Zoom lens having two lens groups

Details Zoom lens having two lens groups Zoom lens having two lens groups

1999-03-26

2001-02-06

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With variable magnification

C359S692000, C359S687000, C359S690000

Reexamination Certificate

active

06185050

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a zoom lens, and in particular, to a small-sized zoom lens suitable for a lens shutter camera having less restriction of a back focus.
It has been known that a zoom lens having two lens groups composed of a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object is right fit for a small-sized zoom lens used in a compact camera, because it is simple in structure and short in total lens length.
However, a zoom lens for a compact camera is requested to have higher variable power, to cover a wider angle and to be more compact, and a conventional zoom lens having two lens groups which has high variable power ratio of 2.5 or more, for example, has not covered a wide angle or has been insufficient in terms of compactness.
An object of the invention is to solve the problems mentioned above and to provide a zoom lens which has a variable power ratio of 2.5 or more and a field angle of 70° or more at a wide angle end and is compact and excellent in image forming power. It is further preferable that the invention provides a compact camera having a zoom lens. In particular, the invention is preferably applied to a zoom lens having two lens groups and to a camera which is equipped with a zoom lens having two lens groups.
Aforementioned objects can be attained by the following structures.
Structure (1): A zoom lens for photographing including a first lens group having positive refracting power and a second lens group having negative refracting power both arranged in this order from an object, conducting variable power by changing a distance between the first lens group and the second lens group, and having a variable power ratio of 2.5 or greater and a field angle of not less than 70° at a wide angle end, wherein the following conditions are satisfied;
1.25≦Tw/Tt≦1.8  (1)
0.60≦fFc/fw≦0.9  (2)
wherein,
Tw: Telephoto ratio at a wide angle end
Tt: Telephoto ratio at a telephoto end
fw: Focal length at a wide angle end
fFc: Focal length of a first lens group.
Structure (2): A zoom lens having wherein the conditions in Structure (1) are satisfied and the following conditions are satisfied;
0.65≦|fRc|/fFc≦1.3  (3)
wherein,
fRc: Focal length of a second lens group
fFc: Focal length of the first lens group.
Structure (3): A zoom lens wherein the conditions in Structure (1) are satisfied, the first lens group is composed of a first sub-unit having negative refracting power and a second sub-unit having positive refracting power both arranged in this order from an object across the air distance on the optical axis which is the longest in the first lens group, the first sub-unit is composed of two lenses one of which is a negative lens which is closer to an object, the second sub-unit is composed of one negative lens and one positive lens, or composed of one negative lens and two positive lenses, and the second lens group is composed of two lenses.
Structure (4): A zoom lens wherein the conditions in Structure (1) are satisfied, the first lens group is composed of a first sub-unit having positive refracting power and a second sub-unit having positive refracting power both arranged in this order from an object across the air distance on the optical axis which is the longest in the first lens group, the first sub-unit is composed of two lenses, and the second sub-unit is composed of one negative lens and one positive lens, or of a 3-element type composed of one negative lens and two positive lenses, and the second lens group is composed of two lenses.
Structure (5): A zoom lens wherein the conditions in Structure (1) are satisfied, the second lens group is composed of a negative lens and a negative meniscus lens whose concave surface faces an object both arranged in this order from an object, the aforementioned negative lens is an aspherical lens whose lens thickness on its periphery is thinner than that on a spherical surface having the same paraxial curvature, and the following condition is satisfied;
0.50≦|fRc2|/fw≦0.9  (4)
wherein,
fRc2: Focal length of a negative meniscus lens
fw: Focal length at a wide angle end
incidentally, ranges of preferable conditional expressions are as follows.
1.46≦Tw/Tt≦1.7  (1′)
0.72≦|fFc|/fw≦0.85  (2′)
0.75≦|fRc|/fFc≦0.9  (3′)
0.55≦|fRc2|/fw≦0.7  (4′)
Conditional expression (1) stipulates an appropriate range of the ratio of a telephoto ratio at a wide angle end to a telephoto ratio at a telephoto end. The telephoto ratio is defined as a ratio of the total length including a back focus, namely an axial distance from the lens surface closest to an object to the image forming plane to a focal length of a total zoom lens system, and when Lw represents a total length including a back focus at a wide angle end, namely an axial distance from the lens surface closest to an object at a wide angle end to the image forming plane, Lt represents a total length including a back focus at a telephoto end, namely an axial distance from the lens surface closest to an object at a telephoto end to the image forming plane, fw represents a focal length of a total zoom lens system at a wide angle end, and ft represents a focal length of a total zoom lens system at a telephoto end, the following expressions hold,
Tw=Lw/fw, Tt=Lt/ft
therefore, the following expression holds.
Tw/Tt=(Lw/Lt)·(ft/fw)
For compactness which is one of those to be solved in the invention, it is especially necessary that total length Lt including a back focus at a telephoto end is short.
In addition to this, a great variable power ratio and a short focal length at a wide angle end which represent other problems awaiting solution in the invention are concentrated on the lower limit of the conditional expression (1), and when this lower limit is exceeded, it is difficult to obtain a compact zoom lens having a variable power ratio of 2.5 or more and a field angle of not less than 70° at a wide angle end, especially a compact zoom lens having two lens groups. When an upper limit is exceeded, it is difficult to obtain an excellent image forming power.
Conditional expression (2) stipulates an appropriate range of a focal length of the first lens group.
When a focal length at a wide angle end is short as in the invention, a back focus tends to be short at the wide angle end, and there are easily caused problems that a rear lens system turns out to be big to make a camera big too, or fine dust particles sticking to the rear lens appear as images. When a focal length of the first lens group exceeds the lower limit of the conditional expression (2) to be short, it is difficult to secure a back focus necessary at a wide angle end. Further, a large lateral magnification of the second group is needed, which makes aberration correction difficult. When the upper limit is exceeded, a total length including a back focus is made long at all zooming areas, which makes compactness to be lost.
Conditional expression (3) stipulates an appropriate range of a ratio of a focal length of the second lens group to a focal length of the first lens group. When this range is satisfied, an amount of movement of the second lens group in zooming can be made small to attain further compactness, and fluctuations of astigmatism in zooming can further be restrained, which is preferable.
The lens composition in Structure (3) is advantageous for securing a ratio of brightness of the edge of image field necessary at a wide angle end and a back focus by making the first sub-unit having negative refracting power to precede, and the lens composition described makes it possible to obtain a field angle of about 75 degrees.
The lens composition in Structure (4) is advantageous for making a telephoto ratio to be smaller. In the lens composition of Structure (3) or (4), it is

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