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US 2003/0197950 A1Published application
Zoom lens system
AssigneePENTAX Corporation
InventorMasaru Eguchi
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AssigneePENTAX Corporation
InventorMasaru Eguchi
Estimating the term…
A zoom lens system includes a movable positive first lens group, a movable negative second lens group, a movable positive third lens group, and movable or stationary positive fourth lens group. Upon zooming, the distance between the first and second lens groups is increased; the distance between the second and the third lens groups is decreased; and the distance between the third and the fourth lens groups is increased. The zoom lens system satisfies the following conditions: 0.5<|f2|/f3<1.0(f2<0) (1) 2<m3t/m3w<4 (2) wherein fi designates the focal length of the ith lens group (i=1˜3); m3t designates the paraxial lateral magnification of the third lens group at the long focal length extremity when an object at infinity is in an in-focus state; and m3w designates the paraxial lateral magnification of the third lens group at the short focal length extremity when an object at infinity is in an in-focus state.
A zoom lens system comprising a positive first lens group that is movable, a negative second lens group that is movable, a positive third lens group that is movable, and positive fourth lens group which is movable or stationary, in this order from an object,
Dependencies are inferred from the claim text.
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The first group being positive: arranged +-++
Optical elements, systems or apparatus
Term end not estimated
1. Field of the Invention
The present invention relates to a zoom lens system mainly used in an electronic still camera (digital camera) having a zoom ratio more than 4 and including a wide angle range (half angle-of-view of more than 30°).
2. Description of the Prior Art
The present invention will be discussed below in detail with reference to the accompanying drawings, in which:
FIG. 1 is a lens arrangement of a zoom lens system according to a first embodiment of the present invention;
FIGS. 2A, 2B, 2C and 2D show aberrations occurred in the lens arrangement shown in FIG. 1 at the short focal length extremity;
The zoom lens system of the present invention, as shown in the schematic lens-group moving paths of FIG. 21, includes a positive first lens group 10, a negative second lens group 20, a diaphragm S, a positive third lens group 30, and a positive fourth lens group 40, in this order from the object.
In the zoom lens system of this four-lens-group arrangement, upon zooming from the short focal length extremity (W) to the long focal length extremity (T), the distance between the positive first lens group 10 and the negative second lens group 20 is increased, the distance between the negative second lens group 20 and the positive third lens group 30 is decreased, and the distance between the positive third lens group 30 and the positive fourth lens group 40 is increased. The positive fourth lens group 40 can either be stationary (first and second embodiments) as shown by a two-dot chain line, or movable (third and fourth embodiments) as shown by a dotted line. The diaphragm S moves together with the positive third lens group 30. Focusing is carried out by the positive fourth lens group 40. A symbol CG designates a cover glass (plane-parallel plate) for an infrared-cut filter group provided in front of an imaging device.
Condition (1) specifies the refractive-power ratio of the negative second lens group 20 to the positive third lens group 30, which mainly perform zooming for the zoom lens system.