GB2269912A

GB2269912A - Zoom lens system for use with a compact camera having a wide coverage of angles

Info

Publication number: GB2269912A
Application number: GB9320915A
Authority: GB
Inventors: Takayuki Ito
Original assignee: Asahi Kogaku Kogyo Co Ltd
Current assignee: Pentax Corp
Priority date: 1989-11-21
Filing date: 1993-10-11
Publication date: 1994-02-23
Anticipated expiration: 2010-11-21
Also published as: GB2269913B; GB2269683A; GB9320915D0; GB2269912B; GB2269913A; GB9320916D0; GB2269683B; GB9320917D0

Abstract

The lens comprises, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length, zooming being performed by changing the distance between the first and second lens groups. The first lens group comprises, in order from the object side, a front subgroup 1F having a positive focal length, a diaphragm stop A and a positive rear subgroup 1R with a small power composed solely of a plastic positive meniscus lens element with a convex surface facing the image. Conditions relating to focal lengths and magnification are specified. For focussing, the first group moves towards the object with the separation between 1F and 1R increasing. <IMAGE>

Description

2269912 ZOOM LENS SYSTEM FOR USE WITH A COMPACT CAMERA HAVING A WIDE

COVERAGE OF ANGLES This invention relates to a zoom lens system that is suitable for use with a compact camera and which is subject to less constraints on back focus that zoom lens systems for use with single-lens reflex cameras. More particularly, this invention relates to a compact zoom lens system of a structurally simple two-group telephoto type (hereinafter referred to simply as "two-group type") which yet has a wide coverage of angles, i. e., ca. 37 degrees as half view angle, at the short focus end and which is capable of zooming up to a ratio of ca. 2.5 in consideration of its nature as a two-group type system having a wide coverage of angles. This invention also relates to a method of focusing with such a zoom lens system.

Conventional zoom lens systems for use with compact cameras are classified as two types, (A) a two-group type and (B) a three- or fourgroup type. Compared with type (A), zoom lens systems of type (B) have the advantage of requiring a relatively small amount of lens movement but, on the other hand, they are not only large in size but also complex in construction. Because of these obvious differerences from lens systems of a two-group type which are envisaged by the present invention, type (B) will not be described in detail hereinafter.

Compared to type (B), zoom lens systems of type (A) required a somewhat greater amount of lens movement but because of their simple lens configuration and mechanical structure, type (A), zoom lens systems have the advantage of ease in size reduction. Conventionally known zoom lens systems of two-group type include version (AA) that is 2 described in Unexamined Published Japanese Patent Application Nos. Sho-56- 12891 1, Sho-57-201213, Sho-60-48009, Sho-60-170816 and Sho60-191216, version (A-2) that is described in Unexamined Published Japanese Patent Application Nos. Sho-62-9061 1, and Sho-64-57222, and version (A-3) that is described in Unexamined Published Japanese Patent Application Nos. Sho- 62-113120 and Sho-62-264019.

In the zoom lens system of the present invention, a negative lens element is used as the first lens and Unexamined Published Japanese Patent Application No. Sho-63-276013 describes a similar telephoto lens system of a two-group type in which a negative lens element is used as the first lens of the first group.

A method of focusing with such a two-group type zoom lens system is described in Unexamined Published Japanese Patent Application No. Hei-1 189 620.

Version (AA) has a small back focus and requires a large rear lens diameter, so it has had the problem that the overall size of the camera incorporating said lens system cannot be reduced. A further problem with this lens system is that internal reflections between the film plane and the last lens surface and other unwanted phenomena are highly likely to occur.

With a view to solving these problems, the assignee has proposed improved versions of a two-group type the back focus of which is comparatively large in consideration of its use with compact cameras. Such improved versions are (A-2) which is of a five-group-six-element composition and which is capable of a zoom ratio of 1.5 - 1.6 and (A-3) 3 which is of a six-group-seven element composition or seven-group- eightelement composition and which is capable of a zoom ratio on the order of 1.7 2.5. These versions range from a six-element composition capable of a zoom ration of ca. 1.5 - 1.6 to an eight-element composition capable of a zoom ration of at least 2. However, they provide half view angle of only about 30 degrees at the short focus end and are chiefly intended to photograph scenery; in other words, they are short of satisfying the need to take pictures through wide angles with a compact camera. Further, those proposals have been unable to satisfy the need for providing a zoom lens system for use with a compact camera that is even more compact and less expensive.

The zoom lens system described in Unexamined Published Japanese Patent Application No. Sho-63-276013 uses a negative lens element as the first lens as in the present invention. However, this requires the use of lenses having a certain refractive index profile which are difficult to manufacture at low cost by the state-of-the-art technology. in addition, the half view angle that can be attained at the short focus end is no wider than 30 degrees.

The zoom lens system according to an aspect of the present invention which is to be used with a compact camera having a wide coverage of angles comprises basically, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length and performs zooming by changing the distance between the first and second lens group, wherein said first lens group comprises, in order from the object side, a front subgroup 1 F having a positive focal length, a diaphragm stop, and a positive rear subgroup 1 R with a small power, said rear subgroup 1 R being solely composed of a 4 single plastic positive meniscus lens element having a convex surface directed toward the image, the following conditions being satisfied:

(3) 0.05 < fl G/f] R< 0.35 (4) (M,1 _ M1R M_,1)2 < 0.8 where M2L the lateral magnification of the second lens group at the narrow-angle end; and M1R: the lateral magnification of the rear subgroup 1 R.

in one embodiment, the rear subgroup 1 R has at least one aspheric surface that has a divergent amount of asphericity with respect to a paraxial radius of curvature in such a way as to satisfy the following condition:

(5) -20< '&I1R < 0 where &I1R the amount of change in the coefficient of a third-order spherical aberration caused by the aspheric surface in the rear subgroup 1 R.

In another embodiment, the second lens group comprises, in order from the object side, a positive meniscus lens having a convex surface directed toward the image and two negative lens elements each having a concave surface directed toward the object, and satisfies the following condition:

(2) 1.7 < N2Gn where- - N2Gn the average of the refractive indices at the d-line of the two negative lens elements in the second lens group.

According to a second aspect of the invention there is provided a method of focusing with a zoom lens system that comprises, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length and which performs zooming by changing the distance between the first and second lens groups wherein said first lens group comprises, in order from the object side, a front subgroup 1 F having a positive focal length, a diaphragm stop, and a positive rear subgroup 1 R, the rear subgroup 1 R being solely composed of a single plastic positive meniscus lens element having a convex surface directed toward the image, focusing being achieved by moving the first lens group toward the object with the distance between the front subgroup 1 F and the rear subgroup 1 R being increased while satisfying the following conditions:

(3) 0.05 < f1G/fiR < 0.35 (4) (M2L - M1R M2L)2 < 0.8 (1) 0: X1R < 0.7 X1F In one embodiment, focusing is performed with both the diaphragm stop and the rear subgroup 1 R being fixed.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which:

6 Figure 1- is a simplified cross-sectional view of the lens system of Example 1, including a diaphragm stop A, at the wide-angle end for an infinitely distance object; Figures 2a, 2b and 2c are graphs plotting the aberration curves obtained with the lens system of Example 1, with (a) showing the state at the wide- angle end, (b), the middle-angle end, and (c), the narrow-angle end; Figure 3 is a simplified cross-sectional view of the lens system of Example 1, including a diaphragm stop A, for an object-to-image distance of 1 m after focusing by moving only the front subgroup 1 F; Figures 4a, 4b and 4c are graphs plotting the aberration curves obtained with the lens systern shown in Figure 3, with (a) showing the state at the wide-angle end, (b), the middle-angle end, and (c), the narrow-angle end; Figure 5 is a simplified cross-sectional view of the lens system of Example 1, including a diaphragm stop A, for an object-to-image distance of 1 m after focusing by moving the front and rear subgroups 1 F and 1 R in a ratio of 1:03; Figures 6a, 6b and 6c are graphs plotting the aberration curves obtained with the lens system shown in Figure 5, with (a) showing the state at the wide-angle end, (b), the middle-angle end, and (c), the narrow-angle end; Figure 7 is a simplified cross-sectional view of the lens system of Example 2, including a diaphragm stop A, at the wide-angle end for an infinitely distant object; 7 Figures- -8a, 8b and 8c are graphs plotting the aberration curves obtained with the lens system of Example 2, with (a) showing the state at the wide-angle end, (b), the middle-angle end, and (c), the narrow-angle end; Figure 9 is a simplified cross-sectional view of the lens system of Example 2, including a diaphragm stop A, for an object-to-image distance of 1 m after focusing by moving only the front subgroup 1 F; Figures 1 Oa, 1 Ob and 1 Oc are graphs plotting the aberration curves obtained with the lens system shown in Figure 9, with (a) showing the state at the wide-angle end, (b), the middle-angle end, and (c), the narrow-angie end; Figure 11 is a simplified cross-sectional view of the lens system of Example 1, including a diaphragm stop A, for an object-to-image distance of 1 m after focusing by moving the first lens group en masse; Figures 12a, 12b and 12c are graphs plotting the aberrations curves obtained with the lens system shown in Figure 11, with (a) showing the state at the wide-angle end, (b), the middle-angle end, and (c), the narrow-angle end; Figure 13 is a simplified cross-sectional view of the lens system of Example 2, including a diaphragm stop A, for an object-to-image distance of 1 m after focusing by moving the first lens group en masse; and Figures 14a, 14b and 14c are graphs plotting the aberration curves obtained with the lens system shown in Figure 13, with (a) showing the 8 state at the wide-angle end, (b) the middle-angle end, and (c) the narrowangle end.

The first lens in the conventional two-group type zoom lens systems for use with a compact camera has been a positive lens in almost all cases. Basically, the zoom lens system of the present invention is also of a telephoto type (in terms of the relationship between the first and second lens groups) but in order to cover a wide half view angle of ca. 37 degrees at the short focus end while insuring a comparatively long back focus, the positive first lens group is composed of a positive subgroup 1 F and a positive, smal 1-power subgroup 1 R to provide an unconventional unique lens arrangement of a retrofocus type starting with a negative lens on the object side.

Condition (2) relates to the two negative lenses in the second lens group. If the lower limit is exceeded, in particular, it is difficult to compensate for the field curvature on the short focal length side.

Also, as a problem concomitant with the wide angle tendency, there is a marginal light problem. It is important to keep a sufficient amount of marginal light even in the wide angle mode. If the position of the stop diaphragm is between the first and second lens groups. It is easy to manufacture the system and to facilitate the structure. However, in this case. a height of the marginal light having a maximum view angle and passing through the stop diaphragm, so that the increase of the marginal light when the stop diaphragm is at a small aperture is not expected. Accordingly, if the fixed stop diaphragm is provided behind the aforesaid diaphragm and the inlet pupil is set as rearward as possible, it is possible 9 to increase the amount of marginal light when the diaphragm is set at a small aperture.

Two examples of a first aspect of the invention are described below with reference to data sheets, in which: f denotes the focal length; co, half view angle; f,,, back focus; r, the radius of curvature of an individual lens surface; d, the thickness of an individual lens or the aerial distance between adjacent lenses, N, the refractive index of an individual lens at the d-line, v, the Abbe number of an individual lens; and c, C and C signify the asphericity coefficients of fourth-, sixth-and eighth-order aberrations, respectively.

1 Example 1

FNO = 1:15 6.0 - 8.2 f 28.90-50.00-68.00 co = 37.9-24.018.00 fB 8.50-30.15-48.61 u rface No. r d N v 1 -150.930 1.20 1.73077 40.5 2 30.579 2.81 3 -17.266 1.10 1.83481 42.7 4 -3 5.5 3 31 0.20 43.230 6.90 1.68893 31.1 6 -34.060 1.39 7 20.054 4.17 1.48749 70.2 8 -11.261 1.20 1.84666 23.8 9 -18.100 2.00 -47.706 1.35 1.49176 57.4 (plastic) 11 -24.144 9.05 3.50 1.49 12 -28.017 2.63 1.80518 25.4 13 -15.358 1.59 14 -18.080 1.35 1.83481 42.7 -40.859 3.57 16 -11.044 1.40 1.80610 40.9 17 -37.661 2nd Surface: Aspherical 11 th Surface: Aspherical C = 0.53606109 x 10-4 C = 0.31364516 x 10-4 c = 0.67837264 x 10-7 C = 0.40451026 x l& c = 0.78976837 x 10-" C = -0.53872249 x 10-8 Example 2

FNO 1:15 6.0 8.2 f 28.92 50.00 - 68.00 CO = 3 7.7 - 2 3.9 - 17.90 fB 8.5129.90 48.17 Surface No. r d N v 1 1000.000 1.20 1.73077 40.5 2 25.825 4.21 3 A 4.363 1.10 1.83481 42.7 A -27.711 0.20 34.987 6.70 1.72047 34.7 6 -30.577 1.16 7 23.491 4.02 1.48749 70.2 8 -11.372 1.30 1.80518 25.4 9 -20.576 2.03 -41.988 1.35 1.49176 57.4 (plastic) 11 -21.682 9.04 3.50 1.49 12 -31.728 2.69 1.80518 25.4 13 -15.809 1.47 14 -19.145 1.35 1.83400 37.2 -49.397 3.48 16 -11.168 1.40 1.79952 42.2 17 -42.430 2nd Surface: Aspherical 1 Oth Surface: Aspherical C = 0.43771496 x 10-4 C = -0.36242524 x 10' C = 0.33409765 x 10-8 C = -0.28552038 x 10' C = 0.61517350 x 10-8 % = 0.38808464 x 10-8 12 One major problem to be solved for insuring wide angles at the short focus end is associated with the brightness at the edge of an image field. When focusing with prior art zoom lens systems of a two-group type in which a diaphragm stop is provided between the first and second lens groups, the mechanistically simplest way is to move only the first lens group with the diaphragm stop and the second lens group being fixed. However, this method has suffered the disadvantage that the height of intercept of the diaphragm stop by marginal rays at a maximum view angle is too small to provide a desired increase in the brightness at the edge of an image field when the lens is stopped down.

Another problem that occurs when focusing is done by moving the first lens group en masse is that astigmatism and curvature of the field are under compensated by undesirable large amounts if the lens is focused for a near-distance object (see Figures 4 and 6).

A prior art proposal for improving the method of focusing with zoom lens systems of a two-group type is described in Unexamined Published Japanese Patent Application No. Hei-1 -189620 but the half view angle that can be achieved at the short focus end is no wider than about 30 degrees and the first lens used is a positive lens as in the other prior art proposals. In other words, one of the objects of the present invention, namely, providing a wide coverage of angles at the short focus end, cannot be attained by the method described in said patent. The zoom lens system for use with a compact camera that is provided by another aspect of the present invention adopts basically a simple two-group type lens arrangement and yet it achieves not only a broader coverage of angles but also an even higher zoom ratio by modifying the compositions of the first lens group and the diaphragm stop. The so

13 constructed zoom lens system is capable of focusing from an infinitely distant object to a near-distance object with reduced aberrational variations. The present invention also provides a method of focusing with this improved zoom lens system.

According to this aspect of the invention, by modifying the position of a diaphragm stop and the method of focusing, not only can the lens system perform focusing from an infinitely distant object to a near-distance object with reduced variations but also the brightness at the edge of an image field can be increased even when the lens system is stopped down.

Condition (3) relates to the power of the a-ear subgroup 1 R. The front subgroup 1 F responsible for almost all part of the power of the first lens group and if one makes an attempt to design a compact and yet wideangle lens system, the power of the first lens group becomes so strong as to cause increase coma. In order to solve this problem, the rear subgroup 1 R having a small power is positioned a short distance behind the front subgroup 1 F so as to reduce the burden on it, to thereby insure efficient compensation for coma. If the upper limit of condition (3) is exceeded, the power of the rear subgroup 1 R increases so much as to cause extensive coma in that subgroup. In addition, the amount of lens movement will increase if the front subgroup 1 F is used as a focusing lens. If the lower limit of condition (3) is not reached, the power of the front subgroup 1 F will increase so much as to cancel the effect of positioning the rear subgroup 1 R behind the front subgroup 1 F.

Conditions (3), (4) and (5) relate to the rear subgroup 1 R of the first lens group. Since the rear subgroup 1 R has a comparatively small power, it 14 can be made of plastic materials. If lenses of a comparatively small power are made of plastic materials, the amount of defocusing or deterioration in lens performance is small despite possible changes in temperature or humidity. In addition, the overall weight of the lens can be reduced. Further, it is easy to make an aspheric surface of plastic lenses and this contributes to an improvement in lens performance.

Supplemental comments are necessary on the amount of defocusing with plastic lenses that can occur in response to changes in temperature or humidity. Plastics will experience temperature- or humidity-dependent changes in linear expansion coefficient or refractive index that are at least about 10 times as great as ordinary glass materials. If the amount of change in the focal length of a plastic lens is written as &f, the amount of defocusing,&p can be expressed by:

Ap = 'JW - m)' where m' is the lateral magnification of the lens groups exclusive of and subsequent to the plastic lens, and m is the lateral magnification of the combination of the plastic lens and subsequent lens groups.

There, if condition (4) is not satisfied. the a Mount of defocusing in response to changes in temperature or humidity will increase to such a value that the lens system is no longer suitable for use with a compact camera.

If the rear subgroup 1 R of a small power is to be formed of a plastic material, it is preferably designed as a positive lens that satisfies condition (3) for the purpose of reducing the amount of defocusing in response to changes in temperature. At elevated temperatures, the lens barrel will extend to increase the distance between the first and second is lens groups, thereby causing the focus position to shift toward the lens. However, with a positive plastic lens, the focus position will be shifted away from the lens at elevated temperatures. Thus, by designing an appropriate power distribution, the positive plastic lens is more effective than glass lenses in reducing the amount of defocusing due to changes in the focus position that are caused by temperature variations.

As aspheric surface can be formed of plastic lenses more easily than glass lenses. Condition (5) relates to the aspheric surface to be formed in the rear subgroup 1 R. If the upper limit of this condition is exceeded, the rear subgroup 1 R will not have a divergent aspheric surface and the under compensation that occurs in the second lens unit 1 b cannot be effectively corrected. If the lower limit of condition (5) is not reached, overcompensation will result to cause higher-order aberrations. In addition, it becomes difficult to produce a desired aspheric surface.

As mentioned before, condition (2) relates to the two negative lenses in the second lens group. If this condition is not satisfied, it becomes difficult to effectively compensate for curvature of the field at the wideangle end.

Having described the lens arrangement of the system of the present invention according to its first aspect, we now describe the method of focusing with this lens system according to the second aspect of the present invention.

In its broadest scope, the focusing method of the present invention is characterized by moving the front subgroup 1 F and the rear subgroup 1 R of the first lens group independently of each other, with a diaphragm 16 stop being provided between the two subgroups. The diaphragm stop provided between the two subgroups 1 F and 1 R permits the brightness at the edge of an image field to increase when the lens is stopped down. Furthermore, if the first lens group is moved toward the object as the distance between the two subgroups 1 F and 1 R is increased, focusing can be accomplished from an infinitely distant object to a near-distance object with reduced variations in astigmatism and curvature of the field.

Condition (1) relates to the movement of the front subgroup 1 F and the rear subgroup 1 R during focusing. If the upper limit of this condition is exceeded. the amount of movement of the two subgroups become so close to each other that the result is not substantially different from the case of focusing by moving the first lens group en masse, whereby difficulty is encountered with effective compensation for astigmatism and curvature of the field. If the lower limit of condition (1) is not reached, the rear subgroup 1 R will move toward the image, causing undesired overcompensation for curvature of the field. If both the diaphragm stop and the rear subgroup 1 R are adapted to remain fixed during focusing, the lens system will become mechanistically simple enough to facilitate its manufacture on a commercial scale.

Figures 3 and 9 are simplified cross-sectional views of the lens systems of Examples 1 and 2, respectively, for the case where focusing is achieved by moving only the front lens group 1 F. Figure 5 is a simplified crosssectional view of the lens system of Example 1 for the case where the front subgroup 1 F and the rear subgroup 1 R are moved for focusing in a ratio of 1:03.

17 Showbelow are the values that are calculated for the foregoing conditions in each of Examples 1 and 2.

Calculation Value Based on Conditions Conditions EX. 1 EX-2 (3) 0.197 0.217 (4) 0.59 0.70 (5) -5.2 -6.4 (2) 1.820 1.817 The readers attention is directed toward copending Patent Application No. 9025215.6 (Serial No. 2240637) from which the present application was divided, and to Application Nos. and which were also divided from GB 9025215.6.

18

Claims

A zoom lens system for use with a compact camera having a wide coverage of angles that comprises, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length and which performs zooming by changing the distance between the first and second lens groups, wherein said first lens group comprises, in order from the object side, a front subgroup 1 F having a positive focal length, a diaphragm stop and a positive rear subgroup 1 R with a small power, and the rear subgroup 1 R is solely composed of a single plastic positive meniscus lens element having a convex surface directed toward the- image, with the following conditions being satisfied:

(3) 0.05 < f1G1f1R < 0.35 (4) (M21 - M1R M2L)
2 < 0.8 where fie; the focal length of the first lens group; f1R the focal length of the rear subgroup 1 R; M2L the lateral magnification of the second lens group at the narrowangle end; and MIR the lateral magnification of the rear subgroup 1 R.

19 2. A zoom lens system as claimed in Claim 1 wherein the rear subgroup 1 R has a divergent amount of asphericity with respect to a paraxial radius of curvature in such a way as to satisfy the following condition:

(5) -20 < 'AR < 0 where &l1R the amount of change in the coefficient of a third-order spherical aberration caused by the aspheric surface in the rear subgroup 1 R.
3. A zoom lens system as claimed in Claim 1 wherein the second lens group comprises, in order from the object side a positive meniscus lens having a convex surface directed toward the image and two negative lens elements each having a concave surface directed toward the object, with the following condition being satisfied:

(2) 1.7 < N2Gn N2Gn: the average of the refractive indices at the d-line of the two negative lens elements in the second lens group.
4. A method of focusing with a zoom lens system for use with a compact camera having a wide coverage of angles, which lens system comprises, in order from the object side, a first lens group having a positive focal length and a second lens group having a negative focal length and which performs zooming by changing the distance between the first and second lens groups, wherein said first lens group comprises, in order from the object side, a front subgroup 1 F having a positive focal length, a diaphragm stop and a positive rear subgroup 1 R being solely composed of a single plastic positive meniscus lens element having a convex surface directed toward the image, and focusing is achieved by moving the first lens group toward the object with the distance between the front subgroup 1 F and the rear subgroup 1 R being increased while satisfying the following conditions:

(3) 0.05 < f1G/f]R < 0.35 (4) (M2L - M1R M21)2 < 0.8 (1) 0: X1R < 0.7 X1F where fic the focal length of the first lens group; f1G the focal length of the rear subgroup 1 R; M2L the lateral magnification of the second lens group at the narrow angle end; M1R: the lateral magnification of the rear subgroup 1 R; 21 X1R: - the amount of movement of the rear subgroup 1 R during focusing; and X,: the amount of movement of the front subgroup 1 F during focusing.
5. A method of focusing as claimed in Claim 4, wherein focusing is performed with both the diaphragm stop and the rear subgroup 1 R being f i xed.