Two-group zoom lens

A two-group zoom lens is provided which has a first lens group of negative power and a second lens group of positive power, located in this order from an object to be photographed. The first and second lens groups are moved relative to one another to provide various degrees of magnification. The first lens group includes a first lens of negative power, a second lens of negative power, and a third lens of positive power. The second lens group includes a fourth lens of positive power, a fifth lens of positive power, a sixth lens of negative power, and a seventh lens of positive power. The fifth lens is cemented to the sixth lens. The fifth and sixth lenses satisfy the following conditions: (a) 0.1<n.sub.N -n.sub.P <0.4; (b) -1.3<r.sub.c /f.sub.2 <-0.5; and (c) 0.6<d.sub.1-2 /f.sub.w <1.2; wherein, n.sub.P represents a refractive index of the fifth lens, n.sub.N represents a refractive index of the sixth lens, r.sub.c represents a radius of curvature of the cementing surface of the fifth and sixth lenses, f.sub.2 represents a focal length of the second lens group, d.sub.1-2 represents a distance between the first and second lens groups, and f.sub.w represents a focal length of the entire zoom lens at a wide angle extremity.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a zoom lens with two lens groups (referred 
to as a two-group zoom lens) of standard focal length range having a 
magnification of approximately two, used with, for example, a single lens 
reflex camera. 
2. Description of Related Art 
A conventional two-group zoom lens of standard focal length range having a 
magnification of approximately two usually includes a positive and 
negative lens groups, as disclosed in, for example, Japanese Unexamined 
Patent Publication Nos. SHO 59-142515, 1-185607, HEI 1-239516 or HEI 
4-114115, etc. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a less expensive and 
more compact two-group zoom lens than conventional two-group zoom lenses 
hitherto known. To achieve the object mentioned above, according to an 
aspect of the present invention, a two-group zoom lens is provided which 
has a first lens group of negative power and a second lens group of 
positive power, located in this order from an object to be photographed. 
The first and second lens groups are moved relative to one another to 
provide various degrees of magnification. The first lens group includes a 
first lens of negative power, a second lens of negative power, and a third 
lens of positive power. The second lens group includes a fourth lens of 
positive power, a fifth lens of positive power, a sixth lens of negative 
power, and a seventh lens of positive power. The fifth lens is cemented to 
the sixth lens. The fifth and sixth lenses satisfy the following 
conditions: (a) 0.1&lt;n.sub.N -n.sub.p&lt; 0.4; (b) -1.3&lt;r.sub.c /f.sub.2 
&lt;-0.5; and (c) 0.6&lt;d.sub.1-2 /f.sub.w &lt;1.2; wherein, n.sub.p represents a 
refractive index of the fifth lens, n.sub.N represents a refractive index 
of the sixth lens, r.sub.c represents a radius of curvature of the 
cementing surface of the fifth and sixth lenses, f.sub.2 represents a 
focal length of the second lens group, d.sub.1-2 represents a distance 
between the first and second lens groups, and f.sub.w represents a focal 
length of the entire zoom lens at a wide angle extremity. 
According to another aspect of the present invention, a two-group zoom lens 
is provided which has a first lens group of negative power and a second 
lens group of positive power, located in this order from an object to be 
photographed. The first and second lens groups are moved relative to one 
another to provide various degrees of magnification. The first lens group 
includes a first meniscus lens of negative power having a convex surface 
on a side of the first meniscus lens nearer the object, a second meniscus 
lens of negative power having a convex surface on a side of the second 
meniscus lens nearer an object image surface, and a third meniscus lens of 
positive power having a convex surface on a side of the third meniscus 
lens nearer the object. The zoom lens satisfies the following 
relationship: (d) 0&lt;f.sub.1 /r.sub.2-2 &lt;1.2; wherein, f.sub.1 represents a 
focal length of the first lens group, and r.sub.2-2 represents a radius of 
curvature of the second lens surface of the second meniscus lens. 
The present disclosure relates to subject matter contained in Japanese 
patent application No. HEI 5-141961 (filed on Jun. 14, 1993) which is 
expressly incorporated herein by reference in its entirety.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
A two-group zoom lens, according to the present invention, includes a first 
lens group I having three lenses (i.e., first, second and third lenses 
I-1, I-2 and I-3B), and a second lens group II having four lenses (i.e., 
fourth, fifth, sixth and seventh lenses II-1, II-2, II-3 and II-4), with 
II-2 and II-3 being cemented to each other. 
Lenses II-2 and II-3 satisfy the requirements represented by the formulas 
(a) and (b) below. 
EQU 0.1&lt;n.sub.N -n.sub.p&lt; 0.4 (a) 
wherein "n.sub.p " designates the refractive index of lens II-2, and 
"n.sub.N " the refractive index of the sixth lens II-3. 
EQU -1.3&lt;r.sub.c /f.sub.2 &lt;-0.5 (b) 
wherein "r.sub.c " designates the radius of curvature of the cementing 
surface of lenses II-2 and II-3 and "f.sub.2 " the focal length of the 
second lens group II, respectively. 
In the second lens group II, lenses II-2 and II-3 which constitute a 
cemented lens assembly have positive power and negative power, 
respectively. In the present invention, the powers of lenses 5 and 6 are 
preferably intensified not only to make the lens assembly small, but also 
to enhance an aberration correction function. Moreover, due to the 
adhesion of lenses II-2 and II-3, the lens assembly is minimally 
influenced by possible manufacturing errors, resulting in a stable and 
high productivity of the lens assembly with a low manufacturing cost. 
The formula (a) specifies a difference in the refractive index between 
lenses II-2 and II-3. If lenses II-2 and II-3 are made of materials which 
do not satisfy the requirement represented by the formula (a), i.e., if 
the refractive index difference is smaller than the lower limit (i.e., 
n=0.1), spherical aberration cannot be effectively corrected at the 
cemented surface of II-2 and II-3. This leads to an increase in the coma. 
Conversely, if the refractive index difference is larger than the upper 
limit (i.e., n=0.4), the refractive index n.sub.p of the positive lens 
II-2 becomes relatively small, and Petzval's sum increases. This makes it 
difficult to compensate the curvature of image (i.e., field curvature) and 
the astigmatism. In theory, the Petzval's sum can be decreased by 
increasing the refractive index n.sub.N of the negative lens II-3, but 
generally speaking, a lens material having a high refractive index is 
expensive. Accordingly, a solution to the problem in which the refractive 
index n.sub.N of the negative lens II-3 is increased is not practical from 
an economical view point. 
Preferably, in the present invention, the lens II-1 is made of a lens 
material having a refractive index that is relatively high in order to 
correct the aberration. This also ensures that the lens II-2 can be made 
of a relatively inexpensive lens material having a relatively small 
refractive index. 
The formula (b) specifies the radius of curvature of the cementing surface 
(i.e., mating surface) of lenses II-2 and II-3 to effectively correct the 
spherical aberration. 
Since the cementing surface of the second lens group II is a diverging 
surface for correcting spherical aberration using the refractive index 
difference obtained by the formula (a), it is not desirable that the 
curvature of the cementing surface is smaller than the lower limit (i.e., 
-1.3). Conversely, if the curvature of the cementing surface is larger 
than the upper limit (i.e., -0.5), a high-order aberration may undesirably 
result. 
The zooming range is defined by formula (c) below. 
EQU 0.6&lt;d.sub.1-2 /f.sub.w &lt;1.2 (c) 
wherein "d.sub.1-2 " designates the distance between the surface of 
terminal lens in the first lens group facing the image surface and a 
surface of the first lens in the second lens group facing the object side 
at the wide-angle extremity, and "f.sub.w " the focal length of the entire 
lens system at the wide-angle extremity, respectively. 
The wide-angle extremity is extended as the distance d.sub.1-2 between the 
first and second lens groups I and II increases, but if the value of 
d.sub.1-2 /f.sub.w is larger than the upper limit (i.e, 1.2), the quantity 
of marginal ray tends to be undesirably reduced. Conversely, if the value 
of d.sub.1-2 /f.sub.w is smaller than the lower limit (i.e, 0.6), a large 
magnification cannot be obtained. 
The following formula (d) defines shape of the lens I-2 of the first lens 
group I. 
EQU 0&lt;f.sub.1 /r.sub.2-2 &lt;1.2 (d) 
wherein "f.sub.1 " designates the focal length of the first lens group I, 
and "r.sub.2-2 " the radius of curvature of the second surface of lens 
I-2, facing the image surface, respectively. 
As is well known, if the power of the negative lens (i.e., front lens 
group) is increased to make a wide-angle lens or a zoom lens having an 
wide-angle compact at a wide-angle position, the distortion is increased. 
To prevent this, it is known to provide an additional weak positive lens 
on a lens nearest to an object to be photographed, for example, as 
disclosed in Japanese Unexamined Patent Publication Nos. HEI 4-114115, HEI 
2-167515 or HEI 4-261511, etc. According to one of the most significant 
features of the present invention, the surface of the negative second lens 
that is located nearer an image of the object is a convex surface, instead 
of the provision of an additional positive lens. The convex lens surface 
functions to correct the distortion, similar to the additional positive 
lens. Consequently, in the present invention, a compact zoom lens which 
does not have an additional positive lens can be provided in which little 
or no distortion occurs. 
If the degree of convexity of the lens surface of the second lens, as 
nearer the image surface is below the lower limit (i.e, 0) in the formula 
(d), distortion cannot be effectively corrected. Conversely, if the degree 
of convexity of the lens surface of the second lens nearer the image 
surface is above the upper limit (i.e., 1.2), the negative power of the 
second lens will be too weak to realize a wide-angle lens as a whole. In 
theory, it is possible to provide an enhanced concave surface on the 
surface of the second lens, as nearer the object to be photographed, to 
obtain a sufficient positive power of the second lens, but a coma will 
tend to occur. 
First Embodiment: 
FIG. 1 shows a lens arrangement of a two-group zoom lens at a wide-angle 
extremity, according to a first embodiment of the present invention. 
As mentioned above, the first lens group I, located in front of a diaphragm 
S with respect to the object, comprises 1st, 2nd and 3rd lenses I-1, I-2, 
and I-3 whereas the second lens group II located behind the diaphragm S 
with respect to the object is comprises 4th, 5th, 6th and 7th lenses II-1, 
II-2, II-3, and I-4, respectively. The lenses of the first lens group I 
are all meniscus lenses, and lenses II-2 and II-3 of the second lens group 
II are cemented to each other. 
Numerical data of the lens system shown in FIG. 1 is shown in Table 1 
below. Diagrams of various aberrations thereof at the shortest focal 
length, intermediate focal length, and longest focal length extremity are 
shown in FIGS. 2, 3 and 4, respectively. In FIGS. 2 through 4, "SA" 
designates the spherical aberration and "SC" the sine condition. The 
"d-line", "g-line" and "C-line" represent the chromatic and transverse 
chromatic aberration, represented by the spherical aberration, at the 
respective wavelengths. The term "S" represents the sagittal ray, and the 
term "M" represents the meridional ray. In Table 1, "r" designates the 
radius of curvature of each lens surface, "d" the lens thickness or the 
distance between the lenses, "N" the refractive index, and ".nu." the Abbe 
number, respectively. 
TABLE 1 
______________________________________ 
F.sub.NO = 1:4.2.about.4.9.about.5.9 
f = 36.03.about.50.00.about.68.00 
.omega. = 32.1.degree..about.23.3.degree..about.17.4.degree. 
F.sub.B = 41.41.about.50.42.about.62.04 
f.sub.1 = -54.610 
f.sub.2 = 35.238 
f.sub.w = 36.03 
______________________________________ 
Surface No. r d N .nu. 
______________________________________ 
1 38.966 2.00 1.77250 
49.6 
2 17.670 7.08 
3 -47.206 1.60 1.68250 
44.7 
4 -61.564 0.50 
5 21.018 2.54 1.80518 
25.4 
6 24.929 28.52-13.60-3.41 (variable) 
stop 1.21 
7 33.325 3.17 1.75500 
52.3 
8 -66.840 0.10 
9 14.913 4.59 1.50378 
66.8 
10 -44.114 3.68 1.83400 
37.2 
11 13.983 4.36 
12 -40.374 2.31 1.58267 
46.4 
13 -21.741 
______________________________________ 
Second Embodiment: 
FIG. 5 shows a lens arrangement of a two-group zoom lens at a wide-angle 
extremity, according to a second embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 5 is shown in Table 2 
below. Diagrams of various aberrations thereof at the shortest focal 
length, intermediate focal length, and longest focal length extremity are 
shown in FIGS. 6, 7 and 8, respectively. 
TABLE 2 
______________________________________ 
F.sub.NO = 1:4.2.about.4.9.about.5.9 
f = 36.03.about.50.00.about.68.00 
.omega. = 32.2.degree..about.23.4.degree..about.17.4.degree. 
F.sub.B = 42.34.about.51.32.about.62.89 
f.sub.1 = -54.828 
f.sub.2 = 35.259 
f.sub.w = 36.03 
______________________________________ 
Surface No. r d N .nu. 
______________________________________ 
1 36.069 2.00 1.79500 
45.3 
2 17.554 6.46 
3 -71.175 1.60 1.80400 
46.6 
4 -137.630 0.50 
5 22.681 2.63 1.80518 
25.4 
6 29.260 28.63-13.65-3.41 (variable).sub.-- 
stop 1.21 
7 42.968 1.21 1.71299 
53.9 
8 -101.850 0.10 
9 19.444 8.42 1.69680 
56.5 
10 -22.971 1.50 1.83400 
37.2 
11 15.854 3.66 
12 -51.274 2.35 1.64328 
47.9 
13 -23.543 
______________________________________ 
Third Embodiment: 
FIG. 9 shows a lens arrangement of a two-group zoom lens at a wide-angle 
extremity, according to a third embodiment of the present invention. 
Numerical data of the lens system shown in FIG. 9 is shown in Table 3 
below. Diagrams of various aberrations thereof at the shortest focal 
length, intermediate focal length, and longest focal length extremity are 
shown in FIGS. 10, 11 and 12, respectively. 
TABLE 3 
______________________________________ 
F.sub.NO = 1:3.9.about.4.6.about.5.9 
f = 35.96.about.50.00.about.75.00 
.omega. = 32.1.degree..about.23.4.degree..about.15.9.degree. 
F.sub.B = 42.60.about.52.50.about.70.13 
f.sub.1 = -49.063 
f.sub.2 = 34.598 
f.sub.w = 35.96 
______________________________________ 
Surface No. r d N .nu. 
______________________________________ 
1 39.540 1.65 1.77250 
49.6 
2 16.866 6.51 
3 -103.932 1.60 1.80610 
40.9 
4 -518.960 0.10 
5 22.598 2.77 1.80518 
25.4 
6 31.345 27.99-14.73-3.42 (variable) 
stop 1.20 
7 47.904 3.21 1.69680 
55.5 
8 -47.904 0.10 
9 15.447 4.81 1.51633 
64.1 
10 -43.109 5.61 1.83400 
37.2 
11 15.150 3.68 
12 -67.864 2.44 1.58267 
46.4 
13 -25.449 
______________________________________ 
The values of the formulas (a), (b), (c) and (d) corresponding to the 
first, second and third embodiments, are shown in table 4 below. 
TABLE 4 
______________________________________ 
formulas 
formulas formulas formulas 
(a) (b) (c) (d) 
______________________________________ 
example 1 
0.33022 -1.252 0.825 0.890 
example 2 
0.13720 -0.651 0.828 0.400 
example 3 
0.31767 -1.246 0.817 0.095 
______________________________________ 
As can be seen from Table 4 above, all three embodiments satisfy the 
requirements defined by the formulas (a), (b), (c) and (d). Moreover, 
according to the present invention, the aberrations are fully corrected 
throughout the entire focal length range from the wide-angle extremity to 
the telephoto extremity in a two-group zoom lens. 
As may be understood from the above discussion, according to the present 
invention, a simple, compact and inexpensive two-group zoom lens of 
standard focal length range, having a magnification of around 2 includes 
fewer lenses, i.e., seven lenses. In general, to realize a compact zoom 
lens, it was necessary to increase the negative power of the negative lens 
or lenses of the second lens group, resulting in a deterioration of the 
quality of the zoom lens due to manufacturing error. However, in the 
present invention, lenses II-2 and II-3 having large powers are cemented 
to each other, the zoom lens is not influenced as significantly by 
manufacturing error. Hence, the zoom lenses can be inexpensively and 
stably mass-produced. 
Furthermore, if the first lens group includes three individual lenses in a 
compact zoom lens having two lens groups, the distortion on the wide-angle 
side tends to be a large negative value. However, according to another 
aspect of the present invention, the second lens is made of a negative 
meniscus lens whose surface that is located nearer the image surface is 
convex. Hence, it is possible to eliminate the distortion while keeping 
the whole lens system compact.