JPH0441804B2 - - Google Patents

Description

[Detailed description of the invention]

(Technical Field of the Invention) The present invention relates to a focusing rear conversion lens that can be attached to a lens for a single-lens reflex camera and can be used for general purposes. (Background of the invention) Various lenses that can autofocus have already been commercialized for single-lens reflex cameras, but all of them are specialized lenses that can only autofocus for a specific lens, so they are versatile. There was no such thing, and it was expensive. For this reason, the configuration of a focusing converter that enables general-purpose automatic focusing by attaching a focusing lens system between an objective lens and a camera body is disclosed, for example, in Japanese Patent Laid-Open No. 54-28133. However, it was not practical. In other words, in order to be able to be attached to any objective lens, it must be small enough to be attached to large aperture ratio objectives and short back focus objectives, and yet have excellent imaging performance. It has been extremely difficult to design a converter that satisfies all of these requirements. (Objective of the Invention) An object of the present invention is to be able to be universally attached to various objective lenses, and in particular to be able to be attached to short back focus objective lenses, and to maintain excellent imaging performance while being compact. An object of the present invention is to provide a focusing rear conversion lens, that is, a rear focus conversion lens. (Summary of the Invention) A rear focus conversion lens according to the present invention is installed between an objective lens and a camera body, and is used to enlarge the focal length of a composite system with the objective lens compared to the focal length of the objective lens. A rear conversion lens, comprising a front group with a positive refractive power and a rear group with a negative refractive power, which are movable on the optical axis relative to the objective lens and the camera body,
By moving both groups, it is possible to focus on an object from infinity to a predetermined short distance, and the front group with positive refractive power is arranged closest to the object side and includes a negative meniscus lens with a convex surface facing the object side, and the negative meniscus lens. It has a positive lens placed on the image side of the lens, and the magnification of the focal length when focused at infinity is 〓, and the composite back focus when focused from infinity to a predetermined short distance.
The amount of change in Bf is △Bf, the focal length fR of the rear conversion lens, the distance from the vertex of the lens surface closest to the object side of the rear conversion lens to the image point of the objective lens is do, the focal lengths of the front and rear groups. When f ₁ and f ₂ are respectively, 1.3＜〓＜2.5……(1) 0＜〓〓BBf／f _R〓 ＜0.2……(2) 0.4＜〓Bf／d ₀・〓〓＜0.9 … …(3) −2.0＜f ₂ /f ₁ ＜−0.31 …(4) 0.6＜〓f ₁ /f _R 〓＜1.8 …(5) 0.3＜〓f ₂ /f _R 〓＜0.6 …( 6) 0.67<〓・d ₀ /f _R <0.88 ……(7) is satisfied. That is, the RFC according to the present invention has basically the same structure as the RFC disclosed in the earlier application (Japanese Patent Application No. 57-32194) by the same applicant as the present application,
In particular, the RFC is divided into a convergent front group and a diverging rear group, and a negative meniscus lens with a convex surface facing the object is placed closest to the object as the convergent front group. It is characterized by the fact that Hereinafter, a rear focus conversion lens (hereinafter referred to as RFC) according to the present invention will be explained based on the drawings. FIG. 1 is a sectional view showing a schematic configuration of a state in which an RFC 30 according to the present invention is attached between an objective lens 10 and a single-lens reflex camera body 20. In the figure, peripheral rays from an on-axis object point reaching the film surface 21 are shown. The single-lens reflex camera body 20 includes a swingable reflector 22, a focusing plate 23, a condenser lens 24, a penta roof prism 25, and an eyepiece 2.
6. The reflecting mirror 22 is normally provided obliquely at the position indicated by the dotted line except when the film surface 21 is exposed. In a single-lens reflex camera, in order to secure a swinging space for this swinging reflector 22, the lens mount surface 28 of the single-lens reflex camera body 20 and the film surface 21
The distance from the camera body, the so-called flange back MB, is set to a value unique to the camera body. The distance between the last lens surface of the objective lens and the film surface, ie, the back focus Bf', is designed to be sufficiently longer than the swinging space of the reflecting mirror 22. Therefore, even when the RFC is attached to the objective lens, the back focus Bf of the composite system with the objective lens is
It is necessary to secure a space greater than the swinging space of the reflecting mirror 22, and furthermore, in order to focus on a close-range object,
It is necessary to maintain sufficient backfocus even when the principal point of the negative lens group forming the RFC is moved toward the image side. As described above, the RFC according to the present invention must not only satisfy the conditions as a rear conversion lens, but also must satisfy various conditions in order to sufficiently achieve the focusing function. Specifically, in order to achieve versatility, there is an upper limit to the magnification that RFC is responsible for in order to maintain good focusing accuracy even when attaching a bright objective lens or a dark objective lens. also ensure sufficient back focus and RFC
Since it is not desirable to increase the amount of movement too much, there is also a lower limit to the magnification. Further, since the RFC according to the present invention performs focusing by moving in a limited space between the objective lens and the camera body, it is also limited in this respect. In other words, when focusing at the closest distance,
RFC moves on the optical axis toward the image side. At this time, a sufficient back focus length is required to establish a single-lens reflex lens system.
The RFC lens system must be located as close to the objective lens as possible. On the other hand, the back focus of objective lenses for general single-lens reflex cameras is set to the minimum necessary value in order to secure the swinging space of the quick return mirror, and depending on the lens type, the back focus is extremely short within this range. There are also objective lenses. In order to satisfy the versatility, it is also necessary to be able to attach it to an objective lens with such a long back focus, and taking these into consideration, the distance to the image point by the RFC and the objective lens,
In other words, the object point distance of RFC cannot be made very long, and there is a limit to how unevenly distributed the lens arrangement of RFC can be. In addition, compared to conventional general rear conversion lenses that do not move, there is little difference in the distance from the optical axis at the position where the oblique light beam and the light beam from the on-axis object point cut the rear conversion lens. There is little freedom in aberration correction, and it is extremely difficult to satisfactorily correct various aberrations over the entire focusing range. In addition, in order to widen the focusable area as much as possible by RFC, the lens length as RFC (RFC
There is also a method of securing backfocus by shortening the length (from the frontmost surface to the last surface), but this also has its limits in terms of aberration correction. In other words, it is difficult to ensure a degree of freedom in correcting aberrations by creating a sufficient air gap within the RFC. Therefore, in the present invention, as described above,
By placing a negative meniscus lens with a convex surface facing the object side closest to the objective lens side of the RFC, achromatization efficiency is improved, and in particular, axial chromatic aberration can be easily corrected, and higher-order color It has been found that correction of spherical aberration is also easy. By widening the air distance between the convergent front group and the diverging rear group as much as possible, it is possible to correct the extroverted coma aberration of the rays above the chief ray at intermediate angles of view, and also It is now possible to correct inward coma aberration of the side rays. In addition, regarding aberration fluctuations when the RFC is moved toward the image side on the optical axis and focused at a finite distance, the present invention in which the negative lens precedes the positive lens disclosed in the previous application It has also been found that this method has smaller fluctuations and is advantageous in maintaining stable imaging performance. Hereinafter, each condition according to the present invention described above will be explained. If the upper limit of equation (1) is exceeded, it becomes difficult to correct aberrations and the number of lenses increases. Also, the F number of the composite lens system becomes too large, making it dark. For this reason, sufficient distance measurement accuracy can only be obtained with a bright objective lens, resulting in a lack of versatility. If the lower limit is exceeded, the amount of RFC movement becomes too large when trying to focus at a predetermined close distance.
On the other hand, if a single-lens reflex lens focuses while securing the back focus, the focusable area becomes narrower, which is inappropriate for practical use. If the condition of formula (2) is exceeded, it becomes necessary to increase the distance d ₀ from the top of the front lens surface of the RFC to the image point of the objective lens, and the number of objective lenses to which the RFC can be attached becomes too small, resulting in loss of versatility. It's inappropriate. Furthermore, as f _R becomes shorter, the refractive power of the RFC becomes too strong, making it difficult to correct astigmatism and Petzval sum, and the movement of the RFC increases aberration fluctuations when focusing at the closest distance. Therefore, it is inappropriate. If the upper limit of equation (3) is exceeded, the RFC lens length becomes too short, the Petzval sum becomes too negative, and the degree of freedom for aberration correction is lost. Also, 〓 becomes too small, and the photographing range that can be focused becomes small, which is inappropriate. If the lower limit is exceeded, the magnification becomes too large, making it difficult to correct astigmatism and increasing the number of lenses. Furthermore, the RFC lens length is too long, making it inappropriate. Equation (4) defines the appropriate refractive power and distribution of the rear group with respect to the front group of RFC, and if the upper limit is exceeded, the refractive power of the front group with positive refractive power will become too strong and the back focus will not be sufficient for focusing. This is inappropriate because it would be difficult to ensure that Exceeding the lower limit is inappropriate because it becomes difficult to correct spherical aberration. Also, equations (5) and (6) are based on the above (4)
This supplements the conditions in the formula, and if these conditions are violated, the balance of refractive power between the front group and the rear group will be lost, making it difficult to correct various aberrations. Also, above (7)
The formula is an effective condition for properly correcting the Petzval sum, and is particularly effective for maintaining versatility as a rear conversion lens that enables focusing. If the lower limit of this condition is exceeded, it will be difficult to secure the movement space necessary for focusing.
On the other hand, if the upper limit is exceeded, the negative refractive power as a rear conversion lens becomes too strong, making it difficult to correct aberrations, and aberration fluctuations due to focusing become large. Note that under the condition of equation (4), it is more preferable to set −0.6<f ₂ /f ₁ <−0.31 from the viewpoint of correcting various aberrations. In the above RFC of the present invention, the distance d ₀ between the top lens surface of the RFC and the image point of the objective lens, and the distance between the objective lens mount surface of the camera body and the film surface, so-called flange bag MB, are further specified. It is desirable to satisfy the following conditions: , 0.7<〓d ₀ /MB〓<0.9. Here, for a typical single-lens reflex camera body, MB = 46.5
mm. Furthermore, in order to properly correct the Petzval sum, it is practical to satisfy the condition of 0.4<d ₀ /f _R <0.7. Regarding the specific lens configuration, the front group consists of, in order from the object side, a negative meniscus lens with a convex surface facing the object side and a biconvex positive lens, and both lenses may be separated or cemented together. Alternatively, another positive lens may be provided on the image side of the biconvex positive lens to share the convergent refractive power of the front group. The rear group consists of, in order from the object side, a biconcave negative lens and a positive lens with a surface with a stronger curvature facing the object side, and these lenses further share the divergent refractive power of the rear group on the image side. It is also possible to provide laminated or single negative lenses. (Example) An example of the RFC according to the present invention is shown below. Each example was designed based on the objective lens shown in Table 1. This reference objective lens is described in Japanese Patent Laid-Open No. 52-88020 by the same applicant as the present application. In Table 1, R is the radius of curvature of each lens surface, d
are the center thickness and air gap of each lens, n is the refractive index of each lens, 〓 is the Abbe number of each lens, and the subscript number represents the order from the object side. Further, the specifications of the first to eighth embodiments of the RFC according to the present invention are shown in Tables 2 to 9, respectively. However, in each of these tables, the order from the object side is shown at the left end of the table.
d ₀ represents the distance between the frontmost lens surface of the RFC and the image point of the objective lens, D ₀ represents the distance from the frontmost lens surface of the objective lens to the object point, and D ₁ represents the distance between the objective lens and the image point.
air distance with RFC, f ₁ is focal length of RFC front group G ₁ ,
Let f ₂ represent the focal length of the RFC rear group G ₂ .
In addition, Bf represents the back focus of the composite system of RFC and the reference objective lens, △B _f represents the amount of change in back focus between infinity focusing and close range focusing by RFC, and F represents the amount of change in back focus between RFC and objective lens. M represents the composite focal length and the imaging magnification of the composite system.

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[Table] Lens configuration diagrams of the first to eighth embodiments described above are shown in FIG. 2 to FIG. 9, respectively. The lens configuration diagram of the first example shown in FIG. 2 also shows the lens configuration of the reference objective lens L ₀ in Table 1. Table 1 shows the RFCs of the first to eighth embodiments above.
Various aberration diagrams when attached to the reference objective lens shown in FIG. 10A, B to FIG. 17A, B are shown in order. A in each figure shows various aberration diagrams when focusing on infinity with each RFC installed, and B in each figure shows various aberration diagrams with each RFC installed.
This shows various aberration diagrams when focusing at close range using RFC. Each aberration diagram includes spherical aberration Sph, astigmatism Ast, distortion Dis, and reference wavelength d-line (〓=
chromatic aberration of magnification Lat.Chr and coma aberration Coma of the g-line (=435.8nm) with respect to 58.7nm). In the first to eighth embodiments described above, the front group G ₁
The configuration is such that focusing on a short-distance object is achieved by moving the rear group _G2 integrally toward the image side.
As shown in FIG. 18, it is also possible to configure the front group _G1 and the rear group _G2 to move at different speeds for focusing. In this case, it is possible to further increase the imaging magnification due to close-range focusing, and it is also possible to correct aberration fluctuations during close-range focusing. When the magnification of the RFC is relatively high, sufficient focusing can be achieved only by relative movement between the front group G ₁ and the rear group G ₂ of the RFC. Objective lens for image plane 21 at magnification
Focusing can also be achieved by moving _L0 itself. Now, the RFC according to the present invention is installed between the objective lens L ₀ and the camera body 20, and the total length of the composite system (distance from the frontmost surface of the objective lens to the image plane 21) when focused on an object at infinity is TL , as shown in Fig. 19, the total length when focused on a finite distance object is TL', and the distance D ₁ between the objective lens L ₀ and the front group G ₁ of the RFC changes by 〓D ₁ , from D ₁ to D. ₁ + 〓D ₁ , RFC front group
If the distance D ₂ between G ₁ and the RFC post-group G ₂ changes by 〓D ₂ ,
From D ₂ to D ₂ +〓D ₂ , synthetic back focus
Assuming that B _f becomes B _f +〓B _f , the amount of change in the total length〓TL is expressed as〓TL=TL′−TL=〓D ₁ +〓D ₂ +〓B _f . Here, the coefficient value obtained by dividing the amount of change in the total length (TL) by the amount of change in the synthetic back focus (B _f ) is: = = TL / B _f = D ₁ / B _f + D ₂ / B _f It becomes +1. Then, if we set 〓 ₁ =〓D ₁ /〓B _f 〓 ₂ =〓D ₂ /〓B _f , then 〓=〓 ₁ +〓 ₂ +1 ...(7), and 〓 ₁ and 〓 ₂ are objective lenses L ₀ and RFC pregroup
Distance change with G ₁ = D ₁ and RFC front group G ₁ and rear group G ₂
The amount of change in the interval between and D ₂ The amount of change in each synthetic back focus = B This is the rate of change with respect to _f . Equation (7) above can represent all cases of movement regarding the RFC of the present invention except for the case where the synthesis back focus does not change, that is, when 〓B _f ≠0. For example, when ==0, it means that the objective lens is fixed with respect to the image plane and focusing is performed only by RFC. However, 〓=
If 0, 〓 ₁ = -1, 〓 ₂ = 0, the front and rear groups of RFC
This is a focusing method in which G ₁ and G ₂ move as one,
This is the focusing method of the first to eighth embodiments described above. In the present invention, it is desirable to move both the front group _G1 and the rear group _G2 of the RFC toward the image side during short-distance focusing so as to reduce the distance between the two groups. Regarding the coefficient value 〓 based on the above formula (7), in the present invention, -10<〓≦0 ...(8) 〓 ₁ <0 ......(9) 0<〓 ₂ <1 ...(10) It was found that it is appropriate to satisfy each of the following conditions. Conditional expression (8) is 〓B _f <0, so 〓TL≧0
RFC actively moves the reference objective lens to the object side.
This means that it moves in conjunction with the opposite direction. By doing so, the closest distance can be shortened. Since conditional expression (9) is 〓B _f <0,
This shows increasing the distance between the RFC and the objective lens. By doing this, you can actively
You can increase the imaging magnification by using RFC.
If the upper limit of conditional expression (10) is exceeded, the astigmatism becomes excessively negative, which is inappropriate. If the lower limit is exceeded and 〓 ₂ becomes negative, since 〓B _f <0, the distance between the front and rear groups increases, making it difficult to correct the excessively negative spherical aberration. FIG. 20 is a lens configuration diagram of a ninth embodiment according to the present invention. In this embodiment, the front group G1 of the RFC is replaced by the rear group _G1 .
By moving toward the image side at a faster speed than G ₂ and at the same time moving the objective lens L ₀ toward the object side, short-range focusing is achieved. FIG. 21 is a lens configuration diagram of the 10th embodiment,
Similar to the ninth embodiment shown in FIG. 20, the front group _G1 and the rear group _G2 of the RFC move toward the image side at different speeds, and the objective lens _L0 moves toward the object side. Close-range focusing is performed. FIG. 22 is a lens configuration diagram of the 11th embodiment,
In this example, the objective lens is used for short-distance focusing.
L ₀ is fixed with respect to the image plane, and the front group G ₁ of the RFC moves toward the image side at a faster speed than the rear group G ₂ . Table 10 shows the specifications of the ninth, tenth, and eleventh embodiments.
Shown in 11 and 12. Note that these embodiments are based on the RFCs of the first, fourth, and eighth embodiments described above, respectively.

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[Table] Various aberration diagrams in the closest distance state according to the ninth, tenth, and eleventh embodiments are shown in FIGS. 23 and 24, respectively.
It is shown in FIG. Various aberration diagrams of these examples when focused at infinity are shown in Fig. 10A, Fig. 13A, and Fig. 17.
Since they are the same as in Figure A, they have been omitted. From each aberration diagram, it is clear that each example of the RFC according to the present invention maintains excellent imaging performance not only when focusing at infinity but also when focusing at short distance. Furthermore, in the ninth and tenth embodiments, by moving the objective lens auxiliary to the object side, practically sufficient imaging performance is maintained even at a high magnification of M=-0.15. I understand. Each example can be attached not only to the reference objective lens shown in Table 1 but also to various objective lenses, and while maintaining the same excellent imaging performance,
It is possible to easily focus from infinity to a predetermined short distance. In the ninth to tenth embodiments described above, focusing is performed by relatively moving the convergent front group and the diverging rear group of the present invention, and by using such a focusing method, the spherical Although it is possible to correct short-range fluctuations in both aberrations and astigmatism, for example, if only the positive lens located closest to the image side in the RFC is moved at different speeds, the same applicant as the present application As in the embodiment disclosed in the earlier application (Japanese Patent Application No. 57-67061), it is possible to mainly correct astigmatism without significantly changing spherical aberration. Table 13 below shows the corresponding values for the main conditions according to the present invention for each of the above examples.

[Table] (Effects) As described above, the RFC according to the present invention can be universally attached to any objective lens, and although it is compact, it has excellent performance from infinity to short distances. And if you combine it with an autofocus device,
Since focusing is possible for any objective lens by moving the RFC, the focusing mechanism is common and there is no need to replace the focusing mechanism even if the objective lens is replaced, which is extremely convenient.

[Brief explanation of drawings]

FIG. 1 is a sectional view showing a schematic configuration of a rear focus conversion lens according to the present invention attached between an objective lens and a single-lens reflex camera body, and FIG. 2 is a lens configuration of a first embodiment according to the present invention. The figure shows the positional relationship between the reference objective lens and the rear focus convergence lens in the infinity focused state.
The lens configuration diagram of the eighth embodiment, and FIGS. 10A and B to FIG. 17A and B are various aberration diagrams of the first to eighth embodiments, respectively, and A in each figure shows the infinity focus state, Each figure B shows the state of focusing at the closest distance, and Fig. 18 shows the state of focusing on an object at infinity with another embodiment of the RFC according to the present invention attached between the objective lens and the camera body. Fig. 19 is a schematic illustration of the composition of the synthesis system when focused on a finite distance object, and Fig. 20 is an illustration of the infinity focus state with the RFC of the ninth embodiment attached to the reference objective lens. Lens configuration diagram, Figure 21 is a lens configuration diagram in the infinity focused state with the RFC of the 10th embodiment attached to the reference objective lens,
Figure 22 is a lens configuration diagram in the infinity focused state with the RFC of the 11th embodiment attached to the reference objective lens, and the second
3 to 25 are diagrams of various aberrations in the ninth, tenth, and eleventh embodiments in the closest focus state, respectively. (Explanation of symbols of main parts), L ₀ ... objective lens, RFC ... rear focus convergence lens, G ₁ ... front group, G ₂ ... rear group.

Claims (1)

[Scope of Claims] 1. A rear conversion lens installed between an objective lens and a camera body for enlarging the focal length of a composite system with the objective lens compared to the focal length of the objective lens, It has a front group with positive refractive power and a rear group with negative refractive power that are movable on the optical axis relative to the objective lens and the camera body, and by moving both groups, the range is from infinity to a predetermined short distance. The front group with positive refractive power includes a negative meniscus lens placed closest to the object side and having a convex surface facing the object side, and a positive lens placed on the image side of the negative meniscus lens. Then, the magnification of the focal length in the infinity focus state is 〓, and the composite back focus when focusing from infinity to a predetermined short distance.
The amount of change in Bf is 〓Bf, the focal length of the rear conversion lens is f _R , the distance from the vertex of the lens surface closest to the object side of the rear conversion lens to the image point of the objective lens is d ₀ , the front group and the rear group are When the focal lengths of the groups are f ₁ and f ₂ , respectively, 1.2＜〓＜2.5...(1) 0＜〓〓Bf／f _R 〓＜0.2……(2) 0.4＜〓Bf／d ₀・〓 〓＜0.9 ……(3) −2.0＜f ₂ ／f ₁ ＜−0.31 ……(4) 0.6＜〓f ₁ ／f _R 〓＜1.8 ……(5) 0.3＜〓f ₂ ／f _R 〓＜ 0.6 ……(6) 0.67＜〓・d ₀ /f _R ＜0.88 ……(7) A rear focus convergence lens characterized by satisfying the following conditions.