JPH0317608A - Switching type tri-focal optical system - Google Patents

Abstract

PURPOSE:To offer the tri-focal optical system which can be switched among three different focal distance by providing one auxiliary lens system on the image side of a main lens system so that the auxiliary lens can freely be inserted and extracted and movable to the object side. CONSTITUTION:In a figure, I is the main lens system which has positive refracting power and II is the auxiliary lens system which is arranged on the image- side optical axis X of the main lens system I in a free insertion/extraction state and has negative refracting power. W is a first photography system of the short focal distance by only the main lens system I. Further, M is a 2nd photography system which has an intermediate focal distance and is formed by moving the main lens system I in the state of a 1st photography system W to the object side and inserting and arranging the auxiliary lens II on the image-side optical axis X of the main lens system I. Then, T is a 3rd photography system which has a long focal distance and is formed by decreasing the on-axis air gap between both the main lens system I and auxiliary lens system 2 in the state of the 2nd photography system M from DM to DT as shown in a figure and moved to the object side.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a switchable three-focal optical system that can be miniaturized and can be used in photographic cameras, video cameras, and the like.

(Prior technology) Over the past few years, cameras that can switch focal lengths in two stages, commonly known as bifocal compact cameras, and capable of wide-angle and telephoto shooting, have gained popularity because they are easier to use than zoom cameras. has become widely used.

However, in this case, since there are only two levels of focal length that can be switched, a focal length such as an intermediate level is often desired.

Therefore, a so-called trifocal camera was devised that can switch the focal length in three stages. An example of such a three-focus camera is an optical system that can be switched in three stages, as disclosed in Japanese Patent Laid-Open No. 62-206514.

In this optical system, a first auxiliary lens system with a positive refractive power and a second auxiliary lens system with a negative refractive power are selectively arranged on the image side of the main lens system with a positive refractive power on the optical axis of the main lens. A first photographing system consisting of a combination of the main lens system and the first auxiliary lens system, a second photographing system consisting only of the main lens system, and a second photographing system consisting of the main lens system and the first auxiliary lens system. It is possible to select and use one of the three third photographing systems that are a combination of two auxiliary lens systems.

(Problem to be Solved by the Invention) However, the conventional switching type three-focal optical system described above has three focal points.
In order to accommodate two different photographing systems, two different auxiliary lens systems are installed on the image side of the main lens system, resulting in increased costs.

Furthermore, when changing the focal length, the two auxiliary lens systems are inserted on the optical axis or withdrawn from the optical path as appropriate, which makes the mechanism of the trifocal optical system complicated.

The present invention has been made in view of the above-mentioned circumstances, and its purpose is to provide a switching type three-focal optical system that has a simple mechanism and can switch the focal length in three stages. It is in.

(Means for Solving the Problem) In order to achieve the above object, the switching type 3 according to the present invention
The focusing optical system is a switching type three-focal optical system in which an auxiliary lens system having a negative refractive power is detachably arranged on the optical axis on the image side of a main lens system having a positive refractive power, The main lens system is moved to the object side from the state of the first imaging system, and the auxiliary lens system is inserted and arranged on the optical axis of the image side of the main lens system. A second photographing system whose focal length is longer than that of the first photographing system, and an axial air gap between the main lens system and the auxiliary lens system based on the state of the second photographing system. and a third imaging system that is moved toward the object side to have a focal length longer than the focal length of the second imaging system. It is something to do.

(Function) In the switching type trifocal optical system according to the present invention configured as described above, when the auxiliary lens system is retracted out of the optical path of the main lens system, the main lens system alone 1 photographing system, the main lens system is moved from the state of the first photographing system to the object side, and the auxiliary lens system is inserted on the optical axis on the image side of the main lens system. By changing the axial air spacing of the lens and moving it toward the object side, the third imaging system has a focal length longer than that of the second imaging system. By combining one main lens system and one auxiliary lens system, three different focal length imaging systems can be created.

(Example) Hereinafter, an example of the present invention will be described in detail based on the drawings.

In FIG. 1 showing the basic form of the lens system structure of the switching type three-focal optical system of the present invention, ■ is a main lens system with positive refractive power, and ■ is on the optical axis X on the image side of the main lens system I. (W) is an auxiliary lens system with a negative refractive power that is removably installed in the main lens system I, and (W) is an auxiliary lens system inserted and arranged on the optical axis X on the image side of the main lens system I only. The second photographing system (T) has an intermediate focal length, and the main lens system I and the auxiliary lens system ■ are connected from the state of the second photographing system (M) with the axial air distance between the two lens systems as shown in the figure. , is a third photographing system with a long focal length, which is decreased from DM to DA and moved toward the object side.

Note that S is a diaphragm, which moves together with the main lens system I when switching from (W) to (T). P is the imaging plane.

Next, numerical examples will be shown.

In this example, as shown in FIG. 2, the main lens system I includes a first convex meniscus lens L, a second biconcave lens Lx, and a third
The auxiliary lens system (2) consists of a fourth biconvex lens L4, a fifth concave meniscus lens L, and a sixth concave meniscus lens L6.

In the numerical examples shown below, m is the surface number counted sequentially from the object side, R is the radius of curvature of the lens surface, D is the thickness of the lens or axial air gap, n is the refractive index of the lens with respect to the d-line, and ν is The ahe number of the lens for the d-line, r' is the focal length, Bf' is the back focus, and F8. is F number 2
ω is the angle of view. Numerical example (main lens system I) m R D n ν1
11.8B 3.5.7 1.7725
49.62 21.708 1.23 3 -28.793 1.53 1.672
7 32.24 13.408 1.08 5 28.868 2.54 1.802
35 46.76-22.059Z,, (auxiliary lens system ■) m R D n ν7
1555.83 3.41 1.57501
41.38 -10.462 0.41 9 -11.136 1.10 1.802
35 46.710 -56.045 2.0
511 -12.224 0.93 1.6
9680 55.612 -19.569 (All systems) Photographing system f'Bf' D. F. ．． 2ω
(W) 35.58 30.08 3.6
62.6'(M) 57.19 28.05*
=7.09 5.9 41.4°(T) 67.
42 41.75*=2.59 6.9 35
．． 6. FIG. 3 shows various aberration diagrams of each imaging system calculated based on the numerical values of the above numerical examples. The aberration diagram shown in Figure 3 is well corrected for a trifocal optical system. Note that the lens system comprising the main lens system I with positive refractive power and the auxiliary lens system II with negative refractive power is not limited to that shown in FIG. Tomo1
The auxiliary lens system (1) consists of at least one convex lens and one concave lens.

In addition, in the second photographing system (M), due to the arrangement position of the auxiliary lens system
After being inserted on the optical axis X on the image side, the auxiliary lens system (2) may be moved to the object side or the image side as necessary.

Note that focusing for a finite distance is performed using the main lens system I.
This is done by moving the main lens system I and the auxiliary lens system 2 along the optical axis, or by moving the main lens system I and the auxiliary lens system 2 together along the optical axis.

(Effects of the Invention) As is clear from the above description, according to the present invention, one auxiliary lens system is provided on the image side of the main lens system so that it can be inserted and removed, and both can be moved toward the object side. It has become possible to provide an inexpensive switching two-trifocal optical system that can be switched to three different focal lengths by a mechanism.

[Brief explanation of drawings]

Fig. 1 is a diagram of the basic structure of the lens system of the switching type trifocal optical system of the present invention, Fig. 2 is a sectional view of the lens system of a numerical example of the invention, and Fig. 3 is a diagram of various aberrations of the numerical example. be. (W): First imaging system (M): Second imaging system (T):
Third photographing system I: Main lens system ■: Auxiliary lens system P: Image formation X: Optical axis S: Aperture

Claims (1)

[Claims] A switchable three-focal optical system in which an auxiliary lens system with a negative refractive power is removably installed on the optical axis on the image side of a main lens system with a positive refractive power, the first lens system comprising only the main lens system. Photography system and
The main lens system is moved from the state of the first imaging system to the object side, and the auxiliary lens system is inserted and arranged on the optical axis of the main lens system on the image side, so that the focal length is longer than that of the first imaging system. A second photographing system having a long focal length, and changing the axial air distance between the two lens systems and moving the main lens system and the auxiliary lens system from the state of the second photographing system toward the object side. A switchable three-focal optical system, characterized in that one of the three third photographing systems having a focal length longer than that of the second photographing system can be selectively used.