The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No.  2001-378781 filed Dec. 12, 2001; and
Japanese Patent Application No.  2001-378819 filed Dec. 12, 2001. 
1.  Field of the Invention
The present invention relates to a zoom lens system suitable for video cameras and electronic still cameras using solid state imaging device, in particular to a zoom lens system having the zoom ratio of about 6 or more, the angle of view of 60xc2x0 or more in a wide-angle end state together with high optical performance and compactness. 
2.  Related Background Art
In a camera system, a recording method has been known in which a shot image is converted into electric output by a light detector constructed by arranged photoelectric converters such as CCD (Charge Coupled Device), CMOS (Complementary Metal Oxide Semiconductor), or the like. 
With the recent progress in fine pattern processing in semiconductor devices, an increase in processing speed of a CPU and in high integration of recording medium allows a large amount of image data to be processed with high speed, which has not been possible before.  Moreover, in the light detector also, an increase in integration allows recording with high spatial frequency and an increase in compactness allows compactness of whole camera system. 
However, there has been a problem that because of high integration and compactness of the light detector, a narrowed detection area of a photoelectric converter decreases an electric output to increase the effect of noise.  In order to prevent this, an optical system is made to be large aperture ratio to increase an amount of light reached the light detector.  Moreover, minute lens elements (so-called xe2x80x9cmicro-lens-arrayxe2x80x9d) are arranged right in front of respective photoelectric converters.  The micro-lens-array has a restriction to the position of the exit pupil of the optical system instead of leading a light reaching a blind spot between adjacent photoelectric converters onto a photoelectric converter.  When the position of the exit pupil of the optical system approaches the light detector, the angle between the principal ray reaching the light detector and the optical axis becomes large, so that off-axis light flux going toward periphery of the image makes a large angle relative to the optical axis.  As a result, off-axis light flux not reaching the light detector causes insufficiency of light amount. 
A camera recording an object image using photoelectric converters as a light detector, the so-called digital still camera, has strong points of easy handling image data such that development is not required, and a shot image is easily confirmed.  However, it has weak points to a film camera such that imaging quality is inferior to a film camera, and it requires to be connected to a device such as a personal computer, so that the diffusion of the digital camera has not increased.  Recently, with increase of imaging quality of digital cameras and the diffusion of the device by lowering price, digital cameras have been increasingly popular. 
As for increase of imaging quality, it is indispensable to increase in optical performance of the optical system as well as the above-mentioned increase in integration of the light detector. 
Moreover, increase in a zoom ratio improves the degree of freedom for shooting, for example, permitting a photographer to shoot a close-up or to shoot a wide area when shooting with a close object distance such as indoors. 
In particular, there have been several documents relevant to the subject such as Japanese Patent Application Laid-Open Nos.  6-194572, 2001-133687, 2001-194590, and 11-52246. 
The zoom lens systems disclosed in Japanese Patent Application Laid-Open Nos.  6-194572 and 2001-133687 have been composed of 4 lens groups which are, in order from an object, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, and a fourth lens group with positive refractive power and wherein the first lens group is fixed along the optical axis regardless of state of lens group positions, and zooming is carried out by moving the second lens group through the fourth lens group along the optical axis. 
The zoom lens system disclosed in Japanese Patent Application Laid-Open No.  2001-194590 has realized a high zoom ratio by consisting of, in order from an object, a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, and a plurality of lens groups locating to an image side of the third lens group. 
However, zoom lens systems disclosed before have had a problem that although both high optical performance and a high zoom ratio can be satisfied, compactness of the optical system is difficult to be realized, so that a large-sized optical system disturbs to be carried with. 
The zoom lens system disclosed in Japanese Patent Application Laid-Open No.  6-194572 has realized both large aperture ratio and a high zoom ratio, but the total lens length and the diameter of the lens relative to an image height have been difficult to be compact.  The zoom lens system disclosed in Japanese Patent Application Laid-Open No.  2001-133687 has realized compactness by reducing the number of lens elements, but the high zoom ratio has not been realized. 
The zoom lens system disclosed in Japanese Patent Application Laid-Open No.  2001-194590 has realized both large aperture ratio and a high zoom ratio, but since the lens diameter of a first lens group and a second lens group is extremely large, compactness and a high zoom ratio have not been able to be satisfied at a time. 
The zoom lens system disclosed in Japanese Patent Application Laid-Open No.  11-52246 has been suitable for video cameras and electric still rameras using a solid state imaging device or the like.  However, the zoom lens system has had the zoom ratio of about 3, so that a sufficiently high zoom ratio has not been obtained. 
The present invention is made in view of the aforementioned problems and has an object to provide a zoom lens system having the zoom ratio of about 6 or more, the angle of view of 60xc2x0 or more in the wide-angle end state together with high optical performance and compactness. 
According to one aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, a fourth lens group having positive refractive power, and a fifth lens group having positive refractive power.  When the state of lens group positions varies from a wide-angle end state to a telephoto end state, a separation between the first lens group and the second lens group increases, a separation between the second lens group and the third lens group decreases, a separation between the third lens group and the fourth lens group varies, a separation between the fourth lens group and the fifth lens group increases, and the third lens group and the fourth lens group moving to the object side.  The following conditional expressions (1) through (5) are satisfied;
4.5 less than F1/FW less than 8.0xe2x80x83xe2x80x83(1)
0.8 less than |F2/FW| less than 1.8xe2x80x83xe2x80x83(2)
xe2x80x832.3 less than F3/FW less than 4.5xe2x80x83xe2x80x83(3)
2.0 less than F4/FW less than 5.2xe2x80x83xe2x80x83(4)
0.2 less than (D34Wxe2x88x92D34T)/FW less than 1.0xe2x80x83xe2x80x83(5)
where FW denotes the focal length of the zoom lens system in the wide-angle end state, F1 denotes the focal length of the first lens group, F2 denotes the focal length of the second lens group, F3 denotes the focal length of the third lens group, F4 denotes the focal length of the fourth lens group, D34W denotes the separation between the third lens group and the fourth lens group in the wide-angle end state, and D34T denotes the separation between the third lens group and the fourth lens group in the telephoto end state. 
In one preferred embodiment of the present invention, when the state of lens group positions varies from the wide-angle end state to the telephoto end state, the first lens group moves such that the position of the first lens group in the telephoto end state is to the object side of the position of the first lens group in the wide-angle end state, and the fifth lens group is fixed.  The following conditional expression (6) is preferably satisfied;
0.4 less than M1/FW less than 3.0xe2x80x83xe2x80x83(6)
where M1 denotes a moving amount of the first lens group when the state of lens group positions varies from the wide-angle end state to the telephoto end state. 
In one preferred embodiment of the present invention, the fourth lens group is composed of a cemented lens constructed by a positive lens element cemented with a negative lens element, and the following conditional expression (7) is preferably satisfied;
0.2 less than N4Nxe2x88x92N4Pxe2x80x83xe2x80x83(7)
where N4N denotes a refractive index of the negative lens element in the fourth lens group at d-line (xcex=587.6 nm), and N4P denotes a refractive index of the positive lens element in the fourth lens group at d-line (xcex=587.6 nm). 
In one preferred embodiment of the present invention, the focusing from a distant object to a close object is carried out by moving the fifth lens group to the object side, and the following conditional expression (8) is preferably satisfied;
3.0 less than FS/FW less than 7.0xe2x80x83xe2x80x83(8)
where F5 denotes the focal length of the fifth lens group. 
In one preferred embodiment of the present invention, the fifth lens group is composed of a cemented lens constructed by a positive lens element cemented with a negative lens element, and the following conditional expression (9) is preferably satisfied;
xe2x80x8315.0 less than xcexd5Pxe2x88x92xcexd5Nxe2x80x83xe2x80x83(9)
where xcexd5P denotes Abbe number of the positive lens element in the fifth lens group, and xcexd5N denotes Abbe number of the negative lens element in the fifth lens group. 
In one preferred embodiment of the present invention, the fifth lens group is composed of a single positive lens element, and the following conditional expression (10) is preferably satisfied;
0.7 less than (RR+RF)/(RRxe2x88x92RF) less than 2.0xe2x80x83xe2x80x83(10)
where RP denotes a radius of curvature of the object side surface of the positive lens element in the fifth lens group, and RR denotes a radius of curvature of an image side surface of the positive lens element in the fifth lens group. 
According to another aspect of the present invention, a zoom lens system includes, in order from an object, a first lens group having positive refractive power, a second lens group having negative refractive power, a third lens group having positive refractive power, and a fourth lens group having positive refractive power.  When the state of lens group positions varies from a wide-angle end state to a telephoto end state, at least the first lens group and the fourth lens group moves to the object side such that a separation between the first lens group and the second lens group increases, a separation between the second lens group and the third lens group decreases and a separation between the third lens group and the fourth lens group decreases.  The third lens group is composed of three lens elements which are, in order from the object, a first positive lens element, a second positive lens element, and a negative lens element.  The fourth lens group includes a positive lens element located to the most image side and a negative lens element located to the object side of the positive lens element.  The following conditional expressions (11) through (13) are satisfied;
0.015 less than D3P/FT less than 0.100xe2x80x83xe2x80x83(11)
0.10 less than YO/(BFWxe2x88x92RE) less than 0.25(RE less than 0)xe2x80x83xe2x80x83(12)
0.4 less than xcex944/(FTxc2x7FW)xc2xd less than 1.0xe2x80x83xe2x80x83(13)
where D3P denotes the thickness of the second positive lens element in the third lens group, FT denotes the focal length of the zoom lens system in the telephoto end state, YO denotes a half of the diagonal of the image frame, RE denotes a radius of curvature of the most image side surface of the fourth lens group, BFW denotes a distance from the most image side surface of the fourth lens group to the image plane in the wide-angle end state, xcex944 denotes a moving amount of the fourth lens group (positive upon moving to the object side) when the state of lens group positions varies from the wide-angle end state to the telephoto end state, and FW denotes the focal length of the zoom lens system in the wide-angle end state. 
In one preferred embodiment of the present invention, at least one of the following conditional expressions (14) and (15) is preferably satisfied;
0.1 less than |F3N|/FT less than 0.7xe2x80x83xe2x80x83(14)
0.2 less than |R31|/F3 less than 2.0xe2x80x83xe2x80x83(15)
where F3N denotes the focal length of the negative lens element located in the third lens group, FT denotes the focal length of the zoom lens system in the telephoto end state, R31 denotes a radius of curvature of the image side surface of the negative lens element located in the third lens group, and F3 denotes the focal length of the third lens group. 
In one preferred embodiment of the present invention, the second lens group is composed of three lens elements which are, in order from the object, a first negative lens element having a concave surface facing to the image, a second negative lens element having a concave surface facing to the object, and a positive lens element having a convex surface facing to the object.  The following conditional expression (16) is preferably satisfied;
xe2x88x920.7 less than (R21+R22)/(R21xe2x88x92R22) less than 0xe2x80x83xe2x80x83(16)
where R21 denotes a radius of curvature of the image side surface of the first negative lens element located in the second lens group, and R22 denotes a radius of curvature of the object side surface of the second negative lens element located in the second lens group.