focal distance causes a large change in the height at which off-axis light rays pass through the negative lens group(s) when zooming. This permits correction of off-axis aberrations produced when zooming.