Electronic image pickup equipment

Details Electronic image pickup equipment Electronic image pickup equipment

2002-06-28

2003-03-25

Dang, Hung Xuan (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S689000

Reexamination Certificate

active

06538824

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to electronic image pickup equipment, and more particularly to a video camera or digital camera wherein its thickness in the depthwise direction is reduced by making some contrivance for optical systems such as a zoom lens system. In addition, the zoom lens system is designed to be rear focused.
In recent years, digital cameras (electronic cameras) attract public attention as next-generation cameras now superseding 24 mm×36 mm film (usually called Leica format) cameras. For current digital cameras there are a wide range of categories from a high-performance type for commercial use to a portable popular type.
A chief object of the present invention is to achieve a video or digital camera of the portable popular type category in particular, which is reduced in depth dimensions while high image quality is ensured.
The greatest bottleneck in reducing the depth dimensions of a camera is the thickness of the surface, nearest to the object side, of an optical system, especially a zoom lens system to an image pickup plane. Recently, a so-called collapsible mount type of lens barrel has gone mainstream, wherein an optical system is driven out of a camera body for phototaking and the optical system is housed in the camera body for carrying. However, the thickness of the lens mount with the optical system housed therein varies largely depending of the lens type used, the filters used or the like. To obtain high specifications especially regarding zoom ratios, F-number, etc., it is preferable to make use of a so-called positive precedent type of zoom lens system wherein the lens group located nearest to its object side has positive refracting power. Even when the zoom lens system is housed in a lens mount, however, it is impossible to reduce the thickness of a camera largely, because the respective lens elements have some thicknesses with a large dead space (see JP-A 11-258507). In this regard, a negative precedent type of zoom lens system, especially a zoom lens system comprising two or three lens groups is favorable. However, it is still impossible to reduce the thickness of a camera largely, even when the lens nearest to the object side is a positive lens. This is because each lens group comprises a number of lens elements or the lens elements are thick (see JP-A 11-52246). Some known examples of the zoom lens system suitable for use with electronic image pickup devices, having satisfcatory image-formation capabilities inclusive of zoom ratios, field angles and F-numbers and capable of having the smallest thickness of a lens mount with the zoom lens system housed therein are disclosed in JP-A's 11-194274, 11-287953 and 2000-9997.
To make the first lens group thin, it is preferable to locate an entrance pupil at a shallow position. To this end, on the one hand, it is required to increase the magnification of the second lens group. On the other hand, some considerable burdens are placed on the second lens group. This does not only make it difficult to keep the second lens group thin but also to make correction for aberrations, resulting in an unacceptably increase in the influence of fabrication errors. Thickness and size reductions may be achieved by reducing image pickup device size. To achieve the same number of pixels, however, it is required to reduce pixel size and make up for sensitivity shortages by the optical system. The same also holds for the influence of diffraction.
To reduce the depth dimensions of a camera body, it is preferable in view of a driving mechanism layout to make use of a rear focusing mode wherein the movement of lenses for focusing is carried out by a rear lens group rather than a front lens group. In this case, however, it is required to make a selection from optical systems less susceptible to aberration fluctuations in the rear focusing mode.
SUMMARY OF THE INVENTION
In view of such problems with the prior art as explained above, it is a primary object of the present invention to reduce the thickness of electronic image pickup equipment as much as possible by making selective use of a zoom mode or construction having a compact yet simple mechanism layout and stable yet high image-formation capabilities from an object at infinity to a near-by object, for instance, a rear focusing mode having a reduced number of lens elements, and making lens elements so thin that the total thickness of each lens group can be reduced while the selection of filters is taken into account.
According to the first aspect of the present invention, this object is achieved by the provision of electronic image pickup equipment including a zoom lens system and an electronic image pickup device in the rear of said zoom lens system comprising, in order from an object side of the zoom lens system, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, in which for zooming from a wide-angle end to a telephoto end of the zoom lens system upon focused on an object point at infinity, a separation between the second lens group and the third lens group becomes wide and which can be focused at a nearer-by subject by moving the third lens group toward the object side, characterized in that:
said second lens group comprises, in order from an object side thereof, one positive lens
2
a
, one negative lens
2
b
and a lens subgroup
2
c
comprising at least one lens and said third lens group comprises one positive lens, while the following conditions are satisfied:
0.04
<t
2N
/t
2
<0.18  (1)
−0.5
<f
2a
/f
2c
<1.1  (2)
where t
2N
is an axial distance from an image-side surface of the positive lens
2
a
located on the object side of the second lens group to an image-side surface of the negative lens
2
b
in the second lens group, t
2
is an optical axis distance from an object-side surface of the positive lens
2
a
located on the object side of the second lens group to a surface located nearest to an image side of the lens subgroup
2
c
, and f
2a
, and f
2c
is a focal length in air of the positive lens
2
a
located on the object side of the second lens group, and the lens subgroup
2
c
, respectively.
According to the second aspect of the present invention, there is provided electronic image pickup equipment including a zoom lens system and an electronic image pickup device in the rear of said zoom lens comprising, in order from an object side of the zoom lens system, a first lens group having negative refracting power, a second lens group having positive refracting power and a third lens group having positive refracting power, in which for zooming from a wide angle end to a telephoto end of the zoom lens system upon focused on an object point at infinity, a separation between the second lens group and the third lens group becomes wide and which can be focused at a nearer-by subject by moving the third lens group toward the object side, characterized in that:
said second lens group comprises, in order from an object side thereof, one positive lens
2
a
, one negative lens
2
b
and a lens group
2
c
consisting of one lens and said third lens group comprises one positive lens, while the following conditions are satisfied:
0.04
<t
2N
/t
2
<0.18  (1)
 −0.5
<f
2a
/f
2c
<1.1  (2)
where t
2N
is an optical axis distance from an image-side surface of the positive lens
2
a
located on the object side of the second lens group to an image-side surface of the negative lens
2
b
in the second lens group, t
2
is an optical axis distance from an object-side surface of the positive lens
2
a
located on the object side of the second lens group to a surface located nearest to an image side of the lens group
2
c
, and f
2a
, and f
2c
is a focal length in air of the positive lens
2
a
located on the object side of the second lens group, and the lens subgroup
2
c
, respectively.
An account is now given of why the aforesaid arrangements are

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