JP3082928B2 - Zoom lens - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zoom lens, particularly to a zoom lens suitable for projection.

[Prior Art] First, FIG. 7 shows a configuration diagram of a projection television for projecting an image formed on a general color liquid crystal screen.
1 emits light collimated by a white light source. 2 (2
a, 2b, 2c) are liquid crystal display elements, 3 (3a, 3b) are reflection mirrors,
4, 5 and 6 are red reflection dichroic mirror, green reflection dichroic mirror, blue reflection dichroic mirror,
Reference numeral 7 denotes a projection lens. Under such a configuration, at least two mirrors such as a reflection mirror or a dichroic mirror need to be arranged between the last surface of the projection lens and the liquid crystal display element (between the back focuses). Must.

[Problems to be solved by the invention]

By the way, a so-called two-group zoom lens having a long back focus and a lens having a negative refractive power as a simple zoom lens and then a lens having a positive refractive power can be considered. Second
There is a problem that the amount of movement of the lens group becomes large and the entire zoom lens system becomes long.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a zoom lens having good optical performance while securing a relatively long back focus and reducing the size.

[Means for solving the problem]

The feature of the present invention is that the first lens unit having a positive refractive power and the second lens unit having a negative refractive power
The zoom lens has a lens group, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. When zooming from the wide-angle side to the telephoto side, the distance between the first lens group and the second lens group increases. ,
A zoom lens in which the distance between the second lens group and the third lens group decreases, the distance between the third lens group and the fourth lens group decreases, and the fourth lens group moves to a large conjugate side, The group has a negative lens on the largest conjugate side. The focal length of the entire system at the wide-angle end is f _W , and the focal length of the first lens group is
f _I , the focal length of the second lens group is f _II , the focal length of the third lens group is f _III , the focal length of the fourth lens group is f _IV , and the focal length of the second and third lens groups at the wide angle end is E is the principal point interval
When it is assumed that _{II W} , 0.1 <e _{II W} / f _W <0.6 (1) 1.2 <− (1 / f _I + 1 / f _II ) · f _W <2.0 (2) 0.3 <−f _II / f _W <0.5 (3) 0.3 <f _III / f _IV <3.0 (4) That is, the following conditional expression is satisfied.

〔Example〕

Hereinafter, the zoom lens of the present invention will be described with reference to the drawings.

1 to 3 are lens cross-sectional views corresponding to numerical examples of the present invention described later.

I is located on the screen (large conjugate) side not shown,
A first lens group having a positive refractive power, II is a second lens group having a negative refractive power, III is a third lens group having a positive refractive power, IV is an original image (small conjugate The fourth lens group having a positive refractive power and moving along the locus indicated by the solid line during zooming from the wide-angle side to the telephoto side. Note that W indicates a wide angle end, M indicates a middle position, and T indicates a telephoto end.

The present invention satisfies the conditional expressions (1), (2), (3) and (4) in order to solve the above-mentioned problems.

By the way, in the zoom lens according to the present embodiment, the fourth lens group, which is located on the original image side and has a positive refractive power, is moved to the screen side to perform zooming from the wide-angle side to the telephoto side. Since it is necessary to dispose an optical component such as a dichroic mirror between the final lens surfaces of the four lens groups and the original image as described above, a sufficient distance (hereinafter referred to as "back focus") must be ensured at the wide-angle end. .

Here When considering a thin system a third lens group and the composite lens system of the fourth lens unit as a 34 lens group, the Batsukufuokasu at the wide-angle end when the _{S W S W = {1- (} e I W + e II W ′) / F _I − (1−e _{I W} / f _I ) e _{II W} ′ / f _II ｝ · f _W (where e _{I W} is the first lens group and the second lens group at the wide-angle end) E _{II W} ′ is the principal point interval at the wide angle end between the second lens unit and the 34th lens unit when the third lens unit and the fourth lens unit are combined and treated as the 34th lens unit. .) Since the distance between the first lens group and the second lens group is minimum at the wide-angle end, the value of e _{I W in} equation (A) cannot be too large to make the entire lens system compact. . Therefore (A) formula _{S W = {k-e II} W '· (1 / f I + 1 / f II)} · f W ... (B) ( _{where, k = 1-e I W} / f I + e I _W・ e _{II W} ′ / (f _I・
f _II )) and try to transform it. Then the values and the e _{II W} 'from the equation (B) - the value of the larger of _{(1 / f I + 1 /} f II), Batsukufuokasu also increases in a thick type because thin type Batsukufuokasu S _W of increase It turns out that there is a tendency.

Therefore, if the lower limit of conditional expression (1) is exceeded, e _{II W} ′ will also increase, making it difficult to maintain a long back focus. On the other hand, when the value exceeds the upper limit, the entire lens system becomes large, and it is difficult to reduce the size.

In addition, even if the lower limit of conditional expression (2) is exceeded, it is similarly difficult to keep the back focus long.

On the other hand, when the value exceeds the upper limit of the conditional expression (2), the focal length f _I of the first lens unit also increases, so that the amount of movement of the first lens unit during zooming also increases, thereby achieving a compact lens system. Becomes difficult.

If the lower limit of conditional expression (3) is exceeded, the focal length | f _II | of the second lens group will be reduced, causing an excessive curvature of field, and correcting this aberration in the other lens groups. It becomes difficult. Also, barrel-shaped distortion occurs on a small conjugate plane (liquid crystal display element), and it becomes difficult to correct aberrations, especially at the wide-angle end.

On the other hand, if the value exceeds the upper limit of the expression (3), that is, if the second focal length | f _II | becomes too large, the moving amount of the second lens unit in zooming becomes large, and a predetermined zoom ratio is obtained. Becomes difficult.

The distance from the front lens surface of the third lens group to the principal point position on the third lens group side of the 34th lens group is Of, and the distance from the last lens surface of the fourth lens group to the principal point position on the fourth lens group side is The distance is Or, the focal length of the 34th lens group is _fIIIIV ,
When lens group and the main point interval of the fourth lens group and e _III, a _{Of = e III · f IIIIV /} f IV ... (C) Or = e III · f IIIIV / f III ... (D), (C ) And (D), Of / Or = f _III / f _IV (E). That is, as the value exceeds the lower limit of Expression (4), the thirty-fourth
Since the principal point of the lens group is closer to the large conjugate side, the value of the principal point interval e _{II W} ′ shown in the equation (B) becomes small, and it becomes difficult to keep the back focus long. On the other hand, when the value exceeds the upper limit of the expression (4), it becomes difficult to correct the under spherical aberration in the small conjugate surface generated in the fourth lens group and the barrel distortion at the wide angle end.

By satisfying the above conditions, the object of the present invention can be achieved.However, in a zoom lens composed of positive, negative, positive, and positive, a better optical performance is maintained while securing a relatively long back focus. In order to achieve this, it is preferable that the fourth lens group has a negative lens on the large conjugate side. In other words, by having the negative lens on the largest conjugate side, the principal point on the small conjugate side of the fourth lens group shifts to the small conjugate side, and it is easy to keep the actual thick back focus long.

Further, the radius of curvature of the large conjugate side of the negative lens is r _A
When the radius of curvature on the small conjugate side is r _B , it is preferable to satisfy the following condition: | r _A |> | r _B | (5) That is, by satisfying the condition of the expression (5), barrel distortion generated at the wide-angle end, particularly at the second lens group, can be corrected by the negative lens.

Next, numerical examples of the present invention will be described. In numerical examples
Ri is the radius of curvature of the lens surface of the i-th board surface in order from the screen side, Di is the i-th lens thickness or air space in order from the screen side, and Ni and νi are the refraction of the glass of the i-th lens in order from the screen side, respectively. Rate and Atsube number.

In the first to third embodiments, focusing is performed by moving the first lens unit I along the optical axis. Further, the variable interval D indicates the interval at the time of the screen distance ∞.

The values of each numerical example corresponding to each numerical conditional expression are shown in the following table.

[Effects of the Invention] As described above, according to the present invention, a zoom lens having a long back focus is achieved, and can be used as a projection lens for a color liquid crystal projection TV.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 to 3 are sectional views of lenses of Numerical Embodiments 1 to 3 of the present invention, and FIGS. 4 to 6 are various sectional views of Numerical Embodiments 1 to 3 of the present invention. In the aberration diagram, various aberration diagrams on a small conjugate plane at a distance of 3/100 at the wide angle end are shown. FIG. 7 is a configuration diagram of the projector. In the aberration diagrams, W indicates the wide-angle end, M indicates the middle position, and T indicates the aberration diagram at the telephoto end. 1 is a light source, 2 is a liquid crystal display element, and 3 is a mirror.

Continuation of the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/00-25/04

Claims (2)

(57) [Claims] 1. A first lens having a positive refractive power in order from the largest conjugate side.
A second lens group having a negative refractive power, a third lens group having a positive refractive power, and a fourth lens group having a positive refractive power. The first lens group is used for zooming from the wide-angle side to the telephoto side. The distance between the second lens group and the second lens group increases, the distance between the second lens group and the third lens group decreases, the distance between the third lens group and the fourth lens group decreases, and the fourth lens group moves to a larger conjugate side. In the zoom lens that moves, the fourth lens group includes a negative lens on the largest conjugate side, and the focal length of the entire system at the wide-angle end is f _W ,
The focal length of the first lens group is f _I , the focal length of the second lens group is f _II , the focal length of the third lens group is f _III , the focal length of the fourth lens group is f _IV , and the second lens at the wide-angle end is Group and third
When the principal point distance between the lens group and _{e II W, 0.1 <e II} W / f W <0.6 1.2 <- (1 / f I + 1 / f II) · f W <2.0 0.3 <-f II / f A zoom lens characterized by satisfying the following conditional expression: _W <0.5 0.3 <f _III / f _IV <3.0. 2. When the radius of curvature of the negative lens on the largest conjugate side of the fourth lens group on the large conjugate side is r _A , and the radius of curvature on the small conjugate side is r _B , | r _A |> | r _B | The zoom lens according to claim 1, wherein the following conditional expression is satisfied.