JPS6119006B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a variable power lens system that intermittently changes the focal length. In recent years, 35mm cameras that use external viewfinders have become extremely compact. However, most lens systems are single lenses, and although there are some that are equipped with zoom lenses, they are no longer compact and are now as large as a TTL single-lens reflex camera. Furthermore, it will continue to be extremely difficult to make the zoom lens system for the external viewfinder 35 mm camera compact using conventional zoom lens systems. Furthermore, for 35 mm cameras, a means has been proposed for changing the focal length intermittently to keep the lens system compact. In US Pat. No. 3,388,650, an auxiliary lens is inserted between the main lens system and the image plane, and the main lens system is moved as a unit to change the magnification. An object of the present invention is to provide a lens system in which the focal length can be changed intermittently at two or more points using a new method different from the conventional method. A further object of the present invention is to provide a lens system that has a compact configuration, has an excellent variable magnification effect, and is well-corrected for aberrations. In the lens system according to the present invention, the main lens system itself is divided into a plurality of lens groups, a part of the divided lens groups is replaced with a lens group having a different refractive index, and along with this replacement, at least The above objective is achieved by moving one other lens group on the optical axis. In the lens system according to the present invention, which will be described later, a first lens group having a converging action, a second lens group having a diverging action, and a third lens group having a converging action are sequentially arranged from the object world side, and the second lens group has a converging action. By replacing the lens group with a lens group having a different negative refractive power and simultaneously moving the first lens group on the optical axis, magnification can be changed while keeping the position of the image plane constant.
With such a lens configuration, the lens system to be replaced can have almost the minimum necessary outer diameter, making it possible to make the lens system extremely compact. below,
The invention will now be described in detail. The power (reciprocal of the focal length) of each lens group of the positive first lens group, negative second lens group, and positive third lens group arranged in order from the object world side is φ ₁ and φ, respectively.
₂ , φ ₃ , the distance between the principal points of the first lens group and the second lens group is e ₁ , and the distance between the principal points of the second lens group and the third lens group is e ₂ . Let the power of the negative second lens group after switching be φ' ₂ , the distance between the principal points be e' ₁ and e' ₂ , and let e ₂ =e' ₂ for simplicity. Therefore, in order to make the image plane position of the lens system the same before and after switching, φ' ₂ = φ ₁ {1/1-e ₁ φ ₁ -1/1-e' ₁ φ ₁
}+ _φ2 may be determined so as to satisfy _φ'2 . At this time, the focal length of the entire system naturally changes. Here, when e ₁ < e′ ₁ , |φ ₂ | < |φ′ ₂
| Therefore, the focal length of the entire system after switching becomes larger than before switching. That is, by moving the first positive lens group in the object side direction and switching the second negative lens group, the focal length changes from the short focal length side to the long focal length side. This is because the overall length of the lens is short on the short focal length side and long on the long focal length side, which is extremely advantageous for compactness, particularly for reducing the diameter of the front lens, and for easily correcting aberrations. Next, if an aperture diaphragm is placed just before or after the switching lens group, the effective diameter of the switching lens group is almost determined only by the F-number beam, so this lens group has a small outer diameter, and the switching lens on the camera side It requires less storage space and helps make the camera body more compact. Furthermore, the aperture diaphragm position can be kept unchanged by switching lenses, and the aperture diameter can also be made virtually the same, which simplifies the diaphragm mechanism or shutter mechanism (when the diaphragm position and shutter position are the same).
It becomes possible to make it more compact. Next, in the first positive lens group, the positions where the light flux passes are quite different between the short focal length side and the long focal length side, so
It is necessary to remove aberrations to some extent only with this group, and it is necessary to consist of at least positive and negative lenses. Further, since it is necessary for the third positive lens group to shorten the back focus and make the lens system compact, it is preferable to use a combination of a positive lens group and a negative lens group from the object side. Here, as for the compactness of the lens system, the first lens surface, which is the lens surface closest to the object world on the short focal length side,
The distance from the surface to the image plane needs to be smaller than at least 1.7 times the overall focal length on the short focal length side, and in this embodiment it is 1.17 to 1.23, which makes it extremely compact. In the first to third embodiments of the present invention shown below, the focal length f = 41, the angle of view 2ω = 55.6°, and the focal length f =
58.5, a two-point variable magnification lens with an angle of view of 2ω = 40.6°, and an F number of 3.5. Also, the fourth
In this embodiment, the lens is a variable magnification lens that can be switched at two points: focal length f = 40.4 and angle of view 2ω = 43.2°, and focal length f = 58.6 and angle of view 2ω = 30.6°, and the F number is 3.5. In each embodiment, Ri is the radius of curvature of the i-th surface, D _i is the axial wall thickness or axial air gap between the i-th surface and the i+1-th surface, N is the refractive index, and ν _d is the Atsube number. It is. Furthermore, L(W) is a value obtained by dividing the distance from the RI surface, which is the lens surface closest to the object world on the short focal length side, to the image plane by the focal length of the entire system on the short focal length side. A sectional view of the lens of the first embodiment is shown in Fig. 1, its various aberrations (spherical aberration, sine condition, astigmatism, distortion) are shown in Fig. 2, and a sectional view of the lens of the second embodiment is shown in Fig. 3. The various aberration diagrams are shown in FIG. 4, the lens cross-sectional view of the third embodiment is shown in FIG. 5, and the various aberration diagrams are shown in FIG.
FIG. 7 shows a cross-sectional view of the lens of the fourth embodiment, and FIG. 8 shows its various aberration diagrams. In each figure from FIG. 1 to FIG. 8, A shows a state at a short focal length, and B shows a state at a long focal length. Since the operations of the lens systems shown in the first to fourth embodiments are the same, the first embodiment will be used for explanation. As shown in FIGS. 1A and 1B, the lens system includes, from the object world side, a first lens group 1 having a positive focal length, a second lens group 2-1, 2-2 having a negative focal length,
It consists of a third lens group 3 having a positive focal length.
FIG. 1A shows the lens state on the short focus side, but in order to transition from this state to the long focus lens state shown in FIG. 1B, the negative second lens group 2-1 must be
It is replaced with another negative lens group 2-2 having a different refractive power, and the first lens group 1 is moved toward the object world side. At this time, the third lens group remains stationary. In addition, in the figure, S represents an aperture. Lens data of the first to fourth examples are shown below.

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【table】 [Brief explanation of the drawing]

FIG. 1A is a diagram showing a lens cross section on the short focus side of the lens system according to the present invention, FIG. 1B is a diagram showing a lens cross section on the long focal length side, and FIG. 2A is a diagram similar to FIG. 1A.
Various aberration diagrams of the lens system shown in Figure 2B is the same as Figure 1B.
3A is a diagram showing various aberrations of the lens system shown in FIG. 3A is a diagram showing a lens cross section on the short focus side of another lens system according to the present invention, and FIG. 3B is a diagram showing a lens cross section on the long focal length side. , FIG. 4A is a diagram of various aberrations of the lens system shown in FIG. 3A, FIG. 4B is a diagram of various aberrations of the lens system shown in FIG. 3B, and FIG. 5A is a diagram of other lens systems according to the present invention. FIG. 5B is a diagram showing a lens cross section on the long focal length side, FIG. 6A is a diagram showing various aberrations of the lens system shown in FIG. 5A, and FIG. B is a diagram showing various aberrations of the lens system shown in FIG. FIG. 8A is a diagram showing various aberrations of the lens system shown in FIG. 7A, and FIG. 8B is a diagram showing various aberrations of the lens system shown in FIG. 7B. 1...First lens group, 2-1, 2-2...Second
Lens group, 3...Third lens group, S...Aperture, R
_i ...Radius of curvature of the i-th surface, D _i ...i-th surface and i+
On-axis wall thickness or on-axis air gap between one side.

Claims (1)

[Claims] 1. In order from the object side, a first lens group having a converging action;
The second lens group has a diverging action, and the third lens group has a converging action.
In a lens system having a structure in which lens groups are sequentially arranged, the focal length can be changed by switching the second lens group to a diverging lens group having a different refractive power while the first lens group moves on the optical axis. A switching type variable magnification lens system that is characterized by variable magnification. 2. The variable magnification lens system according to claim 1, wherein an aperture stop is arranged near the object field side or near the image field side of the second lens group for switching. 3. On the short focal length side, the distance from the surface of the lens system closest to the object side to the image plane is smaller than 1.7 times the focal length of the entire lens system on the short focal length side. variable magnification lens system.