Zoom Lens System

1. Field of the Invention

The present invention relates to a zoom lens system, and more particularly to a zoom lens system for use in an image projecting apparatus such as a microfilm reader or a microfilm reader/printer which projects microfilm images and has a limited conjugate length.

2. Description of the Prior Art

A microfilm includes a plurality of images thereon which are not always recorded in the same direction. Therefore, an image projecting apparatus such as a microfilm reader or a microfilm reader/printer has a prism disposed between projecting lens and an image receiving plane for optically rotating images. This prism normally is disposed adjacent an enlargement side of the projecting lens for compactness. In such an image projecting apparatus, as shown in FIG. 2, an image of a microfilm held by a microfilm holder 1 comprising two flat glass plates is projected onto a screen 7 through a projecting lens system 2, an image rotating prism 3 and mirrors 4-6. A dope prism may be used as the image rotating prism 3 for compactness of the prism itself. In order for the entire projected image to enter this prism 3, it is desirable to arrange the projecting lens system 2 and prism 3 as close to each other as possible, and to arrange the pupil position of the projecting lens 2 as close to the prism 3 as possible.

Zoom lens systems used as such a projecting lens system of the image projecting apparatus are disclosed in Japanese Patent Publication Kokai No. 62-237414, U.S. Pat. No. 4,733,951, U.S. Pat. No. 4,745,204, U.S. Pat. No. 4,743,102 and U.S. Pat. No. 4,750,820.

The zoom lens system disclosed in Japanese Patent Publication Kokai No. 62-237414, as shown in FIG. 3B, is a positive-negative, two group zoom lens system comprising a positive lens group 8 at an enlargement side and a negative lens group 9 at a reduction side. An aperture stop S is fixedly disposed at the enlargement side of the lens system. Reference F denotes a microfilm plane. With such a positive-negative, two group zoom lens system, however, when the zoom ratio is increased to twofold, the positive lens group moves a great distance with a zooming operation and, since the aperture stop S is fixed, a main beam of a maximum angle of view has a very large incident angle .alpha. 0 with respect to the positive lens group 8 at a shortest focal length side. This entails an increase in the outer diameter of the lenses and difficulties in compensating for aberration. U.S. Pat. No. 4,733,951 also discloses a positive-negative, two zoom lens system which has a similar drawback.

On the other hand, the zoom lens systems disclosed in U.S. Pat. No. 4,745,204, U.S. Pat. No. 4,743,102 and U.S. Pat. No. 4,750,820 are negative-positive, two group lens systems comprising a negative lens group at an enlargement side and a positive lens group at a reduction side. Such a negative-positive lens system has a wide angle of view but, because of its construction, has a principal point located relatively close to the reduction side, and hence the disadvantage of having an increased overall length of the lens system.

Having regard to the above, an object of the present invention is to provide a zoom lens system of excellent performance having a zoom ratio on the order of twofold.

A further object of the present invention is to provide a compact zoom lens system.

The above objects are fulfilled, according to the present invention, by a zoom lens system comprising, in order from an enlargement side, a first lens group of a negative power which is fixed during a zooming operation and includes, in order from the enlargement side, a positive lens element and a negative lens element; a second lens group of a positive power which is movable during a zooming operation and includes, in order from the enlargement side, a biconvex lens element, a negative meniscus lens element having a concave surface directed to the enlargement side, a positive lens element, a biconcave lens element and a biconvex lens element; and a third lens group of a negative power which is movable during a zooming operation and includes, in order from the enlargement side, a biconcave lens element and a positive lens element.

These and other objects or features of the present invention will become apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a sectional view of a zoom lens system having a limited conjugate length according to the present invention,

FIG. 2 is a schematic view of an optical path of a microfilm projecting apparatus to which the zoom lens system of the present invention is applied,

FIG. 3A is a schematic view showing a paraxial power arrangement of the zoom lens system according to the present invention,

FIG. 3B is a schematic view showing a paraxial power arrangement of a conventional two group zoom lens system,

FIG. 4 is a schematic view showing a relationship between movable lens groups in the present invention,

FIG. 5A is a schematic view illustrating a span angle of an axial beam in the zoom lens system of the present invention,

FIG. 5B is a schematic view illustrating a span angle of an axial beam in a conventional two group zoom lens system,

FIGS. 6 through 9 are sectional views of lens constructions in Examples 1 through 4 of the present invention, respectively,

FIGS. 10A, 10B and 10C through 13A, 13B and 13C are views showing aberration curves in Examples 1 through 4, respectively.

FIG. 1 shows a zoom lens system embodying the present invention. This zoom lens system is used as the projecting lens of the image projecting apparatus shown in FIG. 2. The zoom lens system comprises, from an enlargement side, a first lens group I of a negative power which is fixed during a zooming operation, a second lens group II of a positive power which is movable during a zooming operation, and a third lens group III of a negative power which is movable during a zooming operation. An aperture stop S is included in the first lens group I and is fixed during a zooming operation. The second and third lens groups II and III move toward a reduction side during a zooming operation from a longest focal length condition to a shortest focal length condition. In this zoom lens system, the aperture stop S is disposed adjacent the enlargement side of the lens system to be close to the image rotating prism for compactness of the prism. The image rotating prism such as a dope prism is equivalent to a plain parallel plate inclined at an angle to the optical axis, and therefore its performance is varied with its position even within the same image circle. An enlargement of the prism increases axial astigmatism produced in proportion to the bottom length of the prism. Therefore, a reduction in size of the prism helps toward a high quality image. The aperture stop S is fixed during a zooming operation in order to reduce variations in divergence of off-axial beams occurring at the enlargement side during the zooming operation.

This zoom lens system desirably is a compact lens system which satisfies a telephoto ratio TL.infin./f.sub.L <1.2 where TL.infin. represents a distance between an end face of the lens system of the enlargement side and an image plane of the reduction side when an object point is at an infinite location, and f.sub.L represents a longest focal length. For this purpose, a telephoto type lens system may be provided in which the first and second lens groups I and II are arranged relatively close to each other for increasing the composite power of the two lens groups I and II in the positive direction.

As shown in FIG. 3A, with the zoom lens system in this embodiment, a principal ray of a maximum angle of view toward the second lens group II has a reduced incident angle .alpha.1 owing to the negative power of the first lens group I in the shortest focal length condition. This construction eliminates the drawback of the positive-negative, two group zoom lens system shown in FIG. 3B.

Since the first lens group I has a negative power, the movement of the second lens group II and third lens group III effectively contributes toward variation of magnification. This feature will be described with reference to FIG. 4 showing a paraxial power arrangement. Assuming the powers (inverse numbers of the focal lengths) of the first to third lens groups I-III to be .psi..sub.I, .psi..sub.II and .psi..sub.III, respectively, .psi..sub.I =1/f.sub.I <0,

Assuming further that an object is at a distance S.sub.1 (which is smaller than zero) from the first lens group I, the first and second lens groups I and II have a calculated distance e.sub.I therebetween (a distance calculated by regarding principal planes of the lens groups as the bases) in the longest focal length condition, the second and third lens groups II and III have a calculated distance e.sub.II therebetween in the longest focal length condition, the third lens group III and microfilm plane F have a calculated distance e.sub.III therebetween in the longest focal length condition, the first to third lens groups I-III have lateral magnifications (with the object point at the enlargement side) .beta..sub.I, .beta..sub.II and .beta..sub.III, respectively, the second lens group II has an amount of movement .chi. from the longest focal length condition, and the third lens group III has an amount of movement y from the longest focal length condition, ##EQU1##

Since the first lens group I is fixed, .beta..sub.I has a fixed value greater than zero. .beta..sub.II contributes to magnification since object point O.sub.2 is fixed for the second lens group II and is much closer to the second lens group II than to the screen 7 and, as is clear from the equation [b] above, approaches zero in a monotonic increase while taking the value .beta..sub.II <0 when .chi.>0. .beta..sub.III approaches zero in a monotonic decrease while taking a proper value as object point O.sub.3 Thus, with the magnification of the entire system .beta.=.beta..sub.I .multidot..beta..sub.II .multidot..beta..sub.III, both .beta..sub.II and .beta..sub.III have their absolute values approaching zero to effectively contribute to magnification in response to a zooming operation from the longest focal length condition to the shortest focal length condition.

Table 1 shows values of .beta..sub.I, .beta..sub.II and .beta..beta..sub.III in the longest focallength condition (.beta.=-1/13.0), an intermediate focal length condition (.beta.=-1/18.7) and the shortest focal length condition (.beta.=-1/27) in Example 1 which is a typical example.

A specific construction of each lens group will be described by reverting to FIG. 1. The first lens group I includes, from the enlargement side opposed to the screen, a positive lens element and at least one negative lens element. The second lens group II includes a biconvex lens element, a negative meniscus lens element having a concave surface directed to the enlargement side, at least one positive lens element, a biconcave lens element having an intense concave surface directed to the reduction side, and a biconvex lens element. The third lens group III includes a biconcave lens element having an intense concave surface directed to the enlargement side, and a positive lens element.

The first lens group I is a lens system comprising a positive-negative combination, and contributes to compensation for spherical aberration.

The second lens group II is a variation of the Ernoster type which includes a plurality of lens element having positive powers at the enlargement side for shifting the principal point toward the enlargement side to facilitate reduction in the calculated distance eI between the first and second lens groups I and II in the longest focal length condition and increase in the magnifying region, that is to reduce the total distance of movement of the second lens group II.

Further, since the second lens group II has a strong power and is bright, the second, concave meniscus lens component thereof has an intense concave surface directed to the enlargement side for coma compensation. The two biconvex lens components and the negative meniscus lens component at the enlargement side of the second lens group II may be formed as a cemented lens.

The third lens group III is a lens group disposed located remotest from the aperture stop S, and facilitates compensation for astigmatism and distortion.

Further, it is desirable for this zoom lens system to satisfy the following conditional expressions:

where f.sub.I represents the focal length of the first lens group I, f.sub.II represents the focal length of the second lens group II, f.sub.III represents the focal length of the third lens group III, f.sub.L represents the focal length of the entire system in the longest focal length condition, and eI represents the calculated distance between the first and second lens groups I and II in the longest focal length condition.

The conditional expression [1] is one for arranging substantially symmetric powers across the second lens group II.

Compensation for distortion will be difficult in the event ofdeviation from its lower limit. Above its upper limit, the negative power of the first lens group I becomes insufficient with off-axial principal ray to the second lens group II having an enlarged incident angle .alpha..sub.1 with respect to the second lens group II, thereby impeding compensation made for various aberrations through the second lens group II.

The conditional expression [2] shows a condition for maintaining an appropriate distance of movement of the second lens group II and yet for minimizing the number of components of the second lens group II. In the event of deviation from its lower limit, the number of components of the second lens group II will be increased although the distance of movement is reduced.

Above its upper limit, the distance of movement will be increased which makes it impossible to maintain a clearance between the second and third lens groups II and III.

The conditional expression [3] shows a condition for making the movement of the second lens group II proper. As is evident from the foregoing equation [b], the magnification .beta..sub.II of the second lens group II describes a hyperbola, and its asymptote is expressed by the following equation: ##EQU2## It is necessary for .chi..sub.0 to be less than 0 in order to realize a monotonic increase with the condition x>0.

That is, the following condition must be satisfied: ##EQU3##

If this expression is substituted with .psi..sub.I =-0.02108, .psi..sub.II =0.04183 and S.sub.1 =-957.1 in Example 1, e.sub.I /f.sub.L =-0.32.

The lower limit of the foregoing conditional expression [3] is for satisfying this condition and maintaining the rate of change of magnification .beta..sub.2 not too high. In the event of deviation from this lower limit, the rate of change of magnification .beta..sub.II will be increased relative to the movement .chi. of the second lens group II, thereby increasing manufacturing error sensitivity. Above the upper limit, the second lens group II will become movable by an increased amount.

The third lens group III is located remotest from the aperture stop S, and therefore contributes to compensation for aberrations, particularly astigmatism and distortion, of off- axial beams. The image height at the reduction side is variable with a zooming operation from the longest focal length condition to the shortest focal length condition, but so are the positions of off-axial principal rays passing through the third lens group III. Thus, the third lens group III has a function to readily compensate for off-axial aberrations produced through the first and second lens groups I and II and variable with a zooming operation.

Since, in this zoom lens system, the first lens group I is fixed and includes the aperture stop S disposed at the reduction side, the axial beam has a constant span angle u.sub.0 and a constant F-number at the enlargement side during a zooming operation from the longest focal length condition to the shortest focal length condition as shown in FIG. 5A.

By contrast, in the zoom lens system shown in FIG. 5B in which the aperture stop S and the lens group disposed at the enlargement side of the aperture stop S are movable, the axial beam has a span angle u.sub.1 variable with a zooming operation from the longest focal length condition to the shortest focal length condition, which entails variations in brightness.

FIGS. 6 through 9 show specific constructions of Examples 1 through 4 in this embodiment, and Tables 2 through 5 show numeric data thereof.

These tables of data show, in the order from the enlargement side opposed to the screen, numeric values for radii of curvature r.sub.1, r.sub.2, . . . r.sub.n, axial distances d.sub.1, d.sub.2, . . . d.sub.n, refractive indexes of the lens components N.sub.1, N.sub.2, . . . N.sub.n, components, and Abbe numbers of the lens components .nu..sub.1, .nu..sub.2, . . .nu..sub.n. The plain parallel plate at the reduction side is a film holder.

FIGS. 10A, 10B and 10C through 13A, 13B and 13C show aberration curves in Examples 1 through 4. FIGS. 10A, 11A, 12A and 13A show the spherical aberration, astigmatism and distortion of the respective examples in the longest focal length condition. FIGS. 10B, 11B, 12B and 13B show the spherical aberration, astigmatism and distortion of the respective examples in the intermediate focal length condition. FIGS. 10C, 11C, 12C and 13C show the spherical aberration, astigmatism and distortion of the respective examples in the shortest focal length condition.

Table 6 shows a table of numeric values for the foregoing conditional expressions in Examples 1 through 4.

As described above, this zoom lens system is constructed very compact.

The image rotating prism may be constructed very small with the value of (prism length)/(focal length of the entire system at .beta.=-1/13) at 0.92. Further, this zoom lens system effectively compensates for various aberrations with the magnification .beta. ranging from about -1/13 to about -1/27.

It is possible to substantially fix the pupil position at the reduction side during a zooming operation, thereby making the movement of a condenser lens unnecessary for Koeler's illumination. Further, F-numbers at the enlargement side are made constant, thereby maintaining constant illumination on the screen.

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 modifications 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.