JPH11194268A - Rear converter lens and camera provided therewith - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a rear converter lens having excellent optical performance while securing specified back focus in spite of simple constitution. SOLUTION: This rear converter lens Lc is attached to the image side of a principal lens system Ls, displaces the focal distance of an entire system to an enlargement side and has negative refractive power as a whole. It is provided with a 1st negative lens 1 whose strong concave surface relatively faces to the image side, a combined lens obtained by sticking a 2nd negative lens 2 whose strong concave surface relatively faces to the image side and a 3rd positive lens 3 whose strong convex surface relatively faces to an object side, and a combined lens obtained by sticking a 4th positive lens 4 whose both surfaces are convex and a 5th negative lens 5 in order 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 rear converter lens which is detachably mounted on the image side of a main lens system and shifts the focal length of the entire system to the telephoto side.

[0002]

2. Description of the Related Art Conventionally, in single-lens reflex cameras, there is known an adapter which displaces a lens system having a negative refractive power between an interchangeable lens and a film surface and displaces a focal length of the entire system to a telephoto side. Have been. For example,
Japanese Patent Application Laid-Open No. 12421 and Japanese Patent Publication No. 61-13206 disclose such a rear converter lens.

On the other hand, in recent years, with the advancement of performance and digitalization of video decks, video cameras have been improved in image quality. As one method for achieving high image quality of a video camera, there is known a method in which photographing light is separated into each color light by a color separation optical system, and an image is captured by three image pickup devices.

There is known a video camera equipped with a color separation optical system in which a photographing lens can be exchanged.
There is also a demand for mounting such a rear converter lens on such a video camera.

[0005]

However, in order to apply a rear converter lens to a video camera or the like having a color separation optical system, if a certain back focus is secured, the rear converter lens is disposed in front of an imaging surface such as a CCD. There is a problem that the converter lens interferes with an optical member such as a color separation optical system or an optical low-pass filter, so that the optical lens cannot be mounted.

Further, since the main lens system to which the rear converter lens is mounted performs photographing by itself, satisfactory aberration correction has already been performed by itself. However, when a rear converter lens having negative refractive power as described above is attached to the main lens system, the Petzval sum becomes negative, so that the image plane becomes larger in proportion to the size of the angle of view, and the image quality deteriorates. Resulting in.

Although it is possible to correct various aberrations by increasing the number of lenses, as a result, the size of the entire system including the main lens system increases, and the lens configuration becomes complicated. .

The present invention has been made to solve the above problems, and has a compact structure.
It is an object of the present invention to provide a rear converter lens that can maintain good optical performance while securing a predetermined back focus.

[0009]

In order to achieve the above object, the present invention provides a rear converter having a negative refractive power as a whole, which is mounted on the image side of a main lens system and displaces the focal length of the entire system to an enlarged side. In the lens, in order from the object side, a negative first lens whose concave surface is relatively strong on the image side, a negative second lens whose concave surface is relatively strong on the image side, and a convex surface which is relatively strong on the object side And a cemented lens in which a positive fourth lens and a negative fifth lens, both surfaces of which are convex, are attached to each other.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be specifically described with reference to the drawings.

FIGS. 1, 3, and 5 are lens cross-sectional views when rear converter lenses of Numerical Examples 1 to 3 to be described later are mounted on the image side of the main lens system (zoom lens). And 6 are aberration diagrams of the entire system when the rear converter lenses of Numerical Examples 1 to 3 are attached to the photographing zoom lens, respectively. In FIGS. 2, 4, and 6, (a)
Is an aberration diagram at the wide-angle end, (b) is an aberration diagram at the intermediate focal length, and (c) is an aberration diagram at the telephoto end. 2, 4, and 6, d and g are d-line, g-line,
ΔM and ΔS represent a meridional image plane and a sagittal image plane.

In FIGS. 1, 3, and 5, Ls denotes a zoom lens composed of first to fourth lens groups I to IV, Lc
Is a rear converter lens, GB is a color separation prism or C
A glass block such as a face plate of a CD or a low-pass filter, and I is an imaging surface on which an imaging device such as a CCD is to be arranged. In the present embodiment, in particular, the screen size 3.6
It is assumed that the digital camera is a 3CCD digital video camera using three image sensors having a size of 4.8 mm (effective screen size: 6.0 mm: 1/3 inch CCD). That is, FIGS.
In FIG. 1, only one imaging plane I is drawn for simplicity, but there are actually three imaging planes corresponding to each color.

The zoom lens Ls is mounted on a camera body via a mount member C, and the rear converter lens Lc is mounted on a camera body via a mount member C '. Therefore, the image side of the glass block GB is included in the camera body. 3 and 5, the mount members C and C 'are omitted, but have the same configuration as that of FIG.

In this embodiment, the zoom lens Ls
Moves the second lens group II to the image side as indicated by the arrows in each figure and moves the fourth lens group IV to change the image plane due to the magnification during zooming from the wide-angle end to the telephoto end. It has been corrected. Further, at the time of focusing, the zoom lens is a rear focus type zoom lens that moves the fourth lens group IV on the optical axis.

The rear converter lens Lc of the present embodiment
In order to extend the back focus, the first lens 1 is constituted by a negative lens, and a retro-focus type lens is disposed. However, if the negative power of the first lens 1 is excessively increased, the Petzval sum increases negatively, and the image plane becomes over (excessive field curvature occurs). The positive lens is arranged between the lenses up to 5, so that the power sharing of each lens is reduced, and the field curvature is effectively reduced.

A general rear converter lens is characterized in that aberrations generated in the main lens are worsened by the square of the magnification of the rear converter lens. When a rear converter lens is used in a camera that achieves high image quality by decomposing an image by a color separation optical system as in the present embodiment, it is necessary to correct chromatic aberration better. For this reason, in the rear converter lens of the present embodiment, the chromatic aberration is favorably corrected by the cemented lens.

The object of the present invention is achieved by configuring the rear converter lens as in the above-described embodiment. To achieve better aberration correction, the following (a) to ( At least one of the items in f)
It is desirable to satisfy one.

(A) When the air gap between the third lens and the fourth lens constituting the rear converter lens is D34 and the focal length of the rear converter lens is fc, 0.01 <| D34 / fc | <0.05 (1) The following conditional expression must be satisfied.

Conditional expression (1) is a condition for correcting astigmatism and distortion in a well-balanced manner. Exceeding the upper limit of conditional expression (1) is not preferable because the astigmatic difference increases. Conversely, if the lower limit is exceeded, it becomes difficult to correct distortion, and at the same time, the overall length of the rear converter lens becomes too short, and the distance between the main lens and the camera becomes insufficient, making it difficult to physically arrange the rear converter lens. become.

(B) The Abbe numbers of the materials of the fourth lens and the fifth lens constituting the rear converter lens are ν4 and ν5
In this case, the following conditional expression is satisfied: 21 <ν4-ν5 <50 (2)

Conditional expression (2) is a condition for favorably correcting chromatic aberration generated in the rear converter lens.
If the upper limit of conditional expression (2) is exceeded, axial chromatic aberration will be over and correction will be excessive. Conversely, if the value exceeds the lower limit, the axial chromatic aberration becomes under, resulting in insufficient correction.

(C) An image point formed only by the main lens system is defined as an object point of the rear converter lens, and a distance from an image point which is a conjugate point formed by the rear converter lens is defined as L (equivalent air distance). , Effective screen size Y
In this case, the following conditional expression should be satisfied: 2.2 <L / Y <4.6 (3)

Condition (3) relates to the overall length (size) of the rear converter lens. If the upper limit of conditional expression (3) is exceeded, the overall length of the rear converter lens will increase,
This leads to an increase in the size of the entire lens, which is not preferable. Conversely, if the lower limit is exceeded, the overall length of the rear converter lens becomes too short, and the distance between the main lens and the camera becomes so close that it becomes difficult to physically arrange the rear converter lens.

The effective screen size Y is shown in FIGS.
Is the size of the imaging plane I. Effective screen size Y
Is generally represented by its diagonal length because the imaging plane I is rectangular. In the present embodiment, the effective screen size Y
Is 6.0 mm as described above.

(D) The radius of curvature of the first lens on the image side is R1
2. When the radius of curvature of the second lens on the object side is R21, the following conditional expression is satisfied: 0.03 <| R12 / R21 | <0.11 (4)

Particularly, in order to correct the spherical aberration and the coma aberration in a well-balanced manner and to set the back focus to an appropriate value, it is preferable to satisfy the conditional expression (4). When the value exceeds the upper limit of conditional expression (4), the coma aberration becomes large and the back focus becomes insufficient. Conversely, if the value exceeds the lower limit, the spherical aberration becomes under, which is not preferable.

(E) Of the lenses constituting the rear converter lens, when the average refractive index of the material of the positive lens is Np and the average refractive index of the material of the negative lens is Nn, 1.39 <Np <1.65. (5) The conditional expression 1.55 <Nn <1.91 (6) must be satisfied.

The conditional expressions (5) and (6) are conditions for preventing the deterioration of Petzval sum by using a low refractive index glass for the positive lens and a high refractive index glass for the negative lens of the lens constituting the rear converter lens. When a material having a refractive index that does not satisfy the conditional expression is used, the field curvature is deteriorated.

(F) Use of at least one aspherical lens for the rear converter lens in order to better correct aberrations, particularly, off-axis flare.

It is desirable that the aspherical shape is such that the positive refractive power becomes weaker toward the periphery of the lens.

Next, numerical data of the zoom lens shown in FIGS. 1 to 3 and the rear converter lenses of Numerical Examples 1 to 3 will be shown. The numerical data of the zoom lens is common to each numerical example.

In each numerical data, ri is the radius of curvature of the i-th surface in order from the object side, di is the i-th lens thickness and air gap in order from the object side, and ni and νi are the i-th lenses in order from the object side. The refractive index and Abbe number of the second lens.

The surface number in the numerical data of the rear converter lens is a surface number including the lens surface of the zoom lens. Therefore, d25 represents the distance from the last surface (the 25th surface) of the main zoom lens. On the other hand,
The members represented by planes 36 to 36 are the glass blocks GB shown in FIGS.

In each numerical data, the aspherical surface shape is defined as the X axis in the optical axis direction, the H axis in the direction perpendicular to the optical axis, the traveling direction of light as positive, r as the paraxial radius of curvature, and K,
B, C, D, E, and F,

[0035]

[Outside 1] This is represented by

For example, the display of “eZ” is “1”.
0- ^Z ".

[0037]

[Outside 2]

[0038]

[Outside 3]

[0039]

[Outside 4]

[0040]

[Outside 5]

Table 1 shows the relationship between each conditional expression and various numerical values in the numerical examples, and Table 2 shows the specifications of the rear converter lens in each numerical example.

[0042]

[Table 1]

[0043]

[Table 2]

The rear converter lens of the present embodiment is
By setting the lens configuration appropriately, it is possible to secure the mechanical space of the mount part for mounting the rear converter lens and to arrange the color separation prism on the image side, while having a simple and compact configuration with a small number of lenses. A good optical performance with very little aberration variation can be exhibited even when the lens is mounted on a bright main lens system having an F-number of about 1.6.

[0045]

As described above, according to the present invention,
It is possible to realize a rear converter lens having good optical performance while securing a predetermined back focus while having a simple configuration.

[Brief description of the drawings]

FIG. 1 is a sectional view of a zoom lens equipped with a rear converter lens according to Numerical Example 1.

FIG. 2 is a diagram illustrating various aberrations of a zoom lens equipped with a rear converter lens according to Numerical Example 1.

FIG. 3 is a sectional view of a zoom lens equipped with a rear converter lens of Numerical Example 2.

FIG. 4 is a diagram illustrating various aberrations of the zoom lens equipped with the rear converter lens of Numerical Example 2.

FIG. 5 is a sectional view of a zoom lens equipped with a rear converter lens of Numerical Example 3.

FIG. 6 is a diagram illustrating various aberrations of the zoom lens equipped with the rear converter lens of Numerical Example 3.

[Explanation of symbols]

 Ls Main lens system Lc Rear converter lens GB Glass plate I Imaging surface 1 First lens 2 Second lens 3 Third lens 4 Fourth lens 5 Fifth lens

Claims (8)

[Claims] 1. A rear converter lens having a negative refractive power as a whole, which is mounted on the image side of a main lens system and displaces the focal length of the entire system to an enlargement side, is relatively strong on the image side in order from the object side. A cemented lens obtained by bonding a negative first lens having a concave surface, a negative second lens having a relatively strong concave surface on the image side, and a positive third lens having a strong convex surface relatively on the object side; A rear converter lens comprising a cemented lens obtained by bonding a positive fourth lens and a negative fifth lens, both surfaces of which are convex. 2. The air gap between the third lens and the fourth lens is D34, and the focal length of the rear converter lens is f.
2. The rear converter lens according to claim 1, wherein when c is satisfied, the following conditional expression is satisfied: 0.01 <| D34 / fc | <0.05. 3. 3. The conditional expression 21 <ν4-ν5 <50, wherein the Abbe numbers of the materials of the fourth lens and the fifth lens are ν4 and ν5, respectively. The described rear converter lens. 4. An air-equivalent distance between an image point formed only by the main lens system and a conjugate point with respect to the object point formed by the rear converter lens when the image point is an object point. Wherein, when the effective screen size is Y, a conditional expression of 2.2 <L / Y <4.6 is satisfied.
The described rear converter lens. 5. The radius of curvature of the image-side surface of the first lens is R12, and the radius of curvature of the object-side surface of the second lens is R2.
5. The conditional expression 0.03 <| R12 / R21 | <0.11 is satisfied, wherein 1.
The described rear converter lens. 6. When the average refractive index of the material of the positive lens is Np and the average refractive index of the material of the negative lens is Nn among the lenses constituting the rear converter lens, 1.39 <Np <1.65. 6. The lens according to claim 1, wherein a conditional expression 1.55 <Nn <1.91 is satisfied.
The described rear converter lens. 7. The rear converter lens according to claim 1, further comprising at least one aspherical lens. 8. A camera comprising the rear converter lens according to claim 1, wherein the rear converter lens is detachable.