1. Field of the Invention
The present invention relates to a zoom lens and an image projection apparatus including the same, suitable for, for example, a liquid crystal projector having a long back focus and maintaining good pupil consistency with a lighting system.
2. Description of the Related Art
Hitherto, various kinds of liquid crystal projectors (image projection apparatuses), each including a display device such as a liquid crystal display and projecting an image formed in the display device onto a screen, have been proposed.
In particular, the liquid crystal projector is in widespread use for a conference and a home theater, as an apparatus capable of projecting an image outputted from a personal computer or the like onto a large screen. A projection lens for use in the liquid crystal projector is requested to have, for example, the following features.
(a) In a three-panel color liquid-crystal projector including three liquid crystal displays, light emitted from a white light source is generally separated into red, green and blue colors by a color-separation optical system and introduced into the corresponding liquid crystal displays. Three kinds of light emitted from the respective liquid crystal displays are synthesized by a color-synthesis optical system and incident on a projection lens.
Due to its configuration, a space having a prism and the like arranged therein, for synthesizing the three kinds of color light passing through the liquid crystal displays, must be provided between the liquid crystal displays and the projection lens, thereby causing the projection lens to have a certain length of back focus. In addition, use of a reflective liquid-crystal display, i.e. use of a liquid crystal on silicon (LCOS) as the liquid crystal display causes the projection lens to have a longer back focus than upon use of a transmissive liquid crystal display.
(b) When the angle of a light flux emitted from the liquid crystal display and incident on the color-synthesis optical system is changed, the spectral transmittance of the color-synthesis optical system is accordingly changed. As a result, a brightness of each color of the projected image is changed in accordance with the angle of view, thereby causing an image hard to be viewed. In order to reduce the influence of the angle dependency, the projection lens must be a so-called telecentric optical system in which a pupil close to the liquid crystal display (a reduction conjugate surface) lies substantially at infinity.
(c) When pictures (images) of the three color liquid-crystal displays are synthesized and projected onto a screen, pixels of the respective colors must be overlaid one another across the overall screen so as prevent a loss of a resolution sensation caused by phenomena, for example, seeing two of a character.
To achieve this, a color drift (a chromatic aberration of magnification) generated in the projection lens must be satisfactorily compensated for in the visible light zone.
(d) A distortion aberration must be satisfactorily compensated for so as to prevent the projected image from being hard to be viewed because of distortion.
(e) The projection lens must have a small F-number (hereinafter, referred to as an Fno) and be a bright one so as to efficiently take in light emitted from the light source.
(f) Recently, needs for a higher brightness and a high resolution of a projector are present, and at the same time, a projector having small liquid crystal panels installed therein is required to have a reduced size and weight for giving greater importance to its portability and mobility. At the same time, taking account of a use environment of such a small-sized projection apparatus, the projection lens must be a large magnification-varying zoom lens with which an image is projected onto a large screen with a shorter projection distance and the size of a projection screen is easily adjusted.
The liquid crystal projector-use projection lens required to have a long back focus and a variety of high optical properties as described above sometimes has an aspherical lens (a lens having an aspherical surface) incorporated therein as an effective mechanism for satisfactorily compensating various aberrations without increasing the number of lenses.
Also, when focusing attention on easy workability and productability, the aspherical lens should be composed of a plastic material.
Hitherto, a variety of liquid crystal projector-use projection lenses are proposed (see Japanese Patent Laid-Open Nos. 2004-004964, 2001-100100, and 2002-131636).
While Japanese Patent Laid-Open No. 2004-004964 discloses an optical system aiming at a reduced size and a low cost, the system is directed to use in a transmissive liquid-crystal projector and, in addition to a limit for achieving a wider angle and a larger magnification, has an excessively short back focus when used in a reflective liquid-crystal projector.
While Japanese Patent Laid-Open No. 2001-100100 discloses an optical system having a reduced size and inhibiting various aberrations generated due to its magnification of varying, in addition to a limit for achieving a larger magnification, the system has an excessively small back focus when used in the reflective liquid-crystal projector.
While Japanese Patent Laid-Open No. 2002-131636 discloses an optical system having a long back focus, taking account of use for in the reflective liquid-crystal projector while keeping a wide angle of view, the system has a large Fno and includes a single focal-point lens.
Also, as a projection lens for use in the liquid crystal projector, US Published Application. 2001050818 discloses a six-group zoom lens configured by six lens groups as a whole, having a structure in which first to sixth lens groups respectively having negative, positive, positive, negative, positive (or negative), and positive refractive power are arranged in order from its magnification conjugate side (from its front side), and performing zooming by appropriately moving a predetermined lens groups of these lens groups.
With a structure in which the first and sixth lens groups are fixed and all of the inside second to fifth lens groups are moved towards the reduction conjugate side (the rear side of the zoom lens) upon zooming from its wide angle end to telephoto end, the above six-group zoom lens maintains the overall length constant at the time of zooming and serves as a telecentric zoom lens towards the reduction conjugate side while reducing the distortion aberration and the chromatic aberration upon zooming.
Other than the above-described lenses, as a projection lens for use in the known liquid crystal projector, Japanese Patent Laid-Open No. 2001-108900 discloses a six-group zoom lens configured by six lens groups as a whole, having a structure in which first to sixth lens groups respectively having negative, positive, positive, negative, positive, and positive refractive power are arranged in order from its magnification conjugate side (from its front side), and performing zooming by appropriately moving a predetermined lens groups of these lens groups.
With a structure in which the first, fourth, and sixth lens groups are fixed and the inside second, third, and fifth lens groups are moved upon magnification of varying from its wide angle end to telephoto end, the six-group zoom lens maintains the overall length constant and serves as a telecentric zoom lens on the reduction conjugate side while inhibiting variations in various aberrations including the chromatic aberration during magnification of varying.
Also, as a zoom lens for use in the liquid crystal projector, US Published Application. 2003117716 discloses a five-group zoom lens having a structure in which, in order from its front to rear sides, five lens groups respectively having negative, positive, negative, positive, and positive refractive power, and performing zooming by moving a plurality of the lens groups of these lens groups.
Also, while a zoom lens disclosed in Japanese Patent Laid-Open No. 2004-138678 has an optical system having a relatively long back focus, its Fno at its wide angle end is large on the order of 2.3 or 2.4, in other words, it serves as a dark zoom lens. In addition, with a large number of glass lenses, the zoom lens is required to improve its cost and weight.
In conjunction with achieving further miniaturization of the liquid crystal projector, presently, achievement of its short-range projection, that is, its wider angle of view, which contributes especially to a home-theater-use projector as a great advantage is strongly requested.
Also, in order to achieve a higher brightness of a projected picture, the projection lens is required to be bright and have a large aperture ratio.
In general, when the projection lens is made so as to achieve a wider angle of view while maintaining a long back focus, its lens group closest to the magnification side has larger refractive power.
Also, in order to achieve a telecentric zoom lens on the reduction conjugate side, the overall lens group arranged from its aperture to the reduction conjugate side has larger positive refractive power, and the overall lens system has a retrofocus refractive-power arrangement. Hence, an asymmetry of the lens system increases, resulting in difficulty in compensating for especially distortion aberration, chromatic aberration of magnification, and the like.
In addition, a curvature of field increases without minimizing the Petzval sum in accordance with a wider angle of view, resulting in difficulty in compensating for this phenomenon.