JPH0758367B2 - Optical system of focus detection device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an optical system of a focus detection device in a video device such as a camera.

(Prior art and its problems) Conventionally, an optical system is arranged behind the film equivalent surface to transmit a subject image and at the same time divide the image into two similar images and focus from the relative displacement of each image. Many devices for detecting the position have been proposed. For this optical system,
For example, Japanese Patent Laid-Open No. 59-75209 “Optical system of a focus position detecting device”, Japanese Patent Laid-Open No. 60-32012 “Camera focal position detecting device”, or Japanese Patent Laid-Open No. 62-25715 “focus”. The optical system of the detection device ”and the like. Then, in these applications, in order to correct the distortion and the illuminance distribution, it is proposed to make the condenser lens aspherical.

However, such an aspherical condenser lens has various problems.

The first problem is that it is difficult to process the aspherical surface of the condenser lens.

That is, in general, when processing an aspherical surface of a condenser lens, first, the lens material is cut into an aspherical surface with a tool such as a precision lathe to accurately shape the aspherical surface, and then the cutting scratches associated with this cutting are performed. In order to remove the above, the polishing is performed to the extent that the aspherical shape is not destroyed. However, at this time, even if the bite bit is slightly deformed,
In many cases, the aspherical shape did not appear accurately, and the aspherical surface formed correctly by cutting collapsed during polishing.
Therefore, it is very difficult to process the aspherical surface of the condenser lens.

The second problem is that if the condenser lens has an aspherical surface, the aspherical surface shape inspection and the surface accuracy inspection cannot be easily performed during mass production.

That is, the inspection of the aspherical shape can be generally performed by a coordinate measuring machine or an interferometer. However, when an aspherical shape is inspected with a three-dimensional measuring machine, there is a drawback that the measurement takes too much time because there are many measurement points. Also,
Although the aspherical surface shape and surface accuracy can be inspected with an interferometer, when the aspherical surface shape and surface accuracy of a lens with a large aspherical coefficient, that is, a lens with a large deviation from the spherical surface are inspected with an interferometer,
The number of interference fringes increases and inspection becomes difficult. If the condenser lens is formed into an aspherical surface in this way, it is difficult to inspect the aspherical surface shape and the surface accuracy during mass production.

Therefore, whether or not the condenser lens can be formed of a spherical surface becomes a very important issue.

As one of the methods for solving this problem, for example, a plano-convex lens is used as the condenser lens, the convex surface side is directed to the film equivalent surface side, and the main plane is made to substantially coincide with the film equivalent surface.

However, with this method, the field of view that can be used for distance measurement, that is, the distance measurement zone cannot be taken too long, and the condenser lens is too close to the film equivalent surface, so if dust adheres to the surface of the condenser lens, this dust will be mixed. This not only adversely affects the focusing accuracy, but also causes the inability to sufficiently correct the distortion aberration.

(Object) Therefore, an object of the present invention is to provide an optical system of a focus detection device which does not have an aspherical surface, can sufficiently lengthen a distance measuring zone to reduce the influence of dust, and can sufficiently correct distortion. To do.

(Means for Solving the Problems) In order to achieve this object, the present invention provides a condenser lens disposed behind the film equivalent surface, and a meridional surface of the condenser lens behind the condenser lens. The optical system of the focus detection device, which comprises a pair of symmetrically arranged light splitting means, detects the relative position shift of the subject image re-formed by each light splitting means, and detects the focus position. When the extension from the film equivalent surface to the rear surface of the condenser lens due to the refraction material is not set, the path length L is set in the range of 3.2 mm <L <3.6 mm, and the front surface radius of curvature of the condenser lens is set to the rear surface curvature. The optical system of the focus detection device is set to be larger than the radius.

(Examples) Examples of the present invention will be described below with reference to the drawings.

<First Embodiment> FIGS. 1 to 3 show a first embodiment of the present invention.

In FIG. 1, reference numeral 1 is a film equivalent surface, and 2 is a condenser lens disposed behind the film equivalent surface 1 with a space. An auxiliary lens 3 is arranged behind the condenser lens 2 with a space, a diaphragm mask 4 and a separator lens 5 are arranged adjacent to each other behind the auxiliary lens 3, and a sensor 6 is arranged behind the separator lens 5. Are spaced apart. For this sensor 6, a photodiode array linearly arranged on the left and right (upper and lower in the figure) is used.

The separator lens 5 is located behind the condenser lens 2 as described above, and is arranged symmetrically with respect to the meridional surface of the condenser lens 2 (symmetrical with respect to the optical axis 0 in the figure), and the convex lens portions 5a and 5a are provided as a pair of light beams. It has as a dividing means. Moreover, the convex lens portions 5a and 5b divide the image on the right and left sides of the light receiving surface of the sensor 6 to re-image it, and the sensor 6 photoelectrically converts the optical information of the re-formed image and outputs it. By inputting this output to a comparator (not shown) and making a judgment, the relative displacement of the subject image is detected.

Here, the distance between the film equivalent surface 1 and the condenser lens 2 is d _1, and the thickness of the condenser lens 2 is d ₂ .
The distance between the condenser lens 2 and the auxiliary lens 3 is d ₃ , the thickness of the auxiliary lens 3 is d ₄ , the distance between the auxiliary lens 3 and the aperture mask 4 is d _5, and the thickness of the aperture mask 4 is d _6. ,
The distance between the aperture mask 4 and the separator lens 5 is d ₇ , the thickness of the separator lens 5 is d ₈ , the radius of curvature of the film equivalent surface 1 is r _1, and the radii of curvature of the front and rear surfaces of the condenser lens 2 are R ₂ and r ₃ , respectively, and the radii of curvature of the front and rear surfaces of the auxiliary lens 3 are r ₄ and r ₅ , respectively,
Assuming that the radii of curvature of the front surface and the rear surface of the separator lens 5 are r ₆ and r ₇ , an example of these respective values will be described below.

In the optical system of the focus detecting device as shown in FIG. 1, when various parameters are changed to examine the factors that affect the distortion aberration, the factors that affect the distortion aberration are those of the condenser lens 2. In addition to the constant change, the distance between the film equivalent surface 1 and the condenser lens 2
It turns out that d ₁ also has a great influence.

Therefore, the aspherical coefficient k and the distance d ₁ on the rear surface of the condenser lens 2 are changed to correct the distortion aberration as much as possible, and the residual distortion aberration is displayed in μ when the corrected distance measuring zone is set to 2 mm. Then, these relationships are
It will be as shown in the figure. However, normalization to a percentage is not performed (the same expression will be used below).

As is clear from FIG. 2, when the distance d ₁ ≈2 mm, the aspherical surface coefficient becomes zero (k = 0), that is, the rear surface of the condenser lens 2 becomes a spherical surface, and at this time, the distortion aberration μ also becomes zero (the third surface). (See the figure).

The data of each variable of the optical system at this time are as shown in Table 1 below.

<Second Embodiment> FIG. 4 shows a second embodiment of the present invention. In this embodiment, when the distance d ₃ between the condenser lens 2 and the auxiliary lens 3 shown in FIG. 1 is smaller than the distance in the first embodiment, the distance d ₁ and the distortion are the same as in the first embodiment. It shows an example of obtaining the aberration, Also in this example, the distortion aberration was almost zero as shown in FIG.

<Third Embodiment> FIGS. 5 and 6 show a second embodiment of the present invention. The present embodiment shows an optical system in which the auxiliary lens 3 in FIG. 1 is omitted. Also in this embodiment, the residual distortion aberration at each point in the distance measuring zone is obtained as shown in the first embodiment, and is as shown in Table 3.

In Table 3, the distance between the film equivalent surface 1 and the condenser lens 2 is d _1, and the thickness of the condenser lens 2 is d ₂
And the distance between the condenser lens 2 and the aperture mask 4
d ₃ , the thickness of the diaphragm mask 4 is d ₄ , the distance between the diaphragm mask 4 and the separator lens 5 is d ₅ , the thickness of the separator lens 5 is d _6, and the radius of curvature of the film equivalent surface 1 is Where r ₁ is the radius of curvature of the front surface and the rear surface of the condenser lens 2 are r ₂ and r ₃ , respectively, and the radius of curvature of the front surface and the rear surface of the separator lens 5 is r ₄ and r ₅ , respectively. Is.

Also in this example, as shown in FIG. 6, the distortion aberration became almost zero.

<Fourth Embodiment> FIG. 7 shows a fourth embodiment of the present invention. This example shows an example in which the thickness d ₂ of the condenser lens 2 in the third example is changed. Also in this embodiment,
When the residual distortion at each point in the distance measuring zone is obtained as shown in the first embodiment, it is as shown in Table 4.

Also in this example, as shown in FIG. 7, the distortion aberration became almost zero.

Next, for the sake of simplicity, if there is no refractive optical member between the film equivalent surface 1 and the condenser lens 2 in each of the embodiments, that is, the light flux that has passed through the film equivalent surface 1 as in each embodiment. Will be considered so that it can enter the condenser lens 2 directly. The path length from the film equivalent surface 1 to the rear surface of the condenser lens 2 that does not extend due to the thickness of the condenser lens 2 is L,
If the refractive index of the condenser lens 2 is n, this path length L is It is represented by.

By substituting the data for each embodiment into this equation and determining the road length L in each embodiment, the results are shown in Table 5.

As can be seen from this result, the path length L calculated in the four examples having different optical system configurations and optical constants is 3.30.
It is noteworthy that they are concentrated in an extremely narrow range of ~ 3.47 mm. Moreover, if the value is larger than the above value, it is apparent from the prior art that it is difficult to make the condenser lens 2 spherical.

However, if the path length L does not have the above-mentioned value, it does not mean that the condenser lens 2 cannot be made spherical. That is, it was found that when the value is smaller than the above value, the condenser lens 2 can be made aspherical depending on the design.

That is, it was found that the above-mentioned value is a substantially upper limit value at which the condenser lens 2 can be made spherical. Therefore, if a value in the vicinity of this is selected, by making the thickness of the condenser lens 2 as thin as possible, there is a space that can sufficiently reduce the influence of dust and the like adhering to the surface of the condenser lens 2 and the film equivalent surface 1 and the condenser. It can be secured between the lens 2. For example, condenser lens 2
If the thickness is 2 mm, the space is about 2 mm, which is a sufficient distance to remove the influence of dust.

Therefore, from the values of the above-mentioned four average values χ and 3σ, the path length L that can make the condenser lens 2 spherical and reduces the influence of dust as much as possible is χ-3σ <L <χ + 3σ, that is, 3.2 It is considered that the optimum value is mm <L <3.6 mm.

Furthermore, what can be said in common to all the examples is that the front surface radius of curvature of the condenser lens is larger than the rear surface radius of curvature. This is also a condition necessary for making the condenser lens 2 spherical.

This is because the solution that minimizes the distortion aberration when correcting the bending of the condenser lens 2 exists only when the front surface radius of curvature is larger than the rear surface curvature radius, and does not exist in the opposite case.

In the embodiment described above, an example is shown in which no light refraction member is arranged between the film equivalent surface 1 and the condenser lens 2, but the invention is not necessarily limited to this. For example, as shown in FIGS. 8 and 9, an infrared cut filter F which is a plane-parallel plate between the film equivalent surface 1 of the optical system shown in FIGS. 1 and 5 and the condenser lens 2. Can also be placed.

In this case, the distance from the film equivalent surface 1 to the infrared cut filter F is again d ₀₁ , the thickness of the infrared cut filter F is d ₀₂ , and the infrared cut filter F
The distance from the lens to the condenser lens 2 is d ₀₃ , the thickness of the condenser lens is d _04, and the refractive indices of the infrared cut filter F and the condenser lens 2 are respectively.
Assuming that n ₀₂ and n ₀₄ , equation (1) is With the above, even if a plane-parallel plate exists between the film equivalent surface and the condenser lens, the above-mentioned contents are satisfied.

(Effects of the Invention) As described above, the present invention is arranged symmetrically with respect to the condenser lens arranged behind the film equivalent surface and the meridian surface of the condenser lens behind the condenser lens. In the optical system of the focus detection device, which comprises a pair of light splitting means, detects the relative position shift of the subject image re-formed by each light splitting means, and detects the focus position, from the film equivalent surface A configuration in which the path length L is set to a range of 3.2 mm <L <3.6 mm when there is no extension due to the refraction material to the rear surface of the condenser lens, and the front surface radius of curvature of the condenser lens is set to be larger than the rear surface radius of curvature. Since it has no aspherical surface, the distance measuring zone can be made sufficiently long to reduce the influence of dust, and the optical system can sufficiently correct distortion. .

[Brief description of drawings]

FIG. 1 is an explanatory view showing a first embodiment of the optical system of the focus detection device according to the present invention. FIG. 2 is an explanatory diagram showing a change in distortion when the distance between the film equivalent surface shown in FIG. 1 and the condenser lens and the aspherical coefficient of the condenser lens are changed. FIG. 3 is an explanatory diagram showing the distortion aberration of the first embodiment. FIG. 4 is an explanatory diagram showing a second embodiment of the optical system of the focus detection device according to the present invention. FIG. 5 is an explanatory diagram showing a third embodiment of the optical system of the focus detection device according to the present invention. FIG. 6 is an explanatory diagram showing the distortion aberration of the third embodiment. FIG. 7 is an explanatory diagram showing a fourth embodiment of the optical system of the focus detection device according to the present invention. 8 and 9 are explanatory views of the optical system showing the fifth and sixth embodiments of the present invention. 1 ... Film equivalent surface 2 ... Condenser lens 3 ... Auxiliary lens 4 ... Aperture mask 5 ... Separator lens 5a, 5a ... Light splitting means 6 ... Sensor

Claims (1)

[Claims] 1. A condenser lens arranged behind a film equivalent surface, and a pair of light splitting means arranged behind the condenser lens and symmetrical with respect to the meridional surface of the condenser lens. In the optical system of the focus detecting device for detecting the relative position shift of the object image re-formed by the light splitting means of the above, the extension from the film equivalent surface to the rear surface of the condenser lens by the refraction material is performed. The optical system of the focus detecting device is characterized in that the path length L is set in the range of 3.2 mm <L <3.6 mm in the case where there is not any, and the front surface radius of curvature of the condenser lens is set larger than the rear surface radius of curvature. .