Compact Zoom Lens With a Large Zoom Ratio

This application is related to the following commonly assigned application:

(1.) U.S. Ser. No. 08/673,713, filed Sep. 22, 1995, now U.S. Pat. No. 5,726,810, by Mark M. Meyers for COMPACT ZOOM LENS (Docket 72367/SZS);

This invention relates to compact zoom lenses which are suited for use as an objective or taking lens in photographic cameras, video cameras, electronic cameras, or the like.

Zoom lenses generally are formed of a plurality of groups (units) of one or more lens elements. It has been known to make three-unit zoom lenses with a positive front lens unit, a positive middle lens unit, and a negative rear lens unit. However, because of the desire to reduce the size, weight and costs of cameras with zoom lenses, efforts are being made to create more compact zoom lenses (also referred to as zoom lens systems) with a small number of lens elements and a minimum number of independent zooming lens units, while still producing high quality images over a large focal length range.

More specifically, a zoom lens system must maintain high image quality in each zoom position and retain a flat field over a range of field angles. In addition, it is desirable to have a minimum front vertex to film distance in the telephoto position. This provides a small package size and decreases the overall lens system volume. It is also desirable to have a minimum number of lens elements and a minimal number of separate zooming lens units in order to reduce cost and to decrease mechanical complexity. However, the requirement to produce a compact zoom lens system with a minimum number of independently movable zooming lens units and a minimum number of lens elements often conflicts with the requirement to produce high quality images over a large focal length range. This conflict occurs because when (i) the air spaces between lens units are reduced and (ii) the front vertex to the image plane distance is also reduced (provided that the optical power of the lens units does not change), the lens units can not move far enough during zooming. The lack of travel distance, in turn, decreases the lens system's zoom ratio. The zoom ratio ZR is measured by the ratio of lens system's focal lengths, i.e. ZR=f.sub.t /f.sub.w, where f.sub.w is the focal length of the zoom lens system in the wide-angle mode and f.sub.t is the focal length of the lens system in the telephoto position. If the optical power of lens units increases (without the additional lens elements) in order to provide a larger zoom ratio, lens surfaces become stronger, compromising the performance of the lens system.

Thus, as the optical powers of lens units increase to increase the zoom ratio to beyond 2.times., it becomes more difficult to maintain the lens system's performance. Because of this, lens systems with higher zoom ratios require larger numbers of lens elements. This, in turn, increases the cost of the lens systems and often makes the lens system less compact. As stated above, if the number of lens elements in a lens system is reduced, the performance of the lens system suffers.

The compactness of the lens system can be determined by the total length D of the lens system, measured from the front vertex of the lens system to its rear vertex. The smaller the value of D, the more compact the lens system.

The compactness of the zoom lens system can also be measured by the ratio L.sub.v to f.sub.t, where L.sub.v is the distance from the front vertex of the lens system to the film plane in the telephoto position. The smaller the ratio, the more compact the zoom lens system. However, as stated above, the compactness of the lens system should not come as a result of reduced zoom ratio or reduced optical performance.

The following five U.S. patents all describe zoom lens systems which have three lens units of positive, positive and negative optical power, respectfully.

U.S. Pat. No. 4,726,668, issued Feb. 23, 1988, discloses a zoom lens system with three lens units of positive, positive and negative optical powers, respectfully. An aperture stop is located within the middle lens unit. The front and the rear lens units move independently of each other while the second unit, which contains an aperture stop, is held stationary during zooming. This lens system has six lens elements and its total length D from the front vertex of the lens system to its rear vertex is 46.95 at the telephoto position. At the wide-angle position, the maximum amount of distortion is 5% and the amount of astigmatism is 0.8 millimeters. In the telephoto position, the distortion amount is again about 5% and the maximum astigmatism is greater than 0.8 millimeters. It is desirable to have a lens system that has better performance with respect to astigmatism and distortion, fewer lens elements and that is more compact, i.e. a lens system which has a substantially smaller total length D in the telephoto position.

U.S. Pat. No. 5,353,160, issued Oct. 4, 1994, discloses a zoom lens system which has either four or five lens elements arranged into three movable lens units. The lens system has a zoom ratio ZR=f.sub.t /f.sub.w of only 2.0, and its compactness ratio L.sub.v /f.sub.t is 0.93. The amount of distortion is about 2% to 4% at the wide-angle position and about 2% to 4% at the telephoto position for the various embodiments shown. The amount of astigmatism is about 0.4 millimeters in the wide-angle position and about 0.4 to 0.6 millimeters in telephoto position.

U.S. Pat. No. 5,325,235, issued Jun. 28, 1994, has relatively few lens elements (four). Its zoom ratio ZR is only 1.6 and its compactness ratio is about 1.0 (for example, it is 0.96 and 1.13 in the embodiments 3 and 5, respectfully). Thus, this zoom lens system is even less compact and has an even smaller zoom ratio than the zoom lens system described in U.S. Pat. No. 5,353,160. The maximum amount of astigmatism ranges from about 0.5 millimeters to about 1.0 millimeters in both the wide-angle and telephoto modes. Depending on the embodiment, the distortion varies from about 0.5% to about 2% when the lens system is in the telephoto position and from about 2% to about 5% when the lens system is in the wide-angle position.

U.S. Pat. No. 5,353,159, issued Oct. 4, 1994, discloses a zoom lens system with three lens units that move during zooming. The zoom lens system has a zoom ratio ZR of only 1.87, and a compactness ratio of about 1.01. This zoom lens system has 8 or 9 lens elements. The amount of distortion ranges from about 1% to about 4% in both the telephoto and wide-angle positions. The amount of astigmatism ranges from about 0.2 millimeters to about 0.5 millimeters when the lens system is in the wide-angle position and from about 0.2 to 0.7 millimeters when the lens system is in the telephoto position. The second zoom lens system embodiment disclosed in this reference has a lens element with a V-number of 70 in its second lens unit.

U.S. Pat. No. 5,216,547, issued Jun. 1, 1993, discloses a zoom lens system with three lens units that move during zooming. The zoom lens system has a zoom ratio ZR of only 1.87, and a compactness ratio of about 0.94. This zoom lens system has 7 lens elements. The amount of distortion ranges from about 2% to about 3% in both the telephoto and wide-angle positions. The amount of astigmatism or/and field curvature range from about 0.2 millimeters to about 0.5 millimeters when the lens system is in the wide-angle position and from about 0.2 to 0.7 millimeters when the lens system is in the telephoto position. The second zoom lens system embodiment disclosed in this reference has a lens element with a V-number of 70 in its second lens unit.

Other patents also provide a three unit lens system, but they are either substantially less compact than the lens systems of the above described U.S. patents, or have a substantially greater number of lens elements or have a substantially smaller zoom ratio.

An object of the present invention is to provide a zoom lens system which offers superior optical performance, as few as possible independently movable lens units, a relatively large zoom ratio (i.e., a zoom ratio ZR of about 2.4 or larger) and which is more compact than prior art zoom lens systems.

The zoom lens according to the present invention comprises three lens units. More specifically, there is a front lens unit of positive refractive power consisting of a positive lens element and a negative lens element, a middle lens unit of positive refractive power, and a rear lens unit of negative refractive power. During zooming from a wide-angle to a telephoto position, the front and the rear lens units are simultaneously movable towards an object side during zooming from a wide angle to a telephoto position. The middle lens unit moves towards the object side at a relatively slower speed than that at which the front and the rear lens units are moved. That is, the middle lens unit travels a shorter net axial distance than the distance traveled by the front and rear lens units. The "net axial distance" refers to the axial distance between one zoom and another zoom position of a lens unit. The lens elements have sufficient optical powers, spacings and radii of curvature to provide for a large zoom ratio ZR and to maintain an overall compactness so that L.sub.v /f.sub.t <0.9 and ZR=f.sub.t /f.sub.w >2.3, where L.sub.v is the distance from the front vertex of the lens system to the film plane in the telephoto position, f.sub.t is the focal length of the lens system in the telephoto position, and f.sub.w is the focal length of the zoom lens system in the wide-angle position. At least one lens element comprising said middle or said rear lens unit has a V-number greater than 70.

According to a preferred embodiment, this V-number is greater than 80. Also, according to a preferred embodiment the front and the rear lens units are integral so as to be simultaneously movable for zooming at the same speed and direction.

FIGS. 1A-1C a sectional view of the lens system 100 of a first illustrative embodiment. FIG. 1A is a sectional view of the lens system 100 in a wide-angle position. FIG. 1B is a sectional view of the lens system 100 in a middle position. FIG. 1C is a sectional view of the lens system 100 in a telephoto position;

FIGS. 2A-2F are graphical representations of the aberrations of the lens system 100 illustrated in Figures 1A-1C;

FIGS. 2G-2I are plots of the Modulation Transfer Function (MTF) vs. frequency of the lens system 100 in wide-angle, middle, and telephoto positions, respectively;

FIGS. 3A-3C are sectional views of the lens system 200 of a second illustrative embodiment. FIG. 3A is a sectional view of the lens system 200 in a wide-angle position. FIG. 3B is a sectional view of the lens system 200 in a middle position. FIG. 3C is a sectional view of the lens system 200 in a telephoto position;

FIGS. 4A-4F are graphical representations of the aberrations of the lens system 200 illustrated in FIGS. 3A-3C;

FIG. 4G-4I are plots of the Modulation Transfer Function (MTF) vs. frequency of the lens system 200 in wide-angle, middle, and telephoto positions, respectively; and

FIG. 5 is a schematic side-sectional view of a zoom lens system 100, 200 and a mount.

The embodiments of the invention illustrated by two examples (lens systems 100 and 200) are shown respectively in FIGS. 1A-1C and 3A-3C, and which are set fourth in Tables 1A-1C and 2A-2C respectively. In the tables and the drawings, the surfaces R are numbered by subscripts from the front or object side of the lens to the rear or image side of the lens. The thickness T of the lens elements, the surface radii R and the spacings S between elements are also numbered from front to rear. For example, S.sub.1 corresponds to the first air space, S.sub.2 to the second air space, S.sub.3 to the third air space, and S.sub.4 to the fourth airspace. In the tables, spaces and thicknesses are listed on the same line as the surface preceding the space or thickness, as the case may be. All indices N.sub.e are for the mercury e line of the spectrum at a wavelength .lambda..sub.e of 546.1 nm. The symbol v stands for the Abbe V-number for the lens material. The thickness and the spacings provided in the tables are in millimeters.

In the illustrative embodiments of the present invention, a zoom lens system includes 5 lens elements E.sub.1 -E.sub.5 arranged into three axially movable lens units U.sub.1, U.sub.2 and U.sub.3. Lens units U.sub.1 and U.sub.3 are linked together mechanically and move together during zooming, as shown in FIG. 5. Therefore, the space between these two lens units (i.e. U.sub.1 and U.sub.3) does not vary for zooming, and thus have the substantial advantage of requiring a much simpler zooming mechanism than those required by typical zoom lens systems having three independently moving lens units. More specifically, the front lens unit U.sub.1 and the rear lens unit U.sub.3 move integrally. They are simultaneously movable at the same speed and direction, while the middle lens unit U.sub.2 is movable in the same direction at a slower speed. The space between the front lens unit U.sub.1 and the middle lens unit U.sub.2 widens while the airspace between the middle lens unit U.sub.2 and the rear lens unit U.sub.3 narrows as the three lens units move toward the object side (along an optical axis) when the zoom lens system zooms from a wide-angle position (W) towards a telephoto position (T). Although it is preferred that lens units U.sub.1 and U.sub.3 move together, this is not necessary. The lens units U.sub.1 and U.sub.3 can be decoupled to provide an even larger zoom ratio.

According to the illustrative embodiments, the front lens unit U.sub.1 is an achromatized positive power doublet. It is corrected for primary axial color. The focal length FL.sub.1 of the front lens unit U.sub.1 of the first illustrative embodiment is about 50.3 mm and its power is about 1.99.times.10.sup.-2. The focal length FL.sub.1 of the front lens unit U.sub.1 of the second illustrative embodiment is about 60.9 mm and its power is about 1.64.times.10.sup.-2. It is preferable for the doublet (of the front lens unit U.sub.1) to be a cemented doublet with a positive power lens element E.sub.1 located in front (i.e. towards the object side) and a negative power lens element E.sub.2 located behind lens element E.sub.1. The use of a cemented doublet as the front lens unit U.sub.1 allows for the achromatization of one of the zooming lens units. In addition, using a cemented doublet makes for simplified mounting and alignment of the front lens unit U.sub.1 compared to what is required in a design which utilizes airspaced doublets, such as the one disclosed in the above-mentioned U.S. Pat. No. 5,353,160. This simplification occurs because it is easier to center two lens elements which are cemented together before they are mounted in a lens barrel, than it is to center two airspaced lens elements in the lens barrel. This is due to the typical manufacturing variations of mount surfaces and variations of lens element diameters. Displacement of the optical axis of the two non-cemented lens elements caused by lens diameter variations or mounting variation can cause aberrations such as field tilt and axial coma.

Furthermore, using a positive power front lens element E.sub.1 allows the light rays to be bent towards focus (i.e. to converge) at the first surface, whereas a negative power front lens element actually causes the light rays to diverge. Thus, the lens system with the positive power front lens element E.sub.1 can be more compact (i.e. shorter) than a similar lens system with a negative power front lens element. Additionally, the lens system with a positive power front lens element can use lens elements with smaller clear aperture diameters in the front lens unit. Having a positive power frontmost lens element E.sub.1 also helps to achieve a better telephoto ratio (L.sub.v /f.sub.t). Finally, the positive front unit U.sub.1, where the front lens element E.sub.1 is positive, produces lower overall astigmatism, flatter field curves (lower petzval curvature) and generates less image plane distortion than similar zoom lenses with a negative power front lens element in the front lens unit.

According to the illustrative embodiments, the middle lens unit U.sub.2 is located behind the front lens unit U.sub.1. The middle lens unit U.sub.2 is also positive (i.e. it has positive power). It contains two very slightly airspaced lens elements (a meniscus lens element E.sub.3 and a biconvex lens element E.sub.4). The meniscus lens element E.sub.3 is made from a glass with a V-number of 81.5. The focal length FL.sub.2 of the second lens unit U.sub.2 of the first illustrative embodiment is about 20.4 millimeters and its power is about 4.9.times.10.sup.-2. The focal length FL.sub.2 of the middle lens unit U.sub.2 of the second illustrative embodiment is about 23.1 millimeters and its power is about 4.3.times.10.sup.-2.

The rear optical unit U.sub.3 is substantially negative in power. Its focal length in the first illustrative embodiment is about -16.6 millimeters and its power is about -6.0.times.10.sup.-2. In the second illustrative embodiment its focal length is about -19.2 millimeters and its power is about -5.2.times.10.sup.-2. The rear optical unit U.sub.3 contains a relatively strong negative power lens element E.sub.5 with a concave object side surface S.sub.8. It is preferred that the absolute value of the ratio of lens unit powers .phi..sub.3 to .phi..sub.2 is approximately 1.15 to 1.3. The object side surface S.sub.8 of lens element E.sub.5 has a smaller radius of curvature then that of the image side surface S.sub.9. The use of a strong negative lens element E.sub.5 in the rear allows for the zoom lens to have a very effective field flattener when the lens system is in wide-angle position and provides for a high level of aberration correction in the telephoto position. The compact zoom lens system of the present invention also achieves minimization of the front vertex to film plane distance partially via the use of the strong rear lens element E.sub.5 and also due to the relative movement of the middle lens unit U.sub.2 towards this negative lens element E.sub.5.

As stated above, the front lens unit U.sub.1 is corrected for primary axial color. The overall zoom lens (including the middle and the rear lens units) is also color corrected. The color correction of the middle and rear lens units is achieved independently from that of the front lens unit. More specifically, the compact zoom lenses 100, 200 utilize a high V-number glass for the lens element E.sub.3 to allow for a better correction of primary axial color. This becomes especially important in high zoom ratio zoom lenses having a telephoto focal length greater than 60 mm. Thus, with the use of a high V-number glass, applicant has achieved a zoom ratio of 3.5 (lens system 200) with only 5 lens elements. A high V-number glass is defined herein as a glass with an Abbe's V number larger than 71. It is preferred that this V-number be between 75 and 85 and more preferably be between about 80 and 85. Although higher V-number glasses may also be used, they tend to be very expensive.

Although any of the last three lens elements may utilize such high V-number glass to achieve the large zoom ratio, it is preferred to use it with the lens element E.sub.3 because this lens element is relatively small (i.e., it has the smallest volume of the rearmost three lens elements) and because it contains no aspheres.

According to the first illustrative embodiment (lens system 100) the aperture stop AS is in the form of a shutter which is positioned with the front lens unit U.sub.1. That is, the shutter moves together with the front lens unit U.sub.1. According to the second illustrative embodiment (lens system 200) the aperture stop AS is in the form of a shutter which is positioned with the middle lens unit U.sub.2. That is, the shutter moves together with the middle lens unit U.sub.2. The placement of the aperture stop between the front and the middle lens units U.sub.1 U.sub.2 reduces the size of the front lens unit U .sub.1, which simplifies mounting and reduces the size of the telescoping zoom lens barrel.

As shown in FIG. 5, the mechanical mount for this type of zoom lens is generally constructed with the larger mount M.sub.1 connecting the front and rear lens units U.sub.1 and U.sub.3 and a smaller mount M.sub.2 for the middle lens unit U.sub.2 which is nested inside the larger lens mount. Having the shutter attached to the front lens unit U.sub.1 as shown in the first illustrative embodiment, allows the shutter to be mounted on a larger lens mount M.sub.1 which links the front and rear lens units U.sub.1 and U.sub.3 together. A gearing mechanism G and a stepping motor SM could be used to control the opening size of the shutter.

Lens systems 100, 200 have at least two aspheric surfaces. In the second illustrative embodiment there is one aspheric surface in each lens unit. The aspheric equation describing these aspheric surfaces is: ##EQU1## where: X is the distance along the optical axis OA;

Y is the height from the optical axis;

C is the reciprocal of the vertex radius of curvature of the curved lens surface;

k is the conic constant; and

AD through AK are aspheric coefficients of 4th, 6th, 8th, 10th, 12th, 14th, 16th, and 18th order. The values of the aspheric coefficients for the various aspheric lens surfaces for the two lens embodiments are provided in Tables 1B and 2B.

More specifically, in both lens system embodiments, the aspheric surfaces are located on the surfaces with radii R.sub.7 and R.sub.8 associated with lens elements E.sub.4 and E.sub.5, respectively. In the second embodiment the additional aspheric surface is located on a surface with radius R.sub.1 associated with the lens element E.sub.1. The use of aspheric surfaces provides several advantages. First, it helps to minimize the number of lens elements in the lens system.

Secondly, having no more than one aspheric surface per lens unit results in aspheric surfaces being widely separated (in a majority of zoom positions) from one another, which in turn results in low sensitivity of the lens system to tilt, decentration, surface figure error (i.e. power irregularity) and thickness errors. Lens systems which utilize biaspheric elements tend to be very sensitive to alignment, especially tilt and decentration of one lens surface to the other and to the displacement of a lens element from the optical axis. The low sensitivity of the present lens system results in a zoom lens system that is easier and less expensive to manufacture and assemble than a typical zoom lens system of comparable performance.

Thirdly, in addition to decreasing the number of lens elements in the lens system, aspheric surfaces significantly increase the resultant image quality. The resulting image quality of lens systems 100 and 200 is very high given the relatively small number of elements, the large zoom ratio, and the small number of independently moving zooming lens units.

For example, zoom lens system 100 of the first illustrative embodiment provides a 2.5.times. zoom ratio. This zoom lens system has a maximum amount of astigmatism 0.5 and only 3.5% distortion in the wide-angle position, while in the telephoto position, the amount of astigmatism is less than 0.4 millimeters and the maximum distortion amount is only 0.3%. Zoom lens system 200 of the second illustrative embodiment provides a 3.5.times. zoom ratio. This zoom lens system has only 0.5 millimeters astigmatism and 3.5% distortion in the wide-angle position while in the telephoto position, the amount of astigmatism is 0.6 millimeters and the distortion is only 0.2%.

FIGS. 2A-2I and 4A-4I detail the performance of zoom lenses 100 and 200 at wide-angle, mid-zoom and telephoto positions. Included are ray traces for the various field positions, field curvature, distortion and lateral color plots, and MTF (Modulation Transfer Function) vs. frequency graphs for the wide-angle, mid-zoom and telephoto zoom positions. More specifically, FIG. 2A provides ray intercept plots for lens 100 when it is operating in the telephoto position at 0, 0.7, and full field (.omega.=12.8.degree.) of view, respectfully. FIG. 2B provides ray intercept plots for lens 100 when it is operating in the middle position at 0, 0.7, and full field (.omega.=19.degree.) of view. FIG. 2C provides ray intercept plots for lens 100 when the lens system is operating at the wide-angle position (.omega.=27.2.degree.). FIGS. 2D-2F represent Astigmatism, Distortion and Lateral Color when the lens system is operating in the telephoto, middle and wide-angle position, respectfully.

Similarly, FIG. 4A provides ray intercept plots for lens 200 when it is operating in the telephoto position at 0, 0.7, and full field (.omega.=9.6.degree.) of view, respectfully. FIG. 4B provides ray intercept plots for lens 200 when it is operating in the middle position at 0, 0.7, and full field (.omega.=15.7.degree.) of view. FIG. 4C provides ray intercept plots for lens 200 when the lens system is operating at the wide-angle (.omega.=28.5.degree.) position. FIGS. 4D-4F represent Astigmatism, Distortion and Lateral Color when the lens system is operating in the telephoto, middle and wide-angle position, respectfully.

Note that the illustrative examples are corrected for zoom ratios of 2.5.times. and 3.5.times.. They have a compactness ratio L.sub.v /f.sub.t <0.9. The F-numbers range from F/4.9 to F/7.75 in the first illustrative embodiment and from F/6 to F/8.4 in the second illustrative embodiment.

The invention has been described in detail with particular reference to a preferred embodiment thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.