JP2581767B2 - Variable focus lens system - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a variable focus lens system.

[Conventional technology]

As a lens system in which the focal length of the lens itself is variable, a liquid crystal lens, a lens using a crystal having an electro-optical effect (Japanese Patent Laid-Open No. 140908/1986), a lens using an elastically deformable substance, and the like are known. I have. Among these, the liquid crystal lens changes the focal length by enclosing the liquid crystal in an empty space (cell) having a lens shape, and applying an electric field to the liquid crystal to change the refractive index of the liquid crystal itself. is there. or,
In a lens using an electro-optic crystal, a crystal such as KDP or ADP whose refractive index changes by applying an electric field (or magnetic field) is formed in a lens shape, and the focal length is changed by changing the refractive index.

On the other hand, an elastically deformable lens forms a lens with transparent silicon rubber or the like, and changes the focal length by changing the radius of curvature and thickness of the lens by applying a force symmetrically to the side of the lens with respect to the optical axis. is there.

[Problems to be solved by the invention]

However, when these lenses are used to adjust the focus of an endoscope, a microscope, a camera, a television camera, or the like, since the variable range of the focal length of the lens is narrow, the focus adjustment may be performed over a wide object distance. There was a problem that it was not possible.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide a varifocal lens system capable of performing focus adjustment over a wide object distance.

[Means and actions for solving the problem]

The varifocal lens system according to the present invention includes a lens group having a positive refractive power and a lens group having a refractive power arranged on the incident side thereof, and the lens group having a positive refractive power is a variable focus lens. And all or part of the lens group having the positive refractive power or the virtual refractive power portion, and all or part of the lens group having the refractive power disposed on the incident side. The afocal system is characterized in that the absolute value of the afocal magnification is smaller than 1.

Further, the varifocal lens system according to the present invention is characterized in that a portion contributing to the formation of an afocal system in a lens group having a refractive power has a negative refractive power.

The varifocal lens system according to the present invention is characterized in that the lens group having a positive refractive power includes a stop, and the varifocal lens is arranged near the stop.

The varifocal lens system according to the present invention is characterized by satisfying the following conditional expression.

| f ₁ | <f _TOT where f ₁ is the focal length of the lens group having a refractive power, f _TOT
Is the focal length of the entire lens system.

In other words, the basic idea of the present invention is to combine a variable focal length lens with an afocal system to amplify the focus adjusting function of the variable focal length lens itself, thereby enabling focusing on a wide range of objects.

 Hereinafter, the principle of the present invention will be described with reference to FIG.

Figure 1 shows the positive lens group L ₁ on the incident side of the lens unit 1 having a refractive power are arranged, an imaging optical system for forming an object image on a predetermined imaging positions by the two lens groups . Lens group I is the variable focus lens L ₂ and a lens having a positive refractive power
L ₃ Metropolitan and consist, it is assumed that the afocal system and the side focal point coincides after the variable focus lens front focus of the lens group L ₁ of L ₂ is formed. Although the drawing on the lens group L ₁ is drawn as a negative lens, it is sufficient even have a refractive power, positive and negative does not matter.

It is considered that focusing is performed over a predetermined object distance range using this optical system. Object distance of near point (distance from the lens group L ₁ to the object) to S _N, the object distance of the far point S _F,
If the change in object distance that occurs when an object moves from the far point to the near point is Δ, and Δ is expressed in diopters, Becomes Then, the distance to the object measured from the front focal position of the lens group L ₁ far point, respectively the near point s x _F, if x _N, Becomes However, f ₁ is the focal length of the lens group L _1. Here, it is assumed that S _F is close to infinity, and | S _N | ≫ | f ₁ | Becomes

When the object approaches the near point from infinity, the moving amount x ₁ ′ of the image of the lens unit L ₁ is Becomes Now, at this time, in order to prevent change overall imaging position, if the focal length of the lens unit L ₂ when the object distance is S _F and f _2, the focal length, Must change.

Defining the d ₂ by, d ₂ is assumed that changes in the refractive power of the second lens group L ₂ in diopters units.

Next, f ₂ ′ is eliminated from equations (6) and (7), and d _{2 is changed} to f ₁ , f ₂ ,
Expressed as x _N , Becomes The left side is approximately x _N ≫f ₂ , x _N ≫f ₁ So, Is obtained. Here, if d ₀ in equation (4) is used, Is obtained. If f ₁ / f ₂ ≡A is referred to as an afocal ratio, the expression (10) means that when the diopter of the object changes by d ₀ , the viewing of the lens group L ₂ necessary for focusing is performed. The degree change may be as small as the square of the afocal ratio.

Therefore, according to the variable focus lens system of the present invention, | A | <1 be determined ...... become like the focal length of each lens (11), the diopter change in the variable focus lens L ₂ 1 / A Focus adjustment of the change of the object distance corresponding to the diopter change of ² times is possible.

Although the above description lens L ₁ and the variable focus lens L ₂ constitute the afocal system, but this is not necessary. For example, the optical system shown in FIG. ₃ has a configuration in which a positive lens L ₁ , a negative lens L ₂ , and a variable focus lens L ₃ are arranged in this order from the incident side, and there is no portion forming an afocal system. However, the negative lens L ₂ and the variable focus lens L ₃ and virtually two power portion L ₂ by a portion rays as shown in FIG. 4 are parallel to the optical axis ', L ₂ "and L _3' "given separately, the refractive power portion L _2" L ₃ discussions described above for configured afocal system by and L ₃ 'is-class as it is. Therefore, the refractive further positive lens L ₁ to The effect of the afocal system constituted by the power portion L ₂ ′ and the effect of focusing on the refractive power portion L ₃ ″ of the varifocal lens L ₃ are added to the focus adjustment capability of the optical system shown in FIG. Become.

Next, returning to FIG. 1, the total length of the variable focus lens system will be considered. The focal length of the entire lens system
_Assuming that f _TOT and the magnification of the lens unit I having a positive refractive power are β ₂ , since B ₂ <0, f _TOT = f ₁ β ₂ (12). When the focal length of the lens group L ₃ and f _3, Because Becomes

Incidentally, the processing C from the full-length or lens group L ₁ in the lens system to the imaging point, if the distant object position is infinity, the lens group L ₂
If L and L ₃ are a thin-walled close contact system, then C = f ₁ + f ₂ + f ₃ (15) or, Because It is.

Therefore, given A and f _TOT , to reduce C, it is better to make | f ₁ | smaller when A <0, | A | <1 holds. That is, it is _preferable that | f ₁ | <f _TOT (19).

Also, if the lens is placed near the variable aperture lens, the change in the angle of view may be small.

In order to reduce the total length, a lens arrangement as shown in FIG. 5 is effective. It comprises a first lens unit L ₁ having a negative power, a second lens unit L ₂ having a positive power, and a third lens unit L ₃ having a negative power. The second lens unit L ₂ has a variable focus. It includes a lens. This example
Given except third lens group L _3, it can be applied condition that shown in Figure 1. Overall, the magnification amount of only the third lens group L ₃ compared with that shown in Figure 1 can focal length long, it has a feature that the total length if the same focal length in the reverse can be shortened.

Note that almost the same effect can be obtained even out of sequence of the lens group L ₂ and L ₃ in Figure 1. Although the varifocal lens is drawn like a positive lens, even when the varifocal lens itself has a negative refractive power, as shown in FIG. given words, the refractive power is positive, since the combined focal length of the lens unit J by a change in the refractive power of the variable focus lens changes, the composite lens group J and f ₂ in the discussion described for Figure 1 With the focal length of, exactly the same result is obtained. Therefore, the sign of the refractive power of the varifocal lens is arbitrary.

〔Example〕

Hereinafter, the present invention will be described in detail based on illustrated embodiments.

FIG. 7 shows a first embodiment, which is a retrofocus type designed as an objective lens for a fiberscope. A polarizing plate 2 that passes only polarized light that vibrates in the vertical direction is disposed behind a concave lens 1 made of glass, a diaphragm 3 is disposed behind the polarizing plate 2, and a liquid crystal lens 6 is disposed near the diaphragm 3. . Liquid crystal lens 6
Covers the facing surfaces of two glass or plastic lenses 4 and 5 with an alignment film 8 and a transparent electrode 9, respectively, and places a liquid crystal 7 in a void (cell) formed by the facing surfaces. It is constituted by enclosing. An AC voltage circuit 10 is connected to the transparent electrodes 9, 9, but in the state shown in FIG. 7, since no voltage is applied, the liquid crystal 7 is oriented in the direction of a higher refractive index. This shows a state where the focus is adjusted to the near point.
Further, a cemented lens 11 and an image guide fiber bundle 12 are arranged behind the liquid crystal lens 6.

FIG. 8 shows the state of the liquid crystal lens 6 when an AC voltage is applied. At this time, the orientation of the liquid crystal 7 is almost parallel to the optical axis and the refractive index is low. Corresponds to a state where is set to the far point.

Next, when the maximum thickness of the liquid crystal 7 is discussed, the liquid crystal 7 scatters light as the thickness increases, so that the thickness of the liquid crystal 7 is preferably 0.2 mm or less, preferably 0.05 mm or less at the maximum. If the liquid crystal lens 6 is a Fresnel lens, the power of the liquid crystal lens 6 can be increased without increasing the thickness of the liquid crystal 7, but the use of the Fresnel lens is limited because the contrast of an image is reduced. When the Fresnel lens is not used, the power of the liquid crystal lens 6 is limited as follows.

As shown in FIG. 9, assuming that the width of a light beam passing through the liquid crystal lens 6 is D, if the front and rear surfaces of the liquid crystal lens 6 intersect at a circumferential portion having a diameter D, the thickness is the thinnest at the same curvature. can do. The maximum thickness d of the liquid crystal lens 6 at this time is such that the radii of curvature of the entire surface and the rear surface of the liquid crystal lens 6 are R ₁ and R ₁ , respectively.
Assuming ₂ , Then, it is given by the following equation.

d = R ₁ (1−cos θ ₁ ) −R ₂ (1−cos θ ₂ ) (2)
2) Therefore, the values of R ₁ , R ₂ , and D may be selected so that | d | <0.2 (23).

In the case where the liquid crystal lens 6 is a concave lens, as shown in FIG. 10, the front and rear surfaces of the liquid crystal lens 6 have a shape closest to the optical axis of the lens. Also in this case, equations (22) and (23) hold. When the liquid crystal lens 6 is a concave lens, the distance between the transparent electrodes 9, 9 in the outer peripheral portion is wide, and the lead wires 9 ', 9' for the electrodes can be easily attached, which is convenient.

In order to increase the transmittance of the liquid crystal lens 6, an antireflection film is provided between the transparent electrodes 9, 9 and the lenses 4, 5, as shown in FIG.
It is good to provide 13,13 respectively.

Although the spherical liquid crystal lens 6 has been described above, the maximum thickness of the aspheric liquid crystal lens is desirably 0.2 mm or more.

FIG. 12 shows a second embodiment, which is an objective lens for an electronic scope using a solid-state imaging device 15 with a mosaic filter 14. A low-pass filter 16 made of a birefringent plate is arranged on the optical axis to prevent moiré. In this case, the direction of the projected image on the plane perpendicular to the optical axis of the crystal axis of the low-pass filter 16 is Must be different from the vibration direction of polarized light that can transmit light. Otherwise, separation of light rays due to birefringence of the low-pass filter 16 does not occur, and the effect of the low-pass filter cannot be obtained. If a depolarizing plate or a circularly polarizing plate 17 (shown by a dotted line in FIG. 12) is arranged in front of the low-pass filter 16, the direction of the projected image on the plane perpendicular to the optical axis of the crystal axis of the low-pass filter 16 and the polarizing plate 2 May be the same as the vibration direction of polarized light that can transmit light.

FIG. 13 shows a modification of the liquid crystal lens 6 used for the objective lens for the electronic scope.
Optical low-pass filter 16 consisting of a parallel-plane birefringent plate on one of the substrates for making the cell that encapsulates the liquid crystal
And an infrared light cut filter 18 made of infrared absorbing glass formed in the shape of a concave lens, and a liquid crystal 7 is sealed in a cell formed by the surfaces facing each other through a transparent conductive film 9 and an alignment film 8. It was done. In this configuration, the low-pass filter 16 and the lens 18 have a birefringence function and an infrared light cutting function, respectively, and also have a sealing function of the liquid crystal lens 6, which is advantageous in terms of making the entire system compact. .

FIG. 14 shows a third embodiment, which is configured as an objective lens for an electronic scope. In this example, since the depolarizing plate or the circular polarizing plate 17 is provided in front of the polarizing plate 2, a bright image can be obtained even when the light from the object is strongly polarized. When the light is not polarized, an image similar to that obtained when the depolarizing plate or the circularly polarizing plate 17 is not provided can be obtained. The depolarizing plate or the circularly polarizing plate is made of a quartz plate or the like.

FIG. 15 shows a fourth embodiment, which is an objective lens for a fiberscope for side view. In this example, the polarized light generated by the mirror 19 behind the concave lens 1 is changed to non-polarized light by the depolarizing plate or the circularly polarizing plate 17, and the focus is adjusted by the liquid crystal lens 6 after passing through the polarizing plate 2.

FIG. 16 shows a fifth embodiment, which is an objective lens for a side-viewing electronic scope. In this example, the liquid crystal lens 6 is arranged in front of the mirror 19 in order to eliminate the influence of polarized light generated in the mirror 19.

FIG. 17 shows a sixth embodiment, which is an objective lens for an electronic scope. This example shows the front and rear lenses 4,5
And the liquid crystal lens 6 constitute a concave lens. With this configuration, the Petzval sum can be made close to 0, which is advantageous in correcting field curvature aberration.

Next, data of an example designed as an endoscope objective lens is shown.

f ₁ = -0.8123 f ₂ = 3.249 (at infinite object distance) f ₂ = 3.165 (at 7 mm object distance) f ₃ = 3.998 f _TOT = 1.0 Image height = 0.731 C = 6.435 β ₂ = -1.231 d ₂ = 8.2 [Effect of the Invention] As described above, the varifocal lens system according to the present invention has a practically important advantage that the focus can be adjusted over a wide object distance.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of one example of a variable focus lens system according to the present invention, FIGS. 2 to 6 are conceptual diagrams of other examples, FIG. 7 is a diagram showing the configuration of the first embodiment, FIG. 9 is a view showing a state of the liquid crystal lens according to the first embodiment when a voltage is applied, FIGS. 9 to 11 are views each showing another example of the liquid crystal lens, and FIG. 12 is a view showing the configuration of the second embodiment. FIG. 13 is a diagram showing a modification of the liquid crystal lens, and FIGS. 14 to 17 are diagrams showing the configurations of the third to sixth embodiments, respectively. 1 ... concave lens, 2 ... polarizing plate, 3 ... stop, 4, 5, 18 ...
... Lens, 6 ... Liquid crystal lens, 7 ... Liquid crystal, 8 ... Alignment film, 9 ... Transparent electrode, 9 '... Lead wire, 10 ... AC voltage circuit, 11 ... Joint lens, 12 ... Image Guide fiber bundle, 13 anti-reflection film, 14 mosaic filter, 15 solid-state imaging device, 16 low-pass filter,
17 ... depolarizing plate or circular polarizing plate, 19 ... mirror.

Claims (4)

(57) [Claims] 1. A lens group having a positive refractive power and a lens group having a refractive power disposed on an incident side thereof, wherein the lens group having a positive refractive power includes a variable focal length lens. An afocal system formed by all or a part of the lens group having the positive refractive power or a virtual refractive power part, and all or a part of the lens group having the refractive power disposed on the incident side. Wherein the absolute value of the afocal magnification is smaller than 1. 2. The varifocal lens system according to claim 1, wherein a portion of the lens group having the refractive power, which contributes to the formation of the afocal system, has a negative refractive power. . 3. A variable focus lens according to claim 1, wherein said lens group having a positive refractive power includes a stop, and said variable focus lens is disposed near said stop. Lens system. 4. The varifocal lens system according to claim 2, wherein the following conditional expression is satisfied. | f ₁ | <f _TOT where f ₁ is the focal length of the lens group having a refractive power, and f _TOT is the focal length of the entire lens system.