JPS6146808B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a lens system that has the function of changing the focal length of the entire lens system by means of an auxiliary lens that is detachably provided on the optical path between a photographic lens serving as a master lens and an image plane. Conventional methods for changing the focal length of a fixed focal length lens include a front converter method in which an auxiliary lens is placed on the object side of the master lens, a rear converter method in which an auxiliary lens is placed between the master lens and the image plane, and a zoom lens. A built-in extender system is known in which an auxiliary lens is provided in a relay lens section on the image field side of the zoom section. Among these methods, the front converter method requires a large front lens diameter and is also limited in that the auxiliary lens must be an afocal lens, making it difficult to ensure its optical performance. In addition, although the auxiliary lens is small according to the rear converter method, the drawback is that when attaching the auxiliary lens, the master lens must be removed from the camera and then the auxiliary lens is installed, making it complicated to use. There is. In addition, in recent years, a rear converter method has been proposed in which an auxiliary lens is built into the camera body, but even with this method, when inserting an auxiliary lens, the master lens must be moved a predetermined distance, resulting in mechanical problems. It is complicated. Furthermore, when trying to apply a built-in extender used in a zoom lens system to a fixed focal length lens system, there are problems with the installation space and theoretical difficulties. An object of the present invention is to provide a lens system that can change the focal length with a simple mechanism. A further object of the present invention is to provide a lens system that can change focal lengths with a compact configuration without increasing the size of the device. In other words, when the auxiliary lens is inserted between the master lens and the image plane to change the focal length, the positional relationship between the master lens and the image plane remains unchanged. The focal length is changed while keeping the image focused on the surface of the photosensitive medium without moving it relative to the surface of the medium. For this reason, in the present application, a predetermined relationship, which will be described later, is maintained between the master lens and the auxiliary lens. Hereinafter, the present invention will be explained in detail using the drawings. Figures 1A and 1B are diagrams showing the principle of the lens system of the present invention. Figure 1A is a diagram with only a master lens, and Figure 1B is a diagram with an auxiliary lens inserted into the master lens. It is. In FIG. 1, 11 is a master lens, 12 is an auxiliary lens, and 13 is an image plane. Even if the auxiliary lens 12 is inserted between the master lens 11 and the image plane 13, the distance between the master lens and the image plane does not change. The conditions that the lens system has for this purpose are shown below. The power of the master lens 11 is Φ ₁ , its back focus is S′ _F , the power of the auxiliary lens 12 is Φ ₂ , its back focus is S 〓 _F , from the image field side principal point of the master lens 11 to the object world side principal point of the auxiliary lens 12 Let the distance to the point be e′ ₁ . Auxiliary lens 1
_{The power Φ〓 of the entire} lens _{system with} ′ ₁ +1)/Φ〓. By having the auxiliary lens 12,
When the focal length is multiplied by m, the condition to be satisfied is m・1/Φ ₁ =1/Φ〓 (1). Also, if the image plane position does not change even if the auxiliary lens is attached, that is, the master lens 11 and the image plane 1
3 remains unchanged and the focal length changes according to equation (1), the condition that is satisfied is S _F =e′ ₁ +△+S〓 _F ……(2). Here, Δ is the principal point interval between the object world side principal point and the image field side principal point of the auxiliary lens 12. Calculating the power Φ ₂ of the auxiliary lens system, the installation position e ₁ and the back focus _S of the entire system from equations (1) and (2), Φ ₂ = (m-1) ² /m・△ ...( 3) e' ₁ = 1/Φ ₁ + △/m-1 ...(4) S' _F = △・m/1-m ...(5) is obtained. Therefore, by placing a lens system whose principal point distance △ and power are as shown in equation (3) between the master lens and the image plane so as to satisfy equation (4), the master lens and the image plane can be It is possible to change the focal length by a factor of m while remaining stationary. Table 1 below shows numerical values for the first to fourth embodiments of the present invention. In the first and second embodiments, an auxiliary lens with negative power is provided between the master lens and the image plane to adjust the focal length.
On the contrary, in the third and fourth embodiments, an auxiliary lens having positive power is provided to reduce the focal length to 1/1.5 times.

[Table] As is clear from Table 1 and equations (3), (4), and (5), when increasing the focal length, principal point spacing △, lens spacing
The value of either e′ ₁ or back focus S〓 _F becomes negative, making it difficult to realize a lens system with a small number of lenses. On the other hand, shortening the focal length using a positive power auxiliary lens can be easily achieved even with a small number of lenses. Furthermore, the master lens system in the examples described below is a combination of a rear aperture Tetsusar and an Ernostar, but it is well known that the various aberrations of this type of medium angle of view photographic lens can be corrected to a fairly high degree. . Based on this assumption, the auxiliary lens is sequentially adjusted to positive,
The reason for configuring it with negative and positive lenses will be explained. First, regarding the spherical aberration that gives axial imaging characteristics, the third-order spherical aberration of the master lens system in the example is a positive value of approximately 0.7, but the value of the entire system does not change significantly even if an auxiliary lens is attached. It is hoped that In the present invention, the large third-order spherical aberration that occurs on the image side surfaces (the 12th and 16th surfaces in the embodiment) of the first and second positive lenses of the auxiliary lens is substantially reduced compared to the object side (13th surface) of the intermediate negative lens. It is possible to generate an equivalent negative spherical aberration so that the value remains almost unchanged as a whole system. The same applies to coma aberration. On the other hand, third-order astigmatism, which is related to image plane characteristics, takes a positive value when only the master lens is used, and it is desirable that the value is reduced by attaching an auxiliary lens. Here, negative third-order aberration coefficients are given from the object side surfaces (11th, 13th, and 15th surfaces) of the first positive lens, intermediate negative lens, and second positive lens, respectively, to obtain a good value for the entire system. It is said that Next, regarding the distortion related to the similarity between the object and the image, the third-order distortion aberration coefficient is a negative value in a system using only the master lens, and when an auxiliary lens is attached, only the absolute value is equal and the sign is reversed. This is desirable. In the example, a third-order distortion aberration coefficient of -0.48 occurs in the system with only the master lens, but these are mutually controlled at the object side of the first positive lens, the image side of the intermediate negative lens, and the object side of the first positive lens. After canceling, the remaining amount is set to a value of 0.34. In order to properly correct each aberration, a positive lens is used as an auxiliary lens.
It is better to give negative and positive configurations, and even biconvex,
By adopting the three-lens configuration, which is biconcave and biconvex, the lens system becomes optically symmetrical with respect to the aperture position when the auxiliary lens is attached, resulting in a good aberration correction state. It is best to place the aperture between the master lens and the auxiliary lens. Next, an example in which the lens system according to the present invention is specifically thickened will be described. Fig. 2 is a diagram showing a lens cross section of the lens system according to the present invention, Fig. 2A is a lens cross section when only the master lens system is used, and Fig. 2B is a lens cross section when an auxiliary lens system is inserted. FIG. The 1st to 9th surfaces constitute a master lens, the 11th to 16th surfaces constitute an auxiliary lens system, the R10 surface is an aperture, and R10 is an image plane. When the auxiliary lens system is inserted, the magnification of the entire system becomes 1/1.384, and the master lens's focal length of 138.354 is reduced to 100. At that time, the thick-walled back focus changes from 94.436 to 25.670.
The second table shown below is data corresponding to FIG. 2A, and the third table is data corresponding to FIG. 2B.

【table】

【table】

[Table] However, Ri is the radius of curvature of the i-th surface, di is the axial air gap or axial wall thickness between the i-th surface and the i+1-th surface, νd
is the dispersion, and N is the refractive index for the d-line. Further, FIG. 3 shows various aberrations of the lens system shown in FIG. 2A, namely spherical aberration, sine condition, astigmatism, and distortion. Similarly, FIG. 4 shows various aberrations of the lens system shown in FIG. 2B.

[Brief explanation of the drawing]

1A and 1B are diagrams each showing the principle of the present invention, 2A and 2B are lens sectional views each showing an embodiment of the lens system according to the present invention, and 3 is a diagram illustrating the principle of the present invention.
FIG. 4 is a diagram of various aberrations of the lens system shown in FIG. 2B, and FIG. 4 is a diagram of various aberrations of the lens system shown in FIG. 11...Master lens, 12...Auxiliary lens, 1
3... Image plane, S' _F , S〓 _F ... Back focus, e' ₁
...principal point spacing.

Claims (1)

[Scope of Claims] 1. In a lens system in which the focal length of the entire lens system is shifted by an auxiliary lens that is freely installed on the optical path between the master lens and the image plane, the master lens and the image plane _The distance between ^the ₁ = 1/Φ ₁ + △/m-1 However, Φ ₁ ; Power of the master lens e'₁; Distance from the image field side principal point of the master lens to the object world side principal point of the auxiliary lens Φ ₂ ; Power of the auxiliary lens Power △; Distance between principal points of auxiliary lens m; Ratio between the power of only the master lens and the power of the entire system when the auxiliary lens is attached.Furthermore, the auxiliary lens has positive power;
A lens system consisting of negative and positive lenses that has the ability to easily change the focal length. 2 The positive, negative, and positive lenses are, in order, biconvex, biconcave,
A lens system having a function of easily changing the focal length according to claim 1, which has a biconvex shape.