JP2001272599A - Converter lens and optical system using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a converter lens which is attached to the object side of a photographing lens so as to displace the focal distance of an entire system to a wide-angle side, is made compact and provides high optical performance, and an optical system using the same. SOLUTION: This converter lens attachably/detachably attached to the object side of a principal lens is constituted of a 1st negative meniscus lens whose convex surface faces to an object side, a 2nd negative lens whose refractive surface intensely negative to an image surface side faces to the object side and a 3rd positive lens whose both surfaces are convex in order from the object side. Then, the thickness of the i-th lens from the object side or distance Di is appropriately set.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention uses a converter lens which is detachably mounted on the object side of a photographic lens (main lens) and displaces the focal length of the entire system to the wide-angle side (the shorter one), and uses the converter lens. The optical system is particularly suitable for optical devices such as cameras, video cameras, and digital cameras.

[0002]

2. Description of the Related Art Conventionally, a focal length of a whole photographing system is set to a wide-angle side (short side) while being detachably mounted on an object side of a photographing lens (photographing system) and keeping a focal plane of a whole system at a fixed position. Various wide-angle front converter lenses have been proposed.

In many cases, this wide converter lens has a front lens group having a negative refractive power on the object side and a rear lens group having a positive refractive power on the image plane side, which are separated by a predetermined principal point interval. As a whole, it constitutes an afocal system.

For this reason, the simplest lens system can be constituted by two lenses having negative and positive refractive power. However, in order to obtain desired optical performance when mounted on a taking lens, it is necessary to configure each lens group with a plurality of lenses for aberration correction.

For example, in Japanese Patent Application Laid-Open No. 3-127005, a wide converter lens composed of a first lens having a negative refractive power, a negative second lens having a meniscus shape, and a third lens having a positive refractive power is described in order from the object side. Has been proposed.

[0006] The wide converter lens disclosed in this publication has an afocal magnification of about 0.65 times, and does not disclose a technique relating to a case where the afocal magnification is further reduced.

Japanese Patent Application Laid-Open No. 5-88080 discloses a wide converter lens composed of a first lens having a negative refractive power, a second meniscus lens, and a third lens having a positive refractive power in order from the object side. Has been proposed.

[0008] The wide converter lens disclosed in the publication has an afocal magnification of about 0.8 times, and does not disclose any technology relating to a case where the afocal magnification is further reduced.

[0009]

In general, a front type wide converter lens mounted in front of a photographing lens (main lens) is capable of keeping the F-number and focal plane of the entire system unchanged even when a wide converter lens is mounted. It has the advantage of being able to.

However, since the front type wide converter lens is mounted in front of the taking lens, the entire lens system tends to be large and heavy.

In particular, when it is desired to obtain a predetermined optical performance when attached to a photographic lens system, or when each lens group is made to have a large refractive power for miniaturization, if each lens group is composed of a plurality of lenses, Since the position of the lens on the side is far from the entrance pupil of the photographing lens, enlargement of the lens system was inevitable.

According to the present invention, by appropriately setting the lens structure, the photographing lens (main lens) is detachably mounted on the object side, and the focal length of the entire system is displaced toward the wide angle side (short side). It is an object of the present invention to provide a compact converter lens capable of easily obtaining high optical performance and an optical system using the same.

[0013]

According to a first aspect of the present invention, there is provided a converter lens detachably mounted on an object side of a main lens, wherein the converter lens has a convex surface facing the object side in order from the object side. A negative first lens having a meniscus shape, a negative second lens having a stronger negative refracting surface directed toward the image surface side than the object side, a positive third lens having both lens surfaces convex, and an object side Further, when the i-th lens thickness or the air gap is Di, the condition 0 <D4 / D2 <0.2２ (1) is satisfied.

According to a second aspect of the present invention, when the refractive index of the material of the third lens is N3, the condition of N3 <1.65 ‥‥‥ (2) is satisfied. I have.

According to a third aspect of the present invention, in the first or second aspect, the radius of curvature of the i-th lens surface from the object side is R.
Assuming i, -5 <(R2 + R1) / (R2-R1) <-1 {(3) -2 <(R4 + R3) / (R4-R3) <-0.5} (4) -0 0.1 <(R6 + R5) / (R6-R5) <0.1 (5)

A converter lens according to a fourth aspect of the present invention comprises:
In a converter lens detachably mounted on the object side of the main lens, the converter lens has, in order from the object side, a meniscus-shaped negative first lens having a convex surface facing the object side, and a meniscus-shaped negative lens having a convex surface facing the object side. It is characterized by comprising a positive second lens and a positive third lens having a positive refraction surface facing the image plane side.

According to a fifth aspect of the present invention, when the focal length of the ith lens from the object side is fi, the condition of 1 <f2 / f3 <3 ‥‥‥ (6) is satisfied. It is characterized by:

According to a sixth aspect of the present invention, in the fourth or fifth aspect, the radius of curvature of the i-th lens surface from the object side is R.
-3 <(R2 + R1) / (R2-R1) <-1 {(7) 2 <(R4 + R3) / (R4-R3) <10} (8) -3 <(R6 + R5) / (R6−R5) <− 1 ‥‥‥ (9).

According to a seventh aspect of the present invention, in the first aspect of the present invention, the converter lens is a wide converter lens that displaces the focal length of the entire system to a shorter one when mounted on the main lens. It is characterized by.

The optical system according to the invention of claim 8 is the optical system according to claims 1 to 7.
Wherein the converter lens according to any one of the above is mounted on the main lens.

[0021]

FIG. 1 is a sectional view of an optical system when a converter lens according to a first numerical embodiment of the first invention is mounted in front of a taking lens (master lens, main lens). In the figure, C is a wide converter lens, which is composed entirely of an afocal system. FIGS. 2 and 3 are lens cross-sectional views of converter lenses according to Numerical Examples 2 and 3 of the first invention.

FIG. 4 is a lens cross-sectional view when the converter lens according to Numerical Embodiment 4 of the second invention is mounted in front of a taking lens (master lens). In the figure, reference numeral C denotes a wide converter lens, which is composed entirely of an afocal system.

FIGS. 5 and 6 show numerical examples 5 and 6 of the second invention.
3 is a lens cross-sectional view of the converter lens of FIG.

M is a photographing lens, 4 is an image plane, and 5 is an optical axis. The first invention and the second invention are collectively referred to as the present invention.

First, the features of the converter lens of the first invention shown in FIGS. 1 to 3 will be described. The converter lens C of the first invention has a meniscus negative first lens L1 having a convex surface facing the object side, a negative second lens L2 having a stronger negative refracting surface facing the image surface side than the object side, and both lenses. The surface is composed of a positive third lens L3 having a convex surface. The converter lens C is mounted on the object side of the photographing lens M and constitutes a wide converter lens for displacing the focal length of the entire system to a shorter one.

The taking lens (master lens) M has a single focal length or a zoom lens. Image plane 4
Is a fixed position regardless of whether or not the converter lens C is attached to the taking lens M.

In general, the photographing lens itself performs photographing by itself, so that the photographing lens independently performs good aberration correction. Therefore, in order to obtain good optical performance as a whole when the converter lens is mounted on the taking lens, it is necessary to achieve good aberration correction by using the converter lens alone.

On the other hand, in order to reduce the overall length of the converter lens C, the distance between the principal points of the lens group on the object side having a negative refractive power and the lens group on the image plane having a positive refractive power should be reduced. There is a need to. However, when the afocal magnification is fixed, it is necessary to increase the refractive power of each of the positive and negative lens groups in order to reduce the lens interval. At this time, if the refractive power of the lens group is increased, a large amount of aberration generally occurs, and it becomes difficult to obtain good aberration correction with a small number of lenses.

Therefore, in the first invention, even if the refractive power of each lens group is increased and the total length of the converter lens is set short, the three lens shapes constituting the converter lens C are configured as described above, and the conditional expression ( By satisfying the condition (1), the focal length of the entire system when attached to the taking lens M is displaced by a predetermined amount toward the wide angle side, and various aberrations are corrected well.

Next, the lens shape in the first invention and the technical meaning of each conditional expression will be described with respect to the aberration of the entire system when the converter lens is mounted on the taking lens.

The negative meniscus first lens L1 with the convex surface facing the object side shares most of the negative refractive power of the converter lens, and the shape thereof mainly reduces astigmatism and distortion. It has the function of correcting. The negative second lens L2 functions to share the negative refractive power of the converter lens with the first lens, and also causes the strong negative refractive surface on the image plane side to have the negative refractive power disposed on the object side of the converter lens. It has the function of separating the principal point distance between the lens group and the lens group having a positive refractive power arranged on the image plane side.

As a result, it is possible to arrange a strong refractive power over a short entire lens length. The positive third lens L3 with both lens surfaces being convex has an optimal shape for sharing the positive refractive power of the converter lens.

Conditional expression (1) satisfies the distance D4 between the second lens and the third lens with respect to the distance D2 between the first lens and the second lens.
Is the ratio of If the lower limit is exceeded, the diameter of the front lens becomes larger due to an increase in the overall length of the lens, and more negative distortion occurs. On the other hand, when the value exceeds the upper limit, it becomes difficult to correct curvature of field and astigmatism.

In the first invention, it is more preferable to satisfy at least one of the following conditions.

(A-1) When the refractive index of the material of the third lens is N3, the condition of N3 <1.65 ‥‥‥ (2) is satisfied.

Conditional expression (2) satisfies the Petzval sum of the converter lens alone by making the refractive index of the material of the third lens having only positive refractive power not to exceed the upper limit in the converter of the first invention. And the curvature of field when the converter lens is attached to the taking lens is suppressed.

(A-2) When the radius of curvature of the i-th lens surface from the object side is Ri, -5 <(R2 + R1) / (R2-R1) <-1 {(3) -2 <( By satisfying the condition of R4 + R3) / (R4-R3) <− 0.5 ＜ (4) −0.1 <(R6 + R5) / (R6-R5) <0.1 ‥‥‥ (5) is there.

The conditional expressions (3) to (5) are for obtaining good aberration correction while keeping the overall length of the converter lens short.

If any of the conditional expressions (3) to (5) is exceeded, it will be difficult to achieve good aberration correction over the entire screen.

Next, the features of the converter lens of the second invention shown in FIGS. 4 to 6 will be described. The converter lens of the second invention is a meniscus negative first lens LL1 having a convex surface facing the object side, a meniscus positive second lens LL2 having a convex surface facing the object side, and a positive lens having a convex surface facing the image surface side. Of the third lens LL3. The taking lens (master lens) M has a single focal length or a zoom lens. The image plane 4 is at a fixed position irrespective of whether the converter lens is attached to the taking lens M or not.

According to the second aspect of the present invention, even when the refractive power of each lens group is increased and the overall length of the converter lens is set to be short, the three lenses constituting the converter lens C are configured as described above, so that photographing is performed. When mounted on the lens M, the focal length of the entire system is displaced toward the wide angle side by a predetermined amount, and various aberrations are satisfactorily corrected.

Next, the technical meaning of the lens shape in the second invention will be described with respect to the aberration of the entire system when the converter lens is mounted on the taking lens.

The negative meniscus first lens LL1 having a convex surface facing the object side shares the negative refractive power of the converter lens, and has a function of favorably correcting astigmatism and distortion due to its shape. . The meniscus-shaped positive second lens LL2 whose convex surface faces the object side suppresses astigmatic difference by its shape, and has a negative refractive power lens group disposed on the object side of the converter lens and an image plane side. It has the function of separating the principal point distance from the disposed lens unit having a positive refractive power. This shortens the overall length of the lens. The shape of the positive third lens having the strong positive refraction surface facing the image plane has a function of favorably correcting negative distortion generated by the first lens.

In the second invention, it is more preferable to satisfy at least one of the following conditions.

(A-1) When the focal length of the i-th lens from the object side is fi, the condition of 1 <f2 / f3 <3 ‥‥‥ (6) must be satisfied.

Conditional expression (6) represents the ratio between the refractive powers of the second lens and the third lens. If the lower limit is exceeded, it is difficult to obtain a sufficient afocal magnification while keeping the overall length of the lens short. On the other hand, when the value exceeds the upper limit, the power distribution between the two positive lenses becomes inappropriate, and it becomes difficult to satisfactorily correct distortion particularly.

(A-2) When the radius of curvature of the focal length of the i-th lens from the object side is Ri, -3 <(R2 + R1) / (R2-R1) <-1 (7) 2 < (R4 + R3) / (R4-R3) <10 {(8) -3 <(R6 + R5) / (R6-R5) <-1} (9)

The conditional expressions (7) to (9) are for obtaining good aberration correction while keeping the overall length of the converter lens short.

Exceeding any of the conditional expressions (7) to (9) makes it difficult to achieve satisfactory aberration correction over the entire screen.

Next, numerical examples of the converter lens of the first and second inventions will be shown. In each numerical example, Ri is the ith radius of curvature in order from the object side, Di is the ith lens thickness and air spacing in order from the object side, and Ni and νi are the materials of the ith lens in order from the object side. And the Abbe number.

Tables 1 and 2 show correspondences between the above-described conditional expressions and the respective embodiments.

[0052]

[0053]

[Table 1]

[0054]

[0055]

[Table 2]

As described above, according to the present invention, when the focal length of the entire photographing system is displaced toward the wide angle side (shorter) by being mounted in front of the photographing lens, the above condition is satisfied.
A converter lens that uses a single lens to achieve a wide angle with an afocal magnification of about 0.6 times, while maintaining a small lens diameter and good optical performance of the entire system when attached to a photographic lens. Can be achieved.

[0057]

According to the present invention, as described above, by appropriately setting the lens configuration, the photographing lens (main lens) is detachably mounted on the object side, and the focal length of the entire system is set to the wide-angle side (short-range). The converter lens which can be displaced in the direction (1) and which can easily obtain a compact and high optical performance and an optical system using the converter lens can be achieved.

[Brief description of the drawings]

FIG. 1 is a lens cross-sectional view when a converter lens according to Numerical Example 1 of the first invention is mounted in front of a taking lens.

FIG. 2 is a lens sectional view of a converter lens according to Numerical Example 2 of the first invention;

FIG. 3 is a sectional view of a converter lens according to Numerical Example 3 of the first invention;

FIG. 4 is a lens cross-sectional view when a converter lens according to Numerical Example 4 of the second invention is mounted in front of a taking lens;

FIG. 5 is a lens sectional view of a converter lens according to Numerical Example 5 of the second invention;

FIG. 6 is a sectional view of a converter lens according to Numerical Example 6 of the second invention;

[Description of Signs] C Converter Lens M Shooting Lens 4 Image Surface 5 Optical Axis

Claims (8)

[Claims] 1. A converter lens detachably mounted on an object side of a main lens, wherein the converter lens has, in order from the object side, a meniscus-shaped negative first lens having a convex surface facing the object side, and an image as compared with the object side. A negative second lens having a strong negative refractive surface directed to the surface side, a positive third lens having both lens surfaces convex, and the i-th lens thickness or air gap from the object side is Di. A converter lens characterized by satisfying the following condition: 0 <D4 / D2 <0.2. 2. The converter lens according to claim 1, wherein a condition of N3 <1.65 is satisfied when a refractive index of a material of said third lens is N3. 3. When the radius of curvature of the i-th lens surface from the object side is Ri, -5 <(R2 + R1) / (R2-R1) <-1-2 <(R4 + R3) / (R4-R3) < 3. The converter lens according to claim 1, wherein the following condition is satisfied: -0.5 -0.1 <(R6 + R5) / (R6-R5) <0.1. 4. A converter lens detachably mounted on the object side of the main lens, wherein the converter lens has, in order from the object side, a negative meniscus first lens having a convex surface facing the object side, and a convex surface facing the object side. Positive meniscus second lens, positive third lens with positive refracting surface facing the image side
A converter lens characterized by being composed of a lens. 5. The converter lens according to claim 4, wherein a condition of 1 <f2 / f3 <3 is satisfied when a focal length of an i-th lens from the object side is fi. 6. When the radius of curvature of the i-th lens surface from the object side is Ri, -3 <(R2 + R1) / (R2-R1) <-12 <(R4 + R3) / (R4-R3) <10 6. The converter lens according to claim 4, wherein a condition of -3 <(R6 + R5) / (R6-R5) <-1 is satisfied. 7. The converter lens according to claim 1, wherein said converter lens is a wide converter lens which displaces a focal length of the entire system to a shorter one when mounted on a main lens. 8. An optical system wherein the converter lens according to claim 1 is mounted on a main lens.