Zoom lens system

[0002] This application is based on Patent Application No. 2000-145033 filed in Japan, the content of which is hereby incorporated by reference.

[0003] The present invention relates to a zoom lens, for example, a zoom lens suitable as a projection optical system used in a projection device (liquid crystal projector for projecting an image on a screen such as a liquid crystal panel and the like).

[0004] Liquid crystal projectors provided with a liquid crystal panel having a micro lens array to brighten the projection image have recently become very popular. Conventional zoom lenses often used in liquid crystal projectors have an F-number of approximately F2.5, but when a liquid crystal panel having a micro lens array is used, a brighter (small F-number) zoom lens is required.

[0005] It is difficult to reduce the F-number while satisfying optical performance requirements in a projection zoom lens. For example, Japanese Laid-Open Publication No. 10-268193 discloses a zoom lens of the negative-positive-positive-negative-positive type, wherein aberration correction is difficult when the zoom lens is brighter at the wide angle end with an F-number of approximately 1.5.

[0006] An object of the present invention is to provide an improved zoom lens.

[0007] Another object of the present invention is to provide a zoom lens, which is bright and provides high performance, for use in a projector.

[0008] These objects are attained by a zoom lens system comprising, sequentially from the enlargement side thereof: a first lens unit having negative optical power, a second lens unit having positive optical power, a third lens unit having positive optical power, a fourth lens unit having negative optical power, and a fifth lens unit having positive optical power, wherein the first and fifth lens units are stationary units, and during zooming from a longest focal length condition to a shortest focal length position, the second lens unit and the third lens unit move in the direction from the enlargement side toward the reduction side and the fourth lens unit moves in the direction from the reduction side toward the enlargement side, and the following conditions (1)˜(4) are satisfied.

[0009] −2.0<f1/fW<−1.1  (1)

[0010] 1.3<f2/fW<2.7  (2)

[0011] 3.5<f3/fW<20  (3)

[0012] 0.6<f5/fW<1.2  (4)

[0013] Where fW represents the focal length of the total system at the shortest focal length position, f1 represents the focal length of the first lens unit, f2 represents the focal length of the second lens unit, f3 represents the focal length of the third lens unit, and f5 represents the focal length of the fifth lens unit.

[0014] In another aspect of the invention, a projection system comprises a light source, at least one liquid crystal panel, an optical integrating system for focusing a light from the light source on to the liquid crystal panel, and a projection optical system. The projection optical system comprises, sequentially from an enlargement side thereof to a reduction side thereof, a first lens unit having negative optical power, a second lens unit having positive optical power, a third lens unit having positive optical power, a fourth lens unit having negative optical power, and a fifth lens unit having positive optical power, wherein the first and fifth lens units are stationary units, and during zooming from a longest focal length condition to a shortest focal length condition, the second lens unit and the third lens unit move in the direction from the enlargement side to the reduction side and the fourth lens unit moves in the direction from the reduction side to the enlargement side, and at least one of the following conditions (1)˜(4) are satisfied.

[0015] −2.0<f1/fW<−1.1  (1)

[0016] 1.3<f2/fW<2.7  (2)

[0017] 3.5<f3/fW<20  (3)

[0018] 0.6<f5/fW<1.2  (4)

[0019] Where fW represents the focal length of the total system at the shortest focal length position, f1 represents the focal length of the first lens unit, f2 represents the focal length of the second lens unit, f3 represents the focal length of the third lens unit, and f5 represents the focal length of the fifth lens unit.

[0020] These and other objects and features of this invention will become clear from the following description taken in conjunction with the preferred embodiments with reference to the accompanying drawings, in which:

[0021] FIG. 1 is a lens structural diagram of a first embodiment (example 1);

[0022] FIG. 2 is a lens structural diagram of a second embodiment (example 2);

[0023] FIG. 3 is an aberration diagram of example 1;

[0024] FIG. 4 is an aberration diagram of example 2; and

[0025] FIG. 5 is a projection device including a projection optical system of the present invention.

[0026] The zoom lens of the present invention is described hereinafter with reference to the accompanying drawings. The embodiments described below are zoom lenses suitable for use as the projection optical system for a projector device (liquid crystal projector), but needless to say, are suitable for use as an image sensing optical system for a solid state image sensing device (e.g., video camera and digital camera).

[0027] FIGS. 1 and 2 are lens structural diagrams corresponding to the zoom lenses of the first and second embodiments, respectively, and show the lens arrangement at the telephoto end (T). In each lens structural diagram, the arrows mj (j=1, 2, 3, 4, 5) and the arrow mS schematically represent the movement of the No. j unit (Grj) and the aperture stop (ST), respectively, in zooming from the telephoto end (longest focal length end, T) to the wide angle end (shortest focal length end W), and the dashed arrows (m1, m5, mS) represent the fixed positions during zooming. Also, in the lens structural diagrams, a surface with an appended symbol ri (i=1, 2, 3, . . . ) represents the No. i surface counting from the enlargement side (i.e., projection side), and an axial distance with the appended symbol di represents the variable spacing which changes during zooming among the axial spacing di (i=1, 2, 3, . . . ) of the No. i surface counting from the enlargement side.

[0028] The first and second embodiments are 5-element zoom lenses comprising, sequentially from the enlargement side: a first lens unit (Gr1) having negative optical power, a second lens unit (Gr2) having positive optical power, a third lens unit (Gr3) having positive optical power, a fourth lens unit (Gr4) having negative optical power, and a fifth lens unit (Gr5) having positive optical power, and dichroic prism (Pr) which remains stationary during zooming is disposed on the reduction side of the fifth lens unit (Gr5). The zoom elements comprise the stationary elements of the first lens unit (Gr1) and the fifth lens unit (Gr5), and the moving elements of the second lens unit (Gr2), third lens unit (Gr3), and the fourth lens unit (Gr4).

[0029] In these embodiments, during zooming from the telephoto end (T) to the wide angle end (W), the second lens unit (Gr2) and the third lens unit (Gr3) move from the enlargement side to the reduction side (m2, m3), and the fourth lens unit (Gr4) moves from the reduction side to the enlargement side. During zooming, the spacing of the first lens unit (Gr1) and the second lens unit (Gr2) and the spacing of the fourth lens unit (Gr4) and the fifth lens unit (Gr5) increase monotonically and the spacing of the second lens unit (Gr2) and the third lens unit (Gr3) increases monotonically in the first embodiment (FIG. 1), but this monotonicity is reduced in the second embodiment (FIG. 2). Furthermore, although the aperture stop (ST) arranged between the third lens unit (Gr3) and the fourth lens unit (Gr4) is at a fixed, stationary position during a zooming operation, as described in the first embodiment (FIG. 1), the aperture stop (ST) can move during a zooming operation as part of the third lens unit (Gr3) in the second embodiment (FIG. 2). Focusing is accomplished, for example, by moving the focus of the first lens unit (Gr1).

[0030] In the first embodiment (FIG. 1), each lens unit is constructed as described below sequentially from the enlargement side. The first lens unit (Gr1) comprises a negative meniscus lens having a concave surface on the reduction side, and a biconcave negative lens. The second lens unit (Gr2) comprises a positive meniscus lens having a convex surface on the reduction side, and a positive meniscus lens having a convex surface on the enlargement side. The third lens unit (Gr3) is a cemented lens comprising a negative meniscus lens having a concave surface on the reduction side, and a biconvex positive lens. The fourth lens unit (Gr4) has a cemented lens including a biconvex positive lens and a biconcave negative lens, and includes a biconcave negative lens. The fifth lens unit (Gr5) has a cemented lens comprising a biconcave negative lens and a biconvex positive lens, and includes two biconvex positive lenses.

[0031] In the second embodiment (FIG. 2), each lens unit is constructed as described below sequentially from the enlargement side. The first lens unit (Gr1) comprises a negative meniscus lens having a concave surface on the reduction side, and a biconcave negative lens. The second lens unit (Gr2) comprises a positive meniscus lens having a convex surface on the reduction side, and a biconvex positive lens. The third lens unit (Gr3) has a cemented lens including a negative meniscus lens having a concave surface on the reduction side and a positive meniscus lens having a convex surface on the enlargement side, and an aperture stop (ST). The fourth lens unit (Gr4) has a cemented lens including a positive meniscus lens having a convex surface on the enlargement side and a negative meniscus lens having a concave surface on the reduction side, and a biconcave negative lens. The fifth lens unit (Gr5) has a cemented lens including a biconcave negative lens and a biconvex positive lens, a biconvex positive lens, and a positive meniscus lens having a convex surface on the enlargement side.

[0032] In these embodiments, a stationary element having negative optical power is disposed on the outermost position on the enlargement side. The diameter of the lens of the first lens unit (Gr1) can be reduced by having the first lens unit (Gr1), which has a negative optical power, remain stationary during zooming. Even when the first lens unit (Gr1) is extended for focusing, the mechanical structure of the first lens unit (Gr1) can be simple and compact in the diameter direction. Accordingly, the zoom lens is compact in the diameter direction.

[0033] In a 5-piece zoom lens of the negative-positive-positive-negative-positive type wherein the first lens unit (Gr1) and the fifth lens unit (Gr5) are stationary elements and the second lens unit (Gr2) and the third lens unit (Gr3) move from the enlargement side to the reduction side and the fourth lens unit (Gr4) moves from the reduction side to the enlargement side during zooming from the telephoto end (T) to the wide angle end (W), it is desirable that the at least one condition among the conditions (1)˜(4) below is satisfied, and is even more desirable that all conditions (1)˜(4) are satisfied. A bright (small F-number) high-performance zoom lens can be realized which is suitable for a liquid crystal projector having a liquid crystal panel provided with a micro lens array.

[0034] −2.0<f1/fW<−1.1  (1)

[0035] 1.3<f2/fW<2.7  (2)

[0036] 3.5<f3/fW<20  (3)

[0037] 0.6<f5/fW<1.2  (4)

[0038] Where fW represents the focal length of the total system at the wide angle end, f1 represents the focal length of the first lens unit, f2 represents the focal length of the second lens unit, f3 represents the focal length of the third lens unit, f5 represents the focal length of the fifth lens unit.

[0039] Condition (1) stipulates the optical power of the first lens unit (Gr1). When the lower limit of condition (1) is exceeded, the optical power of the first lens unit (Gr1) is too weak, and the first lens unit (Gr1) requires excessive movement when the unit (Gr1) is extended for focusing. For this reason, there is large aberration fluctuation from an infinity focus position to a near field focus position, and it becomes difficult to balance performance at the infinity focus positions and near field focus positions. Conversely, when the upper limit of condition (1) is exceeded, the optical power of the first lens unit (Gr1) becomes excessive, and distortion cannot be controlled.

[0040] Conditions (2) and (3) stipulate the optical power of the second lens unit (Gr2) and the third lens unit (Gr3), respectively. When the lower limit of condition (2) or condition (3) is exceeded, the optical power of the second lens unit (Gr2) or the third lens unit (Gr3) becomes excessively strong, and it becomes difficult to control aberration. Conversely, when the upper limit of condition (2) or condition (3) is exceeded, the optical power of the second lens unit (Gr2) or the third lens unit (Gr3) becomes weak and the amount of movement required for zooming increases. As a result, the total length is increased, and the size of the first lens unit (Gr1) becomes large due to the aperture stop (ST) position.

[0041] Condition (4) stipulates the optical power of the fifth lens unit (Gr5). When the lower limit of condition (4) is exceeded, the optical power of the fifth lens unit (Gr5) becomes excessively strong, and the required back focus for the arrangement of the dichroic prism (Pr) cannot be ensured. Conversely, when the upper limit of condition (4) is exceeded, the optical power of the fifth lens unit (Gr5) becomes weak, the back focus becomes excessively long so as to enlarge the projection system.

[0042] It is desirable that condition (5) below, which stipulates the optical power balance of the second lens unit (Gr2) and the third lens unit (Gr3), is satisfied in order to balance the size and optical performance of the zoom lens. The compactness and high optical performance of the zoom lens can be effectively maintained while providing a bright zoom lens by satisfying condition (5). When the lower limit of condition (5) is exceeded, the total length of the zoom lens is increased, and conversely, when the upper limit of condition (5) is exceeded, it becomes difficult to control various types of aberration.

[0043] 0.6<(fW/f2)+(fW/f3)<0.75  (5)

[0044] Although the various lens elements comprising the previously described embodiments are only refractive type lenses which deflect incident light by refraction (i.e., lenses of a type which accomplish deflection by the interface of similar media having different refractive indices), the present invention is not limited to this arrangement. For example, the lens elements may be constructed of lenses of the diffractive type which deflect incident rays by diffraction, lenses of the refraction-diffraction hybrid type which deflect incident rays using a combination of diffractive action and refractive action, lenses of the refractive distribution type which deflect incident rays by the refractive distribution within the medium and the like. Furthermore, at least one lens surface may be an aspherical surface to obtain effective aberration correction.

[0045] The zoom lens systems of the first and second embodiments, described above, are suitable for use as a projection optical system P1 for a projection device, illustrated in FIG. 5. In a preferred embodiment, the projection device is provided with a light source 50, an optical integrating system I1, three liquid crystal panels 52a, 52b, and 52c, and a projection optical system P1, as described in the first and second embodiments.

[0046] The optical integrating system I1, of the projection system, focuses a light emitted from the light source onto the three liquid crystal panels 52a, 52b, and 52c. At an intermediate position, after the light leaves the optical integrator system I1 and before the light impinges the liquid crystal panels 52a, 52b, and 52c, the light in the optical integrator system I1 is separated into R, G, B color data. The three liquid panels 52a, 52b, and 52care then used to selectively transmit the R, G, B color data, respectively, to the projection optical system P1, which projects an image onto a projection surface. The projection optical system P1 is preferably the zoom lens system as described in the first and second embodiments, and satisfies the requirements set forth in conditions (1)-(5), as previously described.

[0047] Further, In the projection system, the prism Pr disposed on the reduction side of the fifth lens element Gr5 can be used to combine the color data received from each liquid crystal panels 52a, 52b, and 52c, and to transmit the combined color data to the projection optical system P1. Additionally, relay lens configurations can be used in the projection system as needed.

[0048] The structure of the zoom lens of the present invention is described below by way of examples using construction data. Examples 1 and 2 respectively correspond to the first and second embodiments, and the lens structural diagrams (FIGS. 1 and 2) of the first and second embodiments represent the lens structures of the corresponding examples 1 and 2.

[0049] In the construction data of the examples, ri (i=1, 2, 3, . . . ) represents the number i surface and its radius of curvature (mm) counting from the enlargement side, di (i=1, 2, 3, . . . ) represents the axial spacing (mm) of the number i surface counting from the enlargement side, Ni (i=1, 2, 3, . . . ) and νi (i=1, 2, 3, . . . ) respectively represent the d-line refractive index (Nd) and Abbe number (νd) of the number i lens counting from the enlargement side. In the construction data, the axial spacing which changes during zooming is the variable air gap at the telephoto end (long focal length state, T) to the middle (intermediate focal length state, M), and wide angle end (short focal length, W). The total system focal length (f, mm), F-number (FNO), and corresponding value of each condition are described at each focal length state (T), (M), (W).

[0050] FIGS. 3 and 4 are aberration diagrams corresponding to examples 1 and 2, and show various types of aberration (sequentially from the left: spherical aberration, astigmatism, distortion; where Y′ is the maximum image height) relative to an object at infinity on the reduction side at the telephoto end (T) and wide angle end (W). In the spherical aberration diagrams, the solid line represents the d-line spherical aberration, the single-dash line (g) represents the g-line spherical aberration, the double-dash line (c) represents c-line spherical aberration, and the broken line (SC) represents the sine condition. In the astigmatism diagrams, the broken line (DM) represents d-line astigmatism on the meridional plane, and the solid line (DS) represents the d-line astigmatism on the sagittal plane. In the distortion diagrams, the solid line represents the percentage distortion on the d-line. Although the screen surface (projection surface) is primarily the image surface and the display element surface (e.g., liquid crystal surface) is the object surface, when each example was used as a projection zoom lens in a projection device (e.g., liquid crystal projector), the optical performance was evaluated using the each example as a reduction system (e.g., an image sensing optical system) wherein the screen surface is the object surface and optical performance was evaluated at the display element surface.

[0051] First Embodiment

[0052] Second Embodiment

[0053] The present invention realizes a bright (small F-number) m high-performance zoom lens. For example, if the zoom lens of the present invention is used in a liquid crystal projector provided with a liquid crystal panel having a micro lens array, a brighter projection image can be obtained than by a conventional zoom lens.

[0054] Although the present invention has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modification will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.