JPS6145207B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention makes it possible to change the focal length of the entire lens system by using a rear converter lens that is detachably provided on the optical path between the main lens and the image plane so that its optical axis coincides with the main lens. The present invention relates to a lens system that can be used, particularly a lens system for photographing used in a camera or the like. It is generally known that a converter lens is attached to the object world side or the image field side of a photographic lens to expand or reduce the focal length of the photographic lens while keeping the position of the focal plane constant. . In this case, in the front converter lens system, which is installed on the object side of the photographic lens, the size of the converter lens, especially the diameter of the front lens, becomes large, making it impossible to reduce its size. On the other hand, a rear converter lens system in which a converter lens is placed on the image field side of a photographic lens has the possibility of making the converter lens more compact than the front converter system. Rear converter lenses known to date are used in so-called interchangeable lens cameras, such as JP-A-54-63752,
No. 25820, JP-A-54-834, and JP-A-54-
No. 53528 is known. Additionally, Japanese Patent Laid-Open No. 54-97423 is known as a technology that aims to downsize the entire system by incorporating a rear converter lens into the camera body. However, the rear converter lens for the above-mentioned interchangeable lens camera uses five to seven lenses, which poses problems in terms of size and cost. Furthermore, even when the converter is built into the camera body, six lenses are used as converter lenses, and the size of the converter lenses is comparable to that of the main lens, so it is difficult to say that the structure is compact. The present invention aims to improve the above-mentioned drawbacks in a lens system that incorporates a rear converter lens, and provides a lens system that has a rear converter lens that has good performance despite being compact and low in cost. It provides: A further object of the present invention is to provide a lens system having a rear converter lens having a shape that is easy to incorporate. In the lens system according to the present invention, a rear converter lens that fully satisfies performance can be achieved with a configuration of only two lenses. In the lens system according to the present invention, the configuration of the rear converter lens is such that the first positive lens having positive power and the second negative lens having negative power are arranged in this order from the object world side. This is a rear converter lens with excellent performance despite the number of lenses. Furthermore, in the lens system according to the present invention, the image field side surface of the first positive lens is a surface with a convexity facing the image field side, and the object world side surface of the second negative lens is a surface facing the object field side. By making the surface concave to , it is possible to effectively correct various aberrations that occur on these two surfaces. Further, in the lens system according to the present invention, by setting the Abbe number of the first positive lens to a predetermined value, chromatic aberration is corrected, and by setting the refractive index to a predetermined value, the Petzvaar sum can be improved. This is a correction. By adopting such a configuration, the diameter of the rear lens of the rear converter lens can be reduced as the converter lens becomes thinner as the rear converter lens becomes smaller. In the examples below, the total thickness of the rear converter lens is approximately 1/4 of the total thickness of the main lens, and the rear lens diameter of the rear converter lens is smaller than the front lens diameter of the main lens. This makes it extremely compact compared to conventional lens systems. The present invention will be explained in detail below. A converter lens is installed in the optical path of the main lens system and the image plane so that the optical axes are aligned, and when changing the focal length of the lens system while keeping the position of the image plane constant, the main lens system Since the converter lens itself corrects aberrations, the converter lens itself must effectively suppress aberrations. In the rear converter lens according to the present invention, a positive first lens and a negative second lens are arranged in this order from the object side, and the converter lens has a negative value as a whole. The first positive lens is preferably a meniscus lens with a convex surface facing the image field side, and the second negative lens is also preferably a meniscus lens with a convex surface facing the image field side. The radius of curvature of the surface on the object side of the first positive lens is much larger than the radius of curvature of the surface on the image field side, and the radius of curvature of the surface on the image field side of the second negative lens is on the object side. It is much larger than the radius of curvature of the surface. Therefore, various aberrations occur mainly from the image field side surface of the first positive lens and the object world side surface of the second negative lens. The surface of the first positive lens on the object side and the surface of the second negative lens on the image field side have small curvatures, and therefore have a shape close to a flat surface. First, among the basic aberrations, spherical aberration exhibiting axial characteristics occurs largely on the image field side surface of the first positive lens and the object field side surface of the second negative lens. However, the aberrations generated by these surfaces have opposite signs and cancel each other out, and if combined, the aberrations can be corrected to a very small value as a whole. Furthermore, regarding off-axis aberrations such as astigmatism, Petzvaar's sum, coma aberration, and distortion, the first
Large aberrations occur on the image field side surface of the positive lens and the object field side surface of the second negative lens, but similar to axial aberrations, the amounts generated on each surface cancel each other out. Since they have opposite signs, they cancel each other out well. In addition, a small amount of off-axis aberration occurs from the object-world side surface of the first positive lens and the image-field side surface of the second negative lens, but these aberrations also occur in the first lens.
Since the aberration of the positive lens and the aberration of the second negative lens have different signs, they cancel each other out, and the aberration of the entire system is reduced to a small value. In particular, the small curvature of the object-field side surface of the first positive lens and the image-field side surface of the second negative lens suppresses the amount of off-axis aberrations, typically astigmatism, and effectively corrects aberrations. will be carried out. Further, the chromatic aberration generated in the first positive lens cannot be removed from the chromatic aberration caused by the second negative lens if the dispersion value of the first positive lens is a normal value. For this reason, in the present application, the Abbe number νd of the first positive lens is made small, that is, preferably νd<40, in order to improve the chromatic aberration. Furthermore, since the rear converter lens as a whole has a negative value, the Petzvaar sum also tends to take a negative value. That is, the power of the first positive lens is φ ₁ ,
When the refractive index is n ₁ , the power of the second negative lens is φ ₂ , and the refractive index is n ₂ , the Petzvaer sum P is expressed as P=φ ₁ /n ₁ +φ ₂ /n ₂ . However, |φ ₁ |<|φ ₂ |
Therefore, if the values of n ₁ and n ₂ are almost equal, P<0
becomes. Therefore, in order to improve the image surface characteristics, n ₁
It is necessary to make small and n ₂ large. In the present application, by setting the refractive index of n ₁ to be smaller than 1.6, the Petzval sum is made small and the image plane characteristics are kept good. Examples of the lens system according to the present invention will be shown below. The main lens systems of these two embodiments are common, and only the rear converter lens is different. In Examples 1 and 2, the first to seventh surfaces of the surface numbers represent the main lens, the eighth surface represents the diaphragm, and the ninth to 12th surfaces represent the rear converter lens. Further, Ri is the radius of curvature of the i-th surface, di is the axial wall thickness or axial air gap between the i-th surface and the i+1-th surface, νd is the Abbe number, and N is the refractive index for the d-line. First example

[Table] FIG. 1 is a lens sectional view showing only the main lens of the lens system shown in the first embodiment. Figure 2 is the first
Various aberrations of the lens shown in the figure (spherical aberration, sine condition,
FIG. 4 is a diagram showing astigmatism and distortion. Fig. 3 is a diagram when the rear converter lens of the first embodiment is inserted between the main lens shown in Fig. 1 and the image plane, and Fig. 4 shows various aberrations of the lens system shown in Fig. 3. It is a diagram. In the lens system shown in FIG. 3, when inserting the rear converter lens, the main lens is moved a predetermined distance toward the object world in order to keep the image plane position constant. The focal length with only the main lens system is 100, but when the rear converter lens is inserted, the focal length is 151.4. That is, the magnification by the rear converter lens is 1.514. Next, various third-order aberration coefficients at each lens surface of the first example will be shown.

【table】

【table】

[Table] The main lens shown in the second embodiment is the same as the main lens used in the first embodiment, and the lens cross section when the rear converter lens shown in the second embodiment is inserted is shown in Fig. 5. Fig. 6 shows various aberration diagrams. The focal length of the lens system shown in FIG. 5 is 151.4, so the magnification by the rear converter lens is 1.514.

[Brief explanation of the drawing]

FIG. 1 is a lens cross-sectional view showing only the main lens in an embodiment of the lens system according to the present invention, FIG. 2 is a view showing various aberrations of the main lens shown in FIG. 1, and FIG. Cross-sectional view of the entire lens system when the rear converter lens is inserted into the main lens shown in the figure, 4th
The figure shows various aberrations of the lens system shown in Fig. 3, Fig. 5 is a cross-sectional view of the entire lens system when another rear converter lens is inserted into the main lens shown in Fig. 1, and Fig. 6 is a diagram showing various aberrations of the lens system shown in Fig. 3. 6 is a diagram showing various aberrations of the lens system shown in FIG. 5. FIG. Ri...the radius of curvature of the i-th surface, di...the i-th surface and the i-th
On-axis wall thickness or on-axis air gap between +1 faces, Sagi...
...Sagittal section, Meri...Meridional section.

Claims (1)

[Claims] 1. A main lens and a rear converter lens removably disposed on an optical path between the main lens and an image plane, and by moving the rear converter lens in and out of the optical path. , when changing the focal length of the entire lens system, the rear converter lens is arranged in order from the object world side to the first positive lens whose lens surface on the image field side faces the convex surface toward the image field side; Consisting of two independent lenses, a second negative lens with a concave surface facing the field side, and where the refractive index and Atsube number of the glass of the first positive lens are N and νd, respectively, νd<40 N<1.6. A lens system equipped with a rear converter lens characterized by being configured to satisfy the following.