JPH05281471A - Small-sized zoom lens - Google Patents

Abstract

PURPOSE:To excellently compensate various aberrations and facilitate the manufacture by providing a 1st lens group having positive refracting and a 2nd lens group having negative refracting power in order from the object side and using one cemented lens for the 1st lens group. CONSTITUTION:The 1st lens group consists of the one cemented lens. Further, an actual stop is arranged behind the 1st lens group and moved in zooming operation together with the 1st lens group, but in focusing operation, only the 1st group is extended. Further, this zoom lens has a concave surface as its 1st surface and also has an aspherical surface for increasing the negative refracting power off the axis. Namely, the on-axis chromatic aberration can effectively be compensated by using the concave surface as the 1st surface. At this time, the final surface of the 1st lens group is inevitably a convex surface and when the actual stop is placed behind it, large astigmatism is generated by this convex surface. For the purpose, the aspherical surface which increases the negative refracting power is used as the 1st surface which is large in main light beam height to cancel the astigmatism.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compact zoom lens suitable for a lens shutter camera or the like, and more particularly to a simple zoom lens having 3 to 4 lens elements.

[0002]

2. Description of the Related Art As a trend of lens shutter cameras in recent years, thin-type monofocal cameras have become popular, and there is an increasing demand for thinner zoom cameras. Therefore, it has been required for the zoom lens to have a shorter overall length and a smaller size. Traditionally,
A zoom lens having a zoom ratio of about 1.5 to 2.5 times for a lens shutter camera includes a first lens group having a positive refractive power and a second lens group having a negative refractive power in order from the object side. A two-group zoom system is generally used (for example, see Japanese Patent Laid-Open No. 56-128911). The feature of this type of zoom lens is that the back focus and the total lens length are short because of the telescopic refracting power arrangement.
However, with the advancement of the technology for collapsing the zoom lens barrel, the demand for the lens has been more directed to the cost reduction than the reduction of the total length. If the zoom lens can be configured with a smaller number of lenses, not only the cost can be reduced, but also the total lens thickness at the time of storage can be shortened, and further thinning of the camera body can be expected.

As a zoom lens proposed under such a background, a negative lens and a positive lens arranged separately from the first lens group, and a second lens group composed of one to two lenses 3 to A four-lens zoom lens is known (see Japanese Patent Laid-Open No. 3-127008). However, in this case, since the first lens group having a large error sensitivity is composed of two separated lenses, there is a problem that the performance is likely to be deteriorated due to an error such as decentering. In particular, when the first lens group is made up of a small number of lenses, such as two lenses, the refractive power of the negative lens is increased to correct the axial chromatic aberration, and the decentering sensitivity is likely to be increased. Further, in the zoom lens disclosed in the above publication, the diaphragm placed in front of the first lens group moves independently during zooming, so that the mechanism for zooming and focusing becomes complicated.

If the first lens group is composed of only one positive lens, it becomes difficult to correct the axial chromatic aberration, and it is practically not applicable to any camera other than a low-priced camera, so it is omitted here.

[0005]

SUMMARY OF THE INVENTION The present invention has a small number of lens elements, such as 3 to 4, and has a small total lens thickness. To get the lens.

[0006]

[Means for Solving the Problems] To achieve the above object,
The zoom lens of the present invention has, in order from the object side, a first lens group having a positive refractive power and a second lens group having a negative refractive power, and zooming is performed by changing the distance between the first and second lens groups. In the zoom lens for performing the above, the first lens group is composed of one cemented lens.

[0007]

If the zoom lens is constructed with a small number of lenses, the occurrence of chromatic aberration becomes a problem, and in particular, it becomes difficult to correct the axial chromatic aberration that occurs in the first lens group. When the chromatic aberration in the first lens group is not sufficiently corrected, it is necessary to increase the dispersion of the second lens group to cancel the chromatic aberration. However, in such a configuration, the axial and lateral chromatic aberrations on the wide-angle side and the telephoto side largely fluctuate, and the difference in lateral chromatic aberration due to the image height also increases. is there. Therefore, in the two-group zoom method, it is important to correct chromatic aberration in the first lens group. In the present invention, the first lens group is composed of one cemented lens to prevent the occurrence of chromatic aberration. Further, when the positive and negative lenses are separately arranged, there is a problem of error sensitivity as described above,
By using a cemented lens, such inconvenience is eliminated and the structure is easy to manufacture.

[0008]

EXAMPLES Examples of the zoom lens of the present invention having the above-mentioned basic structure will be shown below. In the zoom ratio, brightness, and angle of view of the embodiment, it is desirable to satisfy the following conditions in addition to the above basic configuration. In the zoom lens of the present invention,
It is preferable to arrange a real diaphragm behind the first lens group, move it together with the first lens group during zooming, and extend only the first lens group during focusing. As a result, the mechanism for zooming and focusing becomes simple as compared with the case where the diaphragm is moved alone.

In this zoom lens, the first
It is desirable that the surface is a concave surface, an aspherical surface having a strong negative refractive power off axis is used, and the following condition is satisfied. 0.40 <φw | r ₁ | <0.80 0.15 <φw D ₁ <0.40 where φw: refractive power at wide-angle end of the entire system r ₁ : paraxial radius of curvature of the first surface (r ₁ < 0) D ₁ : On-axis thickness of the first lens group

Further, in the following embodiments, the second lens group is composed of one negative lens. In this case, it is desirable to satisfy the following conditions. 0.60 <| φ ₂ | / φw <1.00 40 <ν ₂ where φ _{2 is} the refracting power of the second lens group (φ ₂ <0) ν _{2 is} the Abbe number of the second lens group.

By making the first surface a concave surface, axial chromatic aberration can be corrected more effectively. At this time, the third
The surface, that is, the final surface of the first lens group is inevitably a convex surface, and when an actual diaphragm is placed behind this surface, strong astigmatism occurs on this convex surface. According to the aberration theory, the surface shape is unlikely to affect the third-order astigmatism in the surface having a small chief ray height. Therefore, it is effective to cancel the astigmatism by using an aspherical surface having a strong negative refractive power on the first surface having a large chief ray height.

The expression (1) is a condition for maintaining the overall aberration balance and compactness while maintaining the effect of making the first surface a concave aspherical surface. Beyond the lower limit of the formula | r
_{As 1} | becomes smaller, the negative distortion on the wide-angle side becomes larger,
If | r ₁ | becomes larger than the upper limit, axial chromatic aberration becomes large. If D ₁ becomes smaller than the lower limit of the equation,
If the chief ray height on the first surface is small and the effect of the aspheric surface is small, and if D ₁ is larger than the upper limit, compactness is lost. In general, when an aspherical surface that strengthens the refracting power of the surface is used, the decentration error sensitivity tends to increase, so that the effect of the present invention using the cemented lens becomes more remarkable.

The equation relates to the refractive power of the second lens group. If | φ ₂ | becomes smaller than the lower limit, the amount of movement of the second lens group becomes large, and compactness is lost.
If | φ ₂ | becomes larger than the upper limit, the difference in spherical aberration between the wide-angle side and the telephoto side becomes large, and correction becomes difficult.

Further, the equation relates to the dispersion of the second lens group, and when ν ₂ becomes smaller than the lower limit, the variation of axial chromatic aberration on the wide angle side and the telephoto side becomes large, and the chromatic aberration of magnification due to the image height becomes large. The difference becomes large and the correction becomes difficult.

In the table, f is the focal length of the entire system, F is the F number, ω is the half angle of view, R is the paraxial radius of curvature, D is the axial upper surface spacing,
N is the refractive index for the d-line, and ν is the Abbe number. Also*
The mark represents an aspherical surface, the shape of which is the vertex of the surface as the origin,
In an orthogonal coordinate system with the X-ray in the optical axis direction, the vertex curvature is c, the conical coefficient is K, and the aspherical surface coefficient is Ai (i = 4, 6,).
8) is represented by Formula 1.

[Equation 1]

Example 1 f = 39.16 to 58.93 F5.4 to 8.2 2ω = 59.8 ° to 41.0 ° Surface No. R D N ν 1 * -20.724 0.60 1 .83400 37.2 2 59.774 7.90 1.69680 55.5 3 * -11.273 A 4 * -25.116 1.20 1.54072 47.2 5 -312.097 f A 39.16 26.96 48.89 19.00 58.93 13.54 Aspheric surface coefficient 1st surface 3rd surface K = 0.59971 x 10 K = 0.26707 x 10 ^{-1 _{A 4 = -0.12126 × 10 -3 A}} 4 = -0.28560 × 10 -4 ^{_{A 6 = -0.13080 × 10 -5 A}} 6 = -0.12464 × 10 -6 A ₈ = 0 A ₈ = 0 4th surface K = 0 A ₄ = 0.22405 × 10 ⁻⁷ A ₆ = -0.40404 × 10 ^-7 A ₈ = 0.27902 × 10 ^-9 φw | r ₁ | = 0.53, φwD ₁ = 0.22, | φ ₂ | /φw=0.78

Example 2 f = 39.12 to 58.96 F5.4 to 8.2 2ω = 59.1 ° to 40.8 ° Surface No. R D N ν 1 * −24.149 3.901 .80100 35.0 2 56.173 5.90 1.69680 55.5 3 * -12.231 A 4 * -25.928 1.20 1.69350 53.2 5-126.850 f A 39.12 26.01 48.26 19.06 58.96 13.66 Aspheric coefficient 1st surface 3rd surface K = 0.56269 × 10 K = 0.41147 A ₄ = −0.11439 × 10 ⁻³ A ₄ = 0.63309 x 10 ^{-5 _{A 6 = -0.15509 × 10 -5 A}} 6 = 0.84162 × 10 -7 A ₈ = 0 A ₈ = 0 4th surface K = 0 A ₄ = -0.37010 × 10 ^-5 A ₆ = -0.10992 × 10 ^-7 A ₈ = 0.10616 × 10 ^-9 φw | r ₁ | = 0.62, φwD ₁ = 0.25, | φ ₂ | /φw=0.83

[0018]

As described above, the zoom lens of the present invention is
As can be seen from the examples, although the number of constituent lenses is as small as about 3 and the lens is compact, it is possible to obtain a compact zoom lens in which various aberrations are well corrected and which is easy to manufacture.

[Brief description of drawings]

FIG. 1 is a sectional view of a zoom lens according to a first embodiment of the present invention.

FIG. 2 is a sectional view of a zoom lens according to a second embodiment of the present invention.

FIG. 3 is an aberration diagram of Example 1 above, in which “d”,
“G” is the spherical aberration for the d-line and g-line, respectively,
“S” and “ΔM” represent the sagittal image plane and the meridional image plane, respectively.

FIG. 4 is an aberration diagram of Example 2 described above.

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[Procedure amendment]

[Submission date] December 21, 1992

[Procedure Amendment 1]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 4]

Claims (2)

[Claims] 1. A zoom lens system having a first lens group having a positive refractive power and a second lens group having a negative refractive power in order from the object side, and varying the distance between the first and second lens groups. In the zoom lens, a small zoom lens characterized in that the first lens group is composed of one cemented lens. 2. A compact zoom lens according to claim 1, further comprising an actual aperture stop, which moves together with the first lens unit during zooming, disposed behind the first lens unit.