JP2000028919A - Middle telephotographic lens - Google Patents

Abstract

PROBLEM TO BE SOLVED: To excellently correct the various kinds of aberration while making a telephone ratio smaller than 1. SOLUTION: This lens is composed of a convergent front group G1 and a divergent rear group G2 in turn from an object side by interposing a diaphragm S. The group G1 is composed of one sheet of 1st positive lens L1 and the cemented lens of a 2nd positive lens L2 and a 3rd negative lens L3 in turn from the object side. The group G2 is composed of one sheet of 4th positive lens L4, the cemented lens of a 5th negative biconcave lens L5 and a 6th positive biconvex lens L6 and one sheet of 7th negative meniscus lens L7 whose concave surface is arranged on the object side in turn from the object side.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a medium telephoto lens which is most suitable for a photographic lens having an angle of view of about 24 °, a short overall length, and excellently correcting various aberrations.

[0002]

2. Description of the Related Art This type of photographic lens has been conventionally classified as a medium telephoto lens, but most of the constitutions have a telephoto ratio of greater than 1. Among them, for example,
Although the angle of view is almost the same and the telephoto ratio is slightly smaller than 1, the telephoto ratio described in Japanese Patent Application Laid-Open No. -109724 is still about 0.92.

In general, a telephoto lens having a narrower angle of view than a medium telephoto lens tends to have a longer overall length in proportion to its focal length. Is required. For this reason, many ordinary telephoto lenses have a configuration in which the telephoto ratio is smaller than 1.

[0004] As described above, the reason why the number of lens structures having a telephoto ratio smaller than 1 in the medium telephoto lens has been small so far is that although the focal length of the medium telephoto lens is slightly different depending on the size of the photographing screen, the overall length is small. Is not particularly long and is not inconvenient for portability, and it is considered that the reason is that the allowable length of the entire length can be set relatively long.

Further, a telephoto ratio is larger than 1, like a standard lens having a relatively close angle of view, rather than selecting a configuration having a telephoto ratio smaller than 1 such as a telephoto type.
One of the reasons is that the so-called symmetric configuration is more advantageous for aberration correction.

Here, the telephoto type can be roughly divided into two groups, positive and negative, from the object side through the entire lens system. The rear principal point is located closer to the object side than the first lens surface. In contrast to a lens system having a telephoto ratio of less than 1, the symmetric type means a system in which the position of the rear principal point is inside the lens system and the telephoto ratio is greater than 1.

[0007]

In such a conventional telephoto type lens arrangement, it was possible to shorten the entire length of the lens, but various aberrations caused by the asymmetry, especially pincushion type distortion, were observed. And chromatic aberration of magnification are liable to occur.

The present invention has been made in view of the above points, and provides a medium telephoto lens in which various aberrations are satisfactorily corrected while the telephoto ratio is made smaller than 1 by forming the lens system of a telephoto type. The purpose is to provide.

[0009]

In order to achieve the above object, the present invention comprises a front group that converges from the object side, and a rear group that diverges with an aperture interposed therebetween. The rear group includes, in order from the object side, one positive fourth lens, a negative fourth lens, and a cemented lens of a positive second lens and a negative third lens. The present invention provides a middle telephoto lens that includes a cemented lens of the fifth lens and the positive sixth lens and one negative seventh lens that satisfies the following conditional expression. (1) 0.6 <F12 / F <0.9 (2) 0.85 <| F345 / F | <1.45 where F: focal length of the entire system F12: composite focal length of the front group F345: rear Group focal length

In the middle telephoto lens, the fifth lens is a biconcave negative lens, the sixth lens is a biconvex positive lens, and the seventh lens is a single lens having a concave surface on the object side. It is preferable that the zoom lens comprises a negative meniscus lens and satisfies the following conditional expression. (3) N6−N5> 0.08 (4) ν4 <37, ν5> 52 where N5, N6: refractive index of the fifth and sixth lenses ν4, ν5: Abbe number of the fourth and fifth lenses

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments and examples of the present invention will be specifically described below with reference to the drawings. As shown in FIG. 1, the middle telephoto lens according to the present invention includes a front group G1 converging from the object side (left side in the figure) and a rear group G2 diverging across an aperture S, The front group G1 includes, in order from the object side, one positive first lens L1 and a positive second lens L2.
And a negative third lens L3.
Reference numeral 2 denotes, in order from the object side, one positive fourth lens L4, a cemented lens of a biconcave negative fifth lens L5 and a biconvex positive sixth lens L6, and a meniscus convex lens having a concave surface on the object side. The zoom lens comprises seven lenses L7, and satisfies the following conditional expression. That is, (1) 0.6 <F12 / F <0.9 (2) 0.85 <| F345 / F | <1.45 (3) N6-N5> 0.08 (4) ν4 <37, ν5 > 52

Where F: focal length of the entire system F12: composite focal length of front group G1 F345: composite focal length of rear group G2 N5, N6: refractive index of fifth and sixth lenses L5, L6 ν4, ν5: Abbe number of the fourth and fifth lenses L4 and L5

Next, the reason for each of the above conditional expressions will be described. Conditional expression (1) relates to the value of the ratio of the combined focal length F12 of the front group G1 to the focal length F of the entire system.

Now, the total length of the lens system is set to the first lens of the first lens.
When defined as the distance from the surface to the image plane, the above ratio F12
If the value of / F exceeds the upper limit of 0.9, the refractive power of the front group G1 is too weak, and if the total length of the lens system is to be kept at a predetermined value, the distance between the front group G1 and the rear group G2 is too wide. This causes a decrease in the peripheral light amount and an increase in the lens diameter of the rear group G2. Therefore, if an attempt is made to reduce the distance between the front group G1 and the rear group G2, the overall length of the lens system must be increased.

If the value of F12 / F falls below the lower limit of 0.6, the refractive power of the front unit G1 becomes too strong, and the overall length of the lens system can be shortened. Large distortion and the like occur. From such a point, it is desirable that the range of the conditional expression (1) is further reduced to about 0.7 <F12 / F <0.8.

Condition 2 relates to the absolute value of the ratio of the combined focal length F345 of the rear group G2 to the focal length F of the entire system. If this value | F345 / F | exceeds the upper limit of 1.45, the refracting power of the rear group G2 becomes too weak, and the rear group G2 is maintained while keeping the overall length of the lens system defined by the conditional expression (1) relatively short. In order to obtain the appropriate refraction effect, the rear group G
As a result, the ray height of the marginal ray 2 must be increased. In order to avoid this, it is necessary to move the rear group G2 to the object side, but in this case, there is a possibility that light rays may be emitted by the lens mount or the like.

Conversely, the value of | F345 / F |
Below 5, the refractive power of the rear group G2 becomes too strong, and it becomes impossible to correct various aberrations, particularly astigmatism, curvature of field, pincushion distortion, and chromatic aberration of magnification.

Also, the distance between the front group G1 and the rear group G2 tends to be large, and in order to make the distance between the front group G1 and the rear group G2 relatively small to secure the peripheral light amount, the front group G1 is required.
Must be increased, which causes spherical aberration, coma and pincushion distortion. From such a point, it is desirable that the range of the conditional expression (2) is further reduced to about 1.0 <| F345 / F | <1.3.

Next, conditional expression (3) will be described. In the middle telephoto lens according to the present invention, the fifth lens group G2
The lens L5 is composed of a biconcave negative lens, the sixth lens L6 is composed of a biconvex positive lens, and the seventh lens L7 is composed of a meniscus negative lens having a concave surface on the object side. And it is easy to obtain good optical performance over the entire screen.

However, due to the concentricity described above, pincushion-type distortion, which is peculiar to the telephoto type, tends to occur. For this reason, especially, the joint surface r9 of the fifth and sixth lenses L5 and L6 is made convex on the object side, and as shown in the conditional expression (3), the sixth lens L6
By setting the difference between the refractive indices N6 and N5 of the first lens L5 and the fifth lens L5 to 0.08 or more, it is preferable to provide a positive refraction surface with concentricity broken to correct pincushion distortion.

In general, it is difficult to correct chromatic aberration of magnification in the telephoto type. However, in many cases, chromatic aberration of magnification occurs in the same direction by both the front and rear groups because of the positive and negative lens groups with the stop S interposed therebetween. This is because it tends to be.

In view of the above, according to the present invention, as shown in conditional expression (4), the fourth lens L4 of the rear group G2 is
The Abbe number ν4 is set to a relatively small value in order to largely reduce the chromatic aberration of magnification generated in the front group G1. At the same time, since the fifth lens L5 of the rear group G2 has a strong negative refractive power, the Abbe number ν
5 is set relatively large. In the cemented lens of the fifth and sixth lenses L5 and L6, the distortion is corrected as described above while the chromatic aberration of magnification is reduced by achromatism.

The seventh lens L7 of the rear group G2 serves to reduce the negative load on the rear group G2 caused by the fifth lens L5.
It also serves to correct the field curvature as a so-called field flattener and, in addition, to correct the overcorrection of the chromatic aberration of magnification by the fifth lens L5 of the off-axis light beam, particularly toward the maximum image height.

[0024]

Next, preferred embodiments of the medium telephoto lens according to the present invention will be described. FIGS. 1, 2 and 3 are structural views of embodiments 1, 2 and 3 of the present invention, and FIGS.
7 is an aberration curve diagram of Examples 1, 2, and 3, respectively.

In the following examples, f: focal length of the entire optical system ω: half angle of view r: radius of curvature d: surface interval nd: refractive index for d line νd: Abbe number for d line ΔM: meridional image Surface ΔS: sagittal image surface S: stop

The following Tables 1, 2, and 3 show parameters of the first, second, and third embodiments, respectively.

[0027]

[Table 1]

[0028]

[Table 2]

[0029]

[Table 3]

[0030]

As described above, the medium telephoto lens according to the present invention has an angle of view of 24 according to the first aspect of the present invention.
As a medium telephoto lens of the degree, a telephoto type which is almost unprecedented is adopted, and a telephoto ratio of about 0.85 can be achieved while satisfactorily correcting various aberrations. In the invention according to claim 2, it is possible to further correct the above-mentioned various aberrations and further improve the optical performance while keeping the overall length short.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a first embodiment of the present invention.

FIG. 2 is a configuration diagram of a second embodiment of the present invention.

FIG. 3 is a configuration diagram of a third embodiment of the present invention.

FIG. 4 is an aberration curve diagram of the first embodiment.

FIG. 5 is an aberration curve diagram of the second embodiment.

FIG. 6 is an aberration curve diagram of the third embodiment.

[Explanation of symbols]

 G1: Front group G2: Rear group L1 to L7: First to seventh lenses r1 to r12: Curvature radii of first to twelfth surfaces d1 to d11: Surface distance between lenses ω: Half angle of view ΔM: Meridional image plane ΔS: sagittal image surface S: stop

Claims (2)

[Claims] 1. A first lens unit comprising: a front group that is convergent from the object side; and a rear group that is divergent with a stop interposed therebetween. The front group includes one positive first lens, The rear group includes, in order from the object, one positive fourth lens and a negative fifth lens and a positive sixth lens. A middle telephoto lens, comprising one negative seventh lens, satisfying the following conditional expression. (1) 0.6 <F12 / F <0.9 (2) 0.85 <| F345 / F | <1.45 where F: focal length of the entire system F12: composite focal length of the front group F345: rear Group focal length 2. The medium telephoto lens according to claim 1,
The fifth lens is a biconcave negative lens, the sixth lens is a biconvex positive lens, and the seventh lens is a single meniscus negative lens having a concave surface on the object side, satisfying the following conditional expressions. A medium telephoto lens characterized by: (3) N6−N5> 0.08 (4) ν4 <37, ν5> 52 where N5, N6: refractive index of the fifth and sixth lenses ν4, ν5: Abbe number of the fourth and fifth lenses