JPH0821952A - Zoom lens for finite distance - Google Patents

Abstract

PURPOSE:To obtain a zoom lens which is used at about 3 times to 10 times variable power rate, is long in back focus and has excellent imaging performance by using a first lens group which has three elements of negative lenses and satisfies specific conditional equations. CONSTITUTION:This zoom lens is formed, successively from an magnifying side (printing paper side), the first lens group L1 having negative refracting power, a second lens group L2 having positive refracting power, a third lens group L3 having negative refracting power and a fourth lens group L4 having positive refracting power. The specified distance between the object images at which the first lens group L1, the second lens group L2, the third lens group L3 and the fourth lens group L4 move to the magnifying side is obtd. at the time of varying power from a short focal length to a long focal length. Further, the first lens group L1 has at least three elements of the negative lenses and satisfies the conditions of equation I, equation II. In the equations I, II, fw denotes the focal length of the entire system on the short focal side; f1 denotes the combined focal length of the first lens group L1; nu denotes the average value of the Abbe number of the negative lenses of the first lens group L1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a finite distance zoom lens used in a photo enlarger or the like.

[0002]

2. Description of the Related Art In addition to a fixed focus lens, a zoom lens which can handle various magnifications with a single lens is also used in a photo enlarger for printing a negative film image on a photographic paper. There are many known four-group zoom lenses that provide excellent imaging performance at high magnifications of about 10 times.

Further, in recent years, a photographic enlarger capable of performing a printing operation while visually confirming an image on a negative film has been introduced, and workability is improved. In such a photographic enlarger, a photometric mirror and an optical path dividing prism are arranged between a lens and a negative film, and an image of the negative film is displayed on a monitor device for confirmation.

Further, comparing the method using the optical path splitting prism with the method using the photometric mirror, the prism using the optical path splitting prism is arranged as it is at the time of printing. Since there are problems such as adverse effects on performance and high cost for mounting a prism on each lens, the method using a photometric mirror is more advantageous for the entire photo enlarger.

As described above, the photographic enlarger having good printing workability has the photometric mirror and the optical path splitting prism disposed between the lens and the negative film, so that the lens and the negative film are different from those of the conventional photographic enlarger. Therefore, a lens having a long back focus is required.

The zoom lenses disclosed in JP-A-62-73222, JP-A-1-142519 and JP-A-2-287414 have a wide range of use magnification and have excellent imaging performance. In JP-A-62-73222 and JP-A-1-142519, the back focus at the longest low magnification of 135 film is 83 mm or less, and a photometric mirror and an optical path splitting prism are arranged. I can't. Further, in Japanese Patent Laid-Open No. 2-287414, the low-magnification back focus is 110 m.
It is as long as about m, but at high magnification it becomes as short as about 55 mm, so
No photometric mirror or optical path splitting prism can be placed over the entire magnification range. The zoom lens disclosed in Japanese Patent Laid-Open No. 59-214009 and Japanese Patent Publication No. 2-48089 has a relatively long back focus and is 90-100 mm even at high magnification.
Although it is possible to arrange the optical path splitting prism, it is difficult to arrange the photometric mirror which requires a longer back focus. In addition, there is room for improvement in correction of chromatic aberration at low magnification (long focal length).

Since the zoom lens disclosed in Japanese Patent Laid-Open No. 5-273468 is a lens in which an optical path dividing prism is arranged, a back focus long enough to dispose a photometric mirror is not secured. Also, the image height at high magnification is y = 6.
There is room for improvement, such as being small at 72 and slightly dark at F6.2.

On the other hand, like a retrofocus type lens,
A zoom lens in which the first lens group has a negative refracting power is advantageous for ensuring a long back focus. For example,
There are many well-known examples of wide-angle zoom lenses for photography such as JP-A-57-11315, JP-A-60-87312 and JP-A-6-82698, but these zoom lenses are similar to those of enlargers. Since the aberration is not corrected so that it can be used with a short object-to-image distance, the performance is significantly deteriorated and it cannot be practically used. Also, the back focus is insufficient in the magnification range used by the enlarger.

[0009]

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a finite distance zoom lens having a long back focus and excellent imaging performance, which is used at a magnification of 3 to 10 times. It is an issue.

[0010]

The above-mentioned problems can be solved by the following zoom lens in the present invention.

In order from the magnifying side, that is, the photographic paper side, the first lens group having a negative refractive power, the second lens group having a positive refractive power, the third lens group having a negative refractive power, and the positive lens group And a fourth lens unit having a refracting power of, the first lens unit, the second lens unit
The lens group, the third lens group, and the fourth lens group are finite-distance zoom lenses having a constant object-image distance moving toward the magnification side, and the first lens group has at least three negative lenses. , The following conditional expression must be satisfied.

0.9f _w <| f ₁ | <1.4 f _w , f ₁ <0 (1) 56 <ν (2) where f _w : focal length of the entire system on the short focus side f ₁ : first lens Synthetic focal length of the group ν: Average value of Abbe number of the negative lens of the first lens group. Further, the first lens group has, in order from the enlargement side, a negative lens, a positive lens, a negative lens, a negative lens and a positive lens. It is desirable to have a five-element structure in four groups including a cemented lens of lenses.

Further, the second lens group has a two-lens-three-lens structure consisting of a cemented positive lens and a positive lens in order from the magnification side, and the third lens group is a cemented negative lens in order from the magnification side. It is preferable that the following conditional expression is satisfied, which is a two-group three-lens structure composed of a negative lens and a two-group three lens composed of a cemented negative lens.

[0014] _{1.4f w <f 2 <1.8f w} (3) 1.5f w <| f 3 | <1.9f w, f 3 <0 (4) where, f _2: the composite focal length f of the second lens group ₃ : Composite focal length of the third lens group

[0015]

A negative-group-leading type zoom lens in which the first lens group has a negative refracting power as in the present invention is difficult to correct aberrations, but is very advantageous for lengthening the back focus. Is.

The first is to increase the back focus.
Although the power of the lens group has to be set strongly, the height of the light beam incident on the first lens group from the optical axis varies greatly depending on the angle of view, so that aberration correction becomes difficult when the power is strong. Therefore, the lenses are arranged as described in claim 1, and the negative lenses are made to share the refracting power to correct the aberrations in a well-balanced manner. Further, the use of a glass material that causes less chromatic aberration and a cemented lens as the final lens suppresses the occurrence of chromatic aberration in the first lens group.

Conditional expression (1) is a condition for performing well-balanced aberration correction while ensuring a necessary back focus. If the power is increased beyond the lower limit of the conditional expression (1), the negative distortion aberration increases and the astigmatism deteriorates. Further, chromatic aberration of magnification due to zooming becomes large, and correction becomes difficult even if the glass material of conditional expression (2) is used. When the power exceeds the upper limit of the conditional expression (1) and the power becomes weak, the aberration correction becomes easy, but it becomes difficult to lengthen the back focus and the photometric mirror cannot be arranged.

Further, if the following conditional expression (5) is used, it becomes possible to correct aberrations in a more balanced manner.

1.00f _w <| f ₁ | <1.32f _w , f ₁ <0 (5) Furthermore, if the following conditional expression (6) is used, a more well-balanced aberration correction becomes possible.

1.15f _w <| f ₁ | <1.18f _w , f ₁ <0 (6) Conditional expression (2) is a conditional expression for reducing the occurrence of chromatic aberration. If this conditional expression is not satisfied, the occurrence of chromatic aberration becomes large, and it is difficult to correct particularly at a long focal length, and it cannot be put to practical use.

Further, as a subordinate, it is desirable that the first lens group is composed of a negative lens, a positive lens, a negative lens, and a cemented lens of a negative lens and a positive lens in order from the magnifying side. .

In the second lens group of the negative group preceding type zoom lens, the axial light flux passes through the position away from the optical axis, so that aberration is likely to occur. Therefore, the aberrations are corrected in a well-balanced manner by forming a cemented positive lens and a positive lens in order from the enlargement side in a two-group, three-element configuration.

Conditional expression (3) is a condition relating to the refractive power of the second lens group, and when the power goes below the lower limit and the power becomes strong, the spherical aberration and the field curvature change greatly due to zooming, and at the intermediate focal length. Undercorrection of spherical aberration. If the power exceeds the upper limit and the power is weakened, the zooming effect is reduced and the amount of movement is increased, resulting in an increase in the size of the lens.

Further, it is more preferable to use the following conditional expression (7).

[0025] _{1.49f w <f 2 <1.70f w} (7) In addition, it is more desirable to use the following conditional expression (8).

[0026] _{1.49f w <f 2 <1.57f w} (8) Condition (4) is a conditional expression relating to the refractive power of the third lens group, the power increases beyond the lower limit, spherical aberration is excessively corrected It becomes difficult to correct coma aberration. If the power becomes weaker than the upper limit, spherical aberration will be undercorrected. Since the refractive power of the third lens group is set to be relatively weak, it is possible to have a two-group configuration with one group, but since the refractive power can be shared and the burden on each lens can be reduced, the three-group configuration with two groups is required. It is desirable to obtain a higher imaging performance.

Further, it is more preferable to use the following conditional expression (9).

[0028] _{1.61f w <| f 3 | <} 1.83f w, f 3 <0 (9) Furthermore, it is more desirable to use the following conditional expression (10).

[0029] _{1.61f w <| f 3 | <} 1.70f w, f 3 <0 (10)

[0030]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Four embodiments of the present invention will be described in detail with reference to FIGS.

The symbols used in the following examples are as follows.

M: magnification f: focal length Bf: distance between the final lens surface and the conjugate surface on the reduction side F: F number y: image height r: radius of curvature of each refraction surface d: refraction surface interval n: for d line Refractive index of lens material νd: Abbe number of lens material L1: first lens group L2: second lens group L3: third lens group L4: fourth lens group [Example 1] M = -1/10 to -1 /6.8 to -1 / 3.2 f = 40.0 to 56.4 to 94.8 Bf = 112.1 to 133.6 to 199.2 F = 5.6 to 6.4 to 8.4 y = 15 to 21.6 to 21.6 The lens configuration of this embodiment is shown in FIG. The results are shown in Tables 1 and 2 below.

[0033]

[Table 1]

[0034]

[Table 2]

In this embodiment, the combined focal length f _{1 of} the first lens group, the focal length f _w of the entire system on the short focal side, the average value ν of the Abbe numbers of the negative lenses of the first lens group, The combined focal length f _{2 of} the second lens group and the combined focal length f ₃ of the third lens group are defined as follows.

F ₁ = -1.17 f _w ν = 59.9 f ₂ = 1.50 f _w f ₃ = -1.63 f _w An aberration diagram of this example is shown in FIG.

[Example 2] M = -1/10 to -1 / 6.8 to -1 / 3.2 f = 39.5 to 55.7 to 93.7 Bf = 115.4 to 137.7 to 206.0 F = 5.6 to 6.4 to 8.4 y = 15 to 21.6 21.6 The lens configuration of this example is shown in FIG. 3, and the lens data is shown in Tables 3 and 4 below.

[0038]

[Table 3]

[0039]

[Table 4]

In this embodiment, the combined focal length f _{1 of} the first lens group, the focal length f _w of the entire system on the short focus side, the average value ν of the Abbe numbers of the negative lenses of the first lens group, The combined focal length f _{2 of} the second lens group and the combined focal length f ₃ of the third lens group are defined as follows.

F ₁ = -1.16 f _w ν = 59.9 f ₂ = 1.57 f _w f ₃ = -1.68 f _w FIG. 4 shows an aberration diagram of this example.

[Example 3] M = -1/10 to -1 / 7.1 to -1 / 3.2 f = 40.5 to 54.9 to 96.5 Bf = 109.6 to 127.5 to 196.6 F = 5.6 to 6.3 to 8.4 y = 15 to 21.6 21.6 The lens configuration of this example is shown in FIG. 5, and the lens data is shown in Tables 5 and 6 below.

[0043]

[Table 5]

[0044]

[Table 6]

In this embodiment, the combined focal length f _{1 of} the first lens group, the focal length f _w of the entire system on the short focus side, the average value ν of the Abbe numbers of the negative lenses of the first lens group, The combined focal length f _{2 of} the second lens group and the combined focal length f ₃ of the third lens group are defined as follows.

F ₁ = -1.31 f _w ν = 63.0 f ₂ = 1.68 f _w f ₃ = -1.62 f _w An aberration diagram of this example is shown in FIG.

[Example 4] M = -1/10 to -1 / 7.2 to -1 / 3.3 f = 40.6 to 54.2 to 92.9 Bf = 116.1 to 136.0 to 208.0 F = 5.6 to 6.3 to 8.5 y = 15 to 21.6 21.6 The lens configuration of this example is shown in FIG. 7, and the lens data is shown in Tables 7 and 8 below.

[0048]

[Table 7]

[0049]

[Table 8]

In this embodiment, the combined focal length f _{1 of} the first lens group, the focal length f _w of the entire system on the short focus side, the average value ν of the Abbe numbers of the negative lenses of the first lens group, The combined focal length f _{2 of} the second lens group and the combined focal length f ₃ of the third lens group are defined as follows.

[0051] shown in _{f 1 = -1.02f w ν = 56.8} f 2 = 1.48f w f 3 = -1.81f w aberration diagram of the present embodiment FIG.

[0052]

As described in detail above, according to the finite distance zoom lens of the present invention, a long back focus of 110 mm or more can be realized while maintaining a high image forming performance from 3 times to 10 times. It becomes possible to arrange a photometric mirror.

Therefore, by using the zoom lens of the present invention in the enlarger, it is possible to improve print workability and print image quality.

[Brief description of drawings]

FIG. 1 is a lens configuration diagram of Example 1. FIG.

FIG. 2 is an aberration diagram of Example 1.

FIG. 3 is a lens configuration diagram of Example 2.

FIG. 4 is an aberration diagram for Example 2.

FIG. 5 is a lens configuration diagram of Example 3.

FIG. 6 is an aberration diagram for Example 3.

FIG. 7 is a lens configuration diagram of Example 4.

FIG. 8 is an aberration diagram for Example 4.

[Explanation of symbols]

 L1 First lens group L2 Second lens group L3 Third lens group L4 Fourth lens group

Claims (3)

[Claims] 1. A first lens group having a negative refracting power, a second lens group having a positive refracting power, a third lens group having a negative refracting power, and a positive refracting power in order from the enlargement side. And a fourth lens group having the following: When changing the magnification from the short focal length to the long focal length, the first lens group, the second lens group, the third lens group and the fourth lens group move to the enlargement side. A zoom lens for a finite distance having a constant object-image distance, wherein the first lens group has at least three negative lenses and satisfies the following conditional expression: _{0.9f w <| f 1 | <} 1.4f w, f 1 <0 56 <ν However, f _w: the focal length f ₁ the entire system at the short focal end is a composite focal length of the first lens group [nu: first Average Abbe number of negative lens in lens group 2. The first lens group, in order from the enlargement side,
The finite-distance zoom lens according to claim 1, wherein the zoom lens has a four-group, five-lens structure including a negative lens, a positive lens, a negative lens, and a cemented lens of the negative lens and the positive lens. 3. The second lens group has a two-group three-element configuration including a cemented positive lens and a positive lens in order from the magnification side, and the third lens group has a cemented negative lens in order from the magnification side. 3. A two-group, three-lens structure consisting of a negative lens and a negative lens, or a one-group, two-lens structure consisting of a cemented negative lens, which satisfies the following conditional expression:
Finite distance zoom lens _{1.4f w <f 2 <1.8f w} 1.5f w according to _{<| f 3 | <1.9f w} , f 3 <0 where, f _2: the composite focal length f ₃ of the second lens group : Composite focal length of the third lens group