JPH0420165B2 - - 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. In addition, the configuration of a focusing converter that enables general-purpose automatic focusing by attaching a focusing lens system between the objective lens and the camera body is disclosed, for example, in JP-A-54-28133 and JP-A-57. −74709
However, none of these proposals concern the optical system itself, and no practical optical system has been disclosed. Therefore, the present inventor developed a rear focus conversion lens that can be attached to a large number of objective lenses, maintains practical imaging performance, and has a wide focusing area.
This was previously presented in Publication No. 129411. However, with this lens, aberration fluctuations were relatively large during focusing due to the movement of the rear focus convergence lens. (Objective of the Invention) The object of the present invention is to provide a compact system that can be universally attached to various objective lenses, particularly attachable to short back focus objective lenses, and that maintains excellent imaging performance while being compact. It is an object of the present invention to provide a focusing rear conversion lens, that is, a rear focusing conversion lens with small aberration fluctuations from infinity focusing even when focusing at a finite distance. (Summary of the Invention) The rear focus conversion lens according to the present invention is installed between an objective lens and a camera body, and is designed to expand the focal length of a composite system with the objective lens to be larger than the focal length of the objective lens. A rear conversion lens having a front group with a negative refractive power and a rear group with a positive refractive power that can move on the optical axis relative to the objective lens and the camera body, the objective lens It is possible to focus on objects from infinity to a predetermined short distance by moving the two groups at different speeds while maintaining the front group at a predetermined position with respect to the image plane, and the front group having negative refractive power It has a negative meniscus lens placed closest to the object side with a convex surface facing the object side, and a positive lens placed on the image side of the negative meniscus lens, and the rear group with positive refractive power has at least one lens. It has a positive lens. Then, β is the magnification of the focal length of the objective lens in the infinity focused state, ΔBf is the amount of change in the composite back focus Bf when focusing from infinity to a predetermined short distance, and the focal length of the rear conversion lens is f. _R , the distance from the vertex of the most object-side lens surface of the rear conversion lens to the image point of the objective lens is d ₀ , and the focal lengths of the front group and rear group are f ₁ and f ₂ , respectively, as follows: It satisfies the conditions. 0.2＜｜f ₁ /f ₂ ｜＜0.5 …(1) 0.3＜｜f _R /f ₂ ｜＜1.6 …(2) 0.6＜｜f _R /f ₁ ｜＜1.8 …(3) 1.3＜ β＜2.5……(4) |ΔBf/f _R ｜＜0.2……(5) 0.4＜｜Bf/d ₀ β｜＜0.9……(6) ) will be explained based on the drawings. FIG. 1 is a sectional view showing the general configuration of the RFC 30 installed between the objective lens 10 and the single-lens reflex camera body 20. Film side 2 is shown in the figure.
The marginal rays from the on-axis object point that reach 1 are shown. The single-lens reflex camera body 20 includes a swingable reflector 2
2, a focusing plate 23, a condenser lens 24, a pentagonal roof prism 25, and an eyepiece 26. 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 the swinging reflector 22, the distance between the lens mount surface 28 and the film surface 21 of the single-lens reflex camera body 20, the so-called flange back MB, is determined by a distance unique to the camera body. determined by the value. 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. In this way, the RFC must satisfy the conditions as a rear conversion lens, and at the same time, it must also 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. There is also a lower limit to the magnification ratio because it is desirable to ensure sufficient back focus and to not increase the amount of movement of the RFC too much. Furthermore, since RFC performs focusing by moving in a limited space between the objective lens and the camera body, it is also limited in this respect. That is, when focusing at the closest distance, the RFC moves on the optical axis toward the image side. At this time, in order to establish a single-lens reflex lens system, a sufficient back focus length is required, so 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 between the RFC and the image point of the objective lens, that is, the RFC object distance, cannot be made very long, and there is a limit to how unevenly distributed the RFC lens arrangement 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. Another method is to shorten the RFC lens length (the length from the frontmost surface of the RFC to the last surface) in order to widen the focusable area by RFC as much as possible, but this method also ensures back focus. However, there are limits to correcting aberrations. In other words, it is difficult to ensure a degree of freedom in correcting aberrations by creating a sufficient air gap within the RFC. In the present invention, RFC from infinity to finite distance
In order to widen the focusing range as much as possible when focusing by moving the RFC and to reduce aberration fluctuations at this time, as shown in Figure 2, the RFC is divided into a front group G1 with a negative refractive power and a front group _G1 with a positive refractive power, respectively. Posterior group with
The RFC is divided into two groups of _G2 , and both groups of RFC move at different speeds on the optical axis toward the camera body to focus on the objective lens and camera body. 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 When focusing on a finite distance object, the distance D 1 between the objective lens L ₀ and the front group G ₁ of the RFC changes by ΔD ₁ , from D ₁ to _{D 1 +} ΔD ₁ ,
The distance D ₂ between RFC front group G ₁ and RFC rear group G ₂ is ΔD ₂
Suppose that the back focus Bf of the synthesis system becomes Bf + ΔBf by changing from D ₂ to D ₂ + ΔD ₂ .
In the present invention, since there is no change in the total length, ΔD ₁ +
It is expressed as ΔD ₂ +ΔBf=O. The movement form of each group can be expressed by coefficient values α ₁ and α ₂ obtained by dividing the amount of change in each interval by the amount of change ΔBf in the synthetic back focus. That is, α ₁ = ΔD ₁ /ΔBf α ₂ = ΔD ₂ /ΔBf where α ₁ and α ₂ are the objective lens L ₀ and the RFC front group G ₁
These are the rate of change of the interval change ΔD ₁ between the RFC front group G ₁ and the rear group G ₂ and the interval change ΔD ₂ between the RFC front group G 1 and the rear group G 2 with respect to the change ΔBf of the synthetic back focus. Furthermore, in the present invention, a negative meniscus lens with a convex surface facing the object side is arranged as the component closest to the objective lens in the front group _G1 having a negative refractive index, and a biconvex lens is arranged on the image side. Therefore, axial chromatic aberration can be easily corrected. Furthermore, by arranging a lens having a concave surface on the object side with an air gap between the convex surface on the image side of this biconvex lens,
This is very effective in correcting the extroverted coma aberration of the rays above the principal ray and the introverted coma of the rays below the principal ray at intermediate angles of view. By configuring the front group _G1 from such a lens arrangement, it is possible to achieve infinity focusing compared to the RFC disclosed in Japanese Patent Application Laid-Open No. 58-129411, in which a positive lens is arranged closest to the object side. Aberration fluctuations between focusing and short-distance focusing can be reduced. Furthermore, by adopting such a lens configuration and a mechanism that divides the RFC into two front and rear groups and moves the front and rear groups at different speeds toward the image side for focusing, aberration fluctuations can be further reduced. It can be kept small. The above conditional expression according to the present invention will be explained in detail below. Condition (1) defines the appropriate refractive power distribution of the rear group G ₂ with respect to the front group G ₁ of the RFC. If the magnification β of the RFC and the distance d ₀ from the frontmost lens surface of the RFC to the image point of the objective lens exceed the upper limit, it is inappropriate because the spherical aberration becomes excessively positive and difficult to correct. . If the lower limit is exceeded, the spherical aberration becomes excessively negative, the sine condition becomes extremely negative, and the Petzval sum also becomes negative, which is inappropriate. Condition (2) is the rear group G ₂ for the total refractive power of RFC.
This defines the appropriate distribution of refractive power.
As long as the RFC scaling factor β and d ₀ are within a practical range,
If the upper limit is exceeded, the spherical aberration becomes excessively positive, which is inappropriate. If the lower limit is exceeded, the spherical aberration becomes excessively negative and the distortion becomes excessively positive, which is inappropriate. Condition (3) supplements conditions (1) and (2), and provides appropriate spherical aberration. If the upper limit of equation (4) 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 movement of the RFC becomes too large when trying to focus at a predetermined close distance, and on the other hand, when focusing while maintaining back focus as a single-lens reflex camera lens, the focusable area becomes narrower. Both are inappropriate for practical use. If the condition of equation (5) 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. Also
As fR 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. It's inappropriate. If the upper limit of equation (6) 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 β
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.
The number of lenses also increases. Moreover, the RFC lens length is too long, making it inappropriate. In addition, the front group G ₁ has negative refractive power and the rear group G ₂ has positive refractive power.
For each form of movement, it is desirable that the above-mentioned coefficient values α ₁ and α ₂ satisfy the following conditions: −1.6<α ₁ <−1.0 (7) 0<α ₂ <0.6 (8). The front group mainly has a focusing function by moving within the range defined by equation (7), and contributes to increasing the imaging magnification. Further, it is desirable that the rear group moves relative to the front group within the range of equation (8). According to these conditions, it is possible to appropriately correct astigmatism in the negative direction, and therefore, it is possible to flatten the image plane in response to negative fluctuations in spherical aberration that tend to occur during close-range focusing. It is possible to maintain a good balance of 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. , 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 following conditions: 0.6<β·d ₀ /fR<1.0 0.4<d ₀ /fR<0.7. (Example) An example of the RFC according to the present invention will be described 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. Regarding the specific lens configuration of each example, the front group G ₁ with negative refractive power is made up of, in order from the object side, a negative meniscus lens L ₁ with its convex surface facing the object side, and a biconvex lens L ₂
These two lenses may be separated or cemented, and furthermore, two biconcave lenses L ₃ and L ₅ are provided on the image side to share the divergent refractive power as a front group. A biconvex lens _L4 is placed between these. These three lenses in the front group may be bonded to each other or may be separated. Further, instead of these three lenses, it is also possible to configure the image side component in the front group by three lenses arranged in positive, negative, and positive order. Rear group with positive refractive power
It is easiest to configure G ₂ with a single positive lens L ₆ with the surface with a stronger curvature facing the object side, but it is possible to add additional lenses to make aberration correction advantageous. The first RFC according to the present invention is shown in Tables 2 to 5 below.
~Specifications of the fourth embodiment are shown. In addition, in each table,
R and r are the radius of curvature of each lens surface, d is the center thickness and air spacing of each lens, n is the refractive index of each lens,
Let ν represent the Atsube number of each lens, and the subscript number represent the order from the object side. However, Table 1~
In Table 4, d ₀ represents the distance between the frontmost lens surface of the RFC and the image point formed by 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 air distance between the objective lens and the RFC. distance, f ₁ is the RFC front group
It is assumed that the focal length of G ₁ and f ₂ represent the focal length of the RFC rear group G ₂ . Also, Bf represents the back focus of the composite system of RFC and reference objective lens,
ΔBf represents the amount of change in back focus between focusing at infinity and close range using RFC, F is the combined focal length of RFC and objective lens,
M represents the imaging magnification of the composite system.

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[Table] Lens configuration diagrams of the above-mentioned first to fourth embodiments are shown in FIGS. 3 to 6, respectively. The lens configuration diagram of the first example shown in FIG. 3 also shows the lens configuration of the reference objective lens L ₀ in Table 1. Table 1 shows the RFCs of the first to fourth embodiments above.
Various aberration diagrams when attached to the reference objective lens shown in FIG. 7A and B to FIG. 10A and 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. In each aberration diagram, spherical aberration (Sph),
Astigmatism (Ast), distortion aberration (Dis), reference wavelength d
line (λ=587.6nm) versus g-line (λ=435.8n
m) lateral chromatic aberration (Lat.Chr) and coma aberration (Coma). From each aberration diagram, it is clear that the RFC according to the present invention maintains a sufficiently good imaging performance for practical use from infinity to short distances. Each example can be attached not only to the reference objective lens shown in Table 1, but also to various other objective lenses, and can be used from infinity to a predetermined short distance while maintaining the same excellent imaging performance. can be focused. (Effects of the Invention) As described above, in the present invention, the lens is mounted between the objective lens and the camera body, and is moved relative to the objective lens and the camera body (image plane) to achieve a predetermined finite distance from infinity. The RFC is divided into two groups: a front group with negative refractive power and a rear group with positive refractive power. It is possible to reduce aberration fluctuations and improve imaging performance. Note that the RFC according to the present invention has negative refractive power in the entire system and can focus up to a specific magnification by moving the front group and the rear group together at different speeds toward the image side. With the objective lens fixedly arranged at an arbitrary finite distance shooting state as a new starting point,
It goes without saying that RFC allows focusing on objects at a closer distance.

[Brief explanation of drawings]

Figure 1 shows a rear focus conversion lens between the objective lens and the SLR camera body.
A sectional view showing the schematic configuration with the RFC installed,
Figure 2 is a schematic diagram of the composition system when the RFC according to the present invention is attached between the objective lens and the camera body and focused on an object at infinity, and Figure 3 is the RFC of the first embodiment according to the present invention. Lens configuration diagram in the infinity focus state when attached to the reference objective lens, Figures 4 to 6
The figures are lens configuration diagrams of the second to fourth embodiments of the RFC according to the present invention, and Figures 7A and B to 10A and B are various aberration diagrams of the first to fourth embodiments. A is each
Showing various aberration diagrams when focusing at infinity with RFC installed,
Each figure B shows various aberration diagrams when focusing at close range with each RFC attached. (Explanation of symbols of main parts), L ₀ ... Objective lens, 20 ... Camera body, RFC ... Rear focus conversion 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 negative refractive power and a rear group with positive refractive power, which is movable on the optical axis relative to the objective lens and the camera body,
By moving the two groups at different speeds while maintaining the objective lens at a predetermined position with respect to the image plane, it is possible to focus on an object from infinity to a predetermined short distance, and the negative refractive power is The front group includes a negative meniscus lens disposed closest to the object side with a convex surface facing the object side, and a positive lens disposed on the image side of the negative meniscus lens, and the rear group has a positive refractive power of at least 1 The rear conversion lens has two positive lenses, the magnification of the focal length of the objective lens when focused at infinity is β, the amount of change in the composite back focus B+ when focusing from infinity to a predetermined short distance is ΔBf, and the rear conversion The focal length of the 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 , and the focal lengths of the front and rear groups are f ₁ and f , respectively. A rear focus conversion lens characterized by satisfying the following conditions when set to ₂ . 0.2＜｜f ₁ /f ₂ ｜＜0.5 …(1) 0.3＜｜f _R /f ₂ ｜＜1.6 …(2) 0.6＜｜f _R /f ₁ ｜＜1.8 …(3) 1.3＜ β＜2.5 ……(4) ｜ΔBf／f _R ｜＜0.2 ……(5) 0.4＜｜Bf／d ₀ β｜＜0.9 ……(6)