JPH0456816A - projection zoom lens - Google Patents

Abstract

PURPOSE:To obtain a zoom lens having long backfocus by specifying the arrangement of refractive power of the zoom lens for executing variable power operation from a wide angle end to a telephoto end. CONSTITUTION:At the time of executing zooming from the wide angle side to the telephoto side, an interval between the 1st and 2nd lens groups is increased, an interval between the 2nd and 3rd lens group is reduced and the 3rd lens group is moved to a large conjugate side. When it is defind that the focal distance of the whole system at the wide angle end is fw, the focal distance of the 1st lens group is f1, the focal distance of the 2nd lens group is f11, and the main point interval between the 2nd and 3rd lens groups at the wide angle end is e11w, the conditions of inequalities I are satisfied. Consequently, the zoom lens capable of securing a comparatively long backfocus and obtaining effective optical performance while securing compact size can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a zoom lens, particularly a zoom lens suitable for projection.

[Conventional technology]

First, FIG. 7 shows a configuration diagram of a projection television that projects an image formed on a general color liquid crystal screen. l
emits collimated light from a white light source. 2 (2
a, 2b, 2c) are liquid crystal display elements; 3 (3a);

3b) is a reflecting mirror; 4, 5, and 6 are a red-reflecting dichroic mirror, a green-reflecting dichroic mirror, a blue-reflecting dichroic mirror, and 7 is a projection lens. In this configuration, it is necessary to install a small number of reflection mirrors or dichroic mirrors (in both cases, two mirrors) between the final surface of the projection lens and the liquid crystal display element (between the back focuses), so a long back is unavoidable. Focus must be maintained.

[Problems to be solved by the invention] By the way, as a simple zoom lens with a long back focus, a lens with negative refractive power comes first, and then a so-called second group consisting of a lens with positive refractive power. A type of zoom lens is conceivable, but the problem arises that the amount of movement of the second lens group during zooming becomes large and the entire zoom lens system becomes long.

SUMMARY OF THE INVENTION In view of these problems, it is an object of the present invention to provide a zoom lens that has a relatively long back focus, is compact, and has good optical performance.

[Means to solve the problem]

The present invention is characterized by having, in order from the larger conjugate side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third 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, the distance between the second lens group and the third lens group decreases, and the third lens group moves to the large conjugate side. , the focal length of the entire system at the wide-angle end is fW, the focal length of the first lens group is fr, the focal length of the second lens group is fII, and the second lens group and the second lens group at the wide-angle end are The distance between the principal points with the 3rd lens group is e
When IIW, 0.3<e r1w/ f W <0.6
- (1) 1.5 < (1/fI + 1/fn
)・fW < 2.0 − (2) 0.3 < −f m
The purpose is to satisfy the following conditional expression: /f W <0.45 − (3).

〔Example〕

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

1 to 3 are cross-sectional views of lenses corresponding to numerical embodiments of the present invention, which will be described later.

■ is the first lens group that is located on the screen (large conjugate) side (not shown) and has positive refractive power, ■ is the second lens group that has negative refractive power, and ■ is the original image (not shown) of the liquid crystal, etc. The third lens group is located on the (small conjugate) side and has positive refractive power, and moves along a movement locus shown by a solid line during zooming from the wide-angle side to the telephoto side. Note that W indicates the wide-angle end, M indicates the middle, and T indicates the telephoto end.

In the present invention, conditional expressions (1), (2), and (3) are satisfied in order to solve the above-mentioned problems.

By the way, in the zoom lens of this embodiment, the third lens group, which is located on the original image side and has positive refractive power, is moved to the screen side to change the magnification from the wide-angle side to the telephoto side. As mentioned above, it is necessary to place an optical component such as a dichroic mirror between the final lens surface of the three lens groups and the original image, so a sufficient distance between them (hereinafter referred to as back focus) must be secured at the wide-angle end. .

When considering a thin-walled system, if the back focus at the wide-angle end is SW, then S w = (1-(e+w+e nw) /fI(1-
e+w/fI) e nw/f nl ・ f
w...(A) (where elW is the principal point distance between the first lens group and the second lens group at the wide-angle end.). The distance between the first lens group and the second lens group is minimum at the wide-angle end, so (A
) The value of e+w in the equation cannot take a very large value in order to make the entire lens system compact. So (A
) formula as Sw=(k-enw・(1/fI+1/fII)
"fW - (B) (where k = 1-e+w/f
Try changing it to I+e+w-enw/(fj-fn)). Then, from this formula (B), the value of enw and -(
It can be seen that when the value of 1/f)+1/f ff) is increased, the back focus sw of the thin-walled system increases, and therefore the back focus of the thick-walled system also tends to become longer.

Therefore, if the lower limit of conditional expression (1) is exceeded, it becomes difficult to maintain the back focus for a long time. On the other hand, if the upper limit is exceeded, the entire lens system becomes large and it becomes difficult to make it compact.

Furthermore, even if the lower limit of conditional expression (2) is exceeded, it becomes difficult to maintain a long back focus.

On the other hand, if the upper limit of conditional expression (2) is exceeded, the focal length fl of the first lens group also increases, and the amount of movement of the first lens group during zooming also increases, making it difficult to achieve a compact lens system. becomes.

Furthermore, when the lower limit of conditional expression (3) is exceeded, the focal length 1fnl of the second lens group becomes small (and the image plane becomes oversized, making it difficult to correct this aberration in other lens groups. Also, barrel-shaped distortion aberration occurs in the small conjugate plane (liquid crystal display element), making it difficult to correct the aberration, especially at the wide-angle end.

On the other hand, if the upper limit of equation (3) is exceeded, that is, if the second focal length 1f■1 becomes too large, the amount of movement of the second lens group during zooming will increase, making it difficult to achieve the predetermined variable power ratio. It's getting difficult.

On the other hand, in a zoom lens consisting of positive, negative, and positive lenses as in the present invention, in order to maintain better optical performance while ensuring a relatively long back focus, the third lens group must include at least one negative lens. With the negative lens (2) having the strongest negative refractive power as a boundary, the lens group on the conjugate side that is larger than the negative lens (2) in the third lens group is designated as lens group (2) When this lens group
When the focal length of lens 1 is fW+ and the radius of curvature of the lens surface on the small conjugate side of negative lens 2 is rA, then 0.6
<rA/fw<0.9...(4)
0.5<f m+/f w <0.8 −
It is preferable to satisfy the following condition (5).

Conditional expression (4) is a conditional expression regarding the radius of curvature of the lens surface on the small conjugate side of negative lens ■2. As a result, unidirectional spherical aberration and astigmatism increase, making it difficult to correct them.

On the other hand, when the upper limit is exceeded, thread-shaped distortion on the small conjugate side of the negative lens ■2, which occurs on the lens surface on the small conjugate side, becomes small (especially at the wide-angle end, and occurs in the second lens group). It becomes difficult to correct barrel distortion.

In addition, conditional expression (5) is a conditional expression that appropriately determines the focal length fml of lens group ■1, and if the lower limit of equation (5) is exceeded, the principal point on the large conjugate side of the third lens group As a result, the value of the principal point spacing emw shown in the above-mentioned equation (B) becomes small, making it difficult to maintain the back focus for a long time. On the other hand, if the upper limit of conditional expression (H) is exceeded, the spherical aberration in the over-unidirectional direction that occurs simultaneously when trying to correct distortion aberration on the small conjugate side lens surface of the negative lens ■2 is corrected in the under-unidirectional direction. This becomes difficult.

Next, numerical examples of the present invention will be shown. In numerical examples R
1 is the radius of curvature of the first lens surface from the screen side, D is the thickness and air gap of the first lens from the screen side, and N1 and ν1 are the radius of curvature of the first lens surface from the screen side, respectively.
These are the refractive index and Atsube number of the glass of the th lens.

In Examples 1 to 3, focusing is performed by moving the first lens group (2) along the optical axis. Further, the variable interval D8 indicates the interval of the screen distance ■.

Note that the values of each numerical example corresponding to each numerical conditional expression are shown in the table below.

〔Effect of the invention〕

As explained above, from the screen side, the first lens group has a positive refractive power, the second lens group has a negative refractive power, and the third lens group has a positive refractive power. A refractive power arrangement of a zoom lens that increases the distance between the groups, decreases the distance between the second and third lens groups, and moves the third lens group toward the screen to change magnification from the wide-angle end to the telephoto end. By specifying this, a zoom lens with a long back focus can be achieved and used as a projection lens for color liquid crystal projection TVs.

[Brief explanation of the drawing]

Figures 1 to 3 are lens cross sections of numerical examples 1 to 3 of the present invention, respectively. Figures 4 to 6 are various aberration diagrams of numerical examples 1 to 3 of the present invention, respectively, and magnification at the wide-angle end. shows various aberration diagrams on a small conjugate plane at a distance of 3/100. FIG. 7 is a block diagram of the projector. In the aberration diagram, W shows the aberration diagram at the wide-angle end, M shows the aberration diagram at the middle, and T shows the aberration diagram at the telephoto end. 1 is a light source, 2 is a liquid crystal display element, and 3 is a mirror.躬1 fig.Fn Hanige=//ri. Ball-shaped convergence, soft twin ΔW -7ノ,?゛Distortion (Nζ'F/z　W2 27. The surface throat is weak, aberration = 2"l, 6゜distortion (in) F,? w = 75.2" (d = /s, 2゜

Claims (1)

[Claims] (1) In order from the larger conjugate side, the first lens having a positive refractive power
a lens group, a second lens group having negative refractive power, and a third lens group having positive refractive power, and when zooming from the wide-angle side to the telephoto side, the distance between the first lens group and the second lens group is increased, the distance between the second lens group and the third lens group is decreased, and the third lens group moves toward the large 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, and the distance between the principal points of the second and third lens groups at the wide-angle end is e_II_W, then 0.3 <e_II_W/f_W<0.6 1.5<-(1/f_ I +1/f_II)・f_W<2
．． A zoom lens that satisfies the following conditional expression: 00.3<-f_II/f_W<0.45. (2) The third lens group has at least one negative lens, with the negative lens having the strongest negative refractive power as the boundary, and the conjugate side of the third lens group having a larger conjugate than this strong negative lens. The focal length of the lens group is f_III_1, and the radius of curvature of the lens surface on the small conjugate side of the strong negative lens is r_A.
The zoom lens according to claim 1, wherein the following conditional expressions are satisfied: 0.6<r_A/f_W<0.9 0.5<f_III_1/f_W<0.8.