JPH0685017B2 - Focus detection device - Google Patents

Description

The present invention relates to a camera focus detection device that receives light that has passed through a taking lens and detects the focus of the taking lens.

Conventionally, various types of focus detection devices as described above are known, and one of them is a condenser lens disposed behind a planned focal plane of a photographing lens, and a pair of imaging lenses are disposed behind the condenser lens and are symmetrical with respect to the optical axis. By arranging, the images of the planned focal planes formed by both imaging lenses are compared to detect the focus of the photographing lens. FIG. 7 shows the basic optical system of such a focus detection device.
L) is the taking lens, (L ₀ ) is the condenser lens, and (L ₁ )
And (L ₂ ) are imaging lenses, and both imaging lenses (L ₁ ) and (L ₂ ) are symmetrical with respect to the optical axis (X) of the taking lens (TL), and both imaging lenses (L ₂ ) The optical axes of L ₁ ) and (L ₂ ) are arranged parallel to the optical axis (X). (F) is the planned focal plane of the taking lens (TL), (FR) is the image plane of both imaging lenses, and the light receiving element array is arranged on the image plane (FR).

With such a configuration, when the taking lens (TL) is in focus on a single subject, its image (A) is formed on the planned focal plane (F), and further the imaging lens (L ₁ ) The first image (A ₁ ) and the second image (A ₂ ) are formed by (L ₂ ). When the taking lens (TL) is in the rear focus state, the image (B) is formed behind the in-focus image (A), and the imaging lenses (L ₁ ) (L ₂ ) The first image (B 1) and the second image (B ₂ ) are formed at positions apart from the first image (A ₁ ) and the second image (A ₂ ) at the time of focusing in the direction perpendicular to the optical axis (X). To be done. Conversely, when in the front focus state, the taking lens (T
The image (C) is formed in front of the image (A) at the time of focusing by L), and the first image (C ₁ ) and the second image (C ₂ ) thereof are both at the optical axis (X) than at the time of focusing. ) Is formed in a position close to. Therefore, by measuring the distance between the two images separated by such a focus detection device using the light receiving element array arranged on the image plane (FR), the focus of the front focus, the focus, and the rear focus can be obtained. Detection information is obtained.

Here, the role of the condenser lens (L ₀ ) is to project the pupils of the pair of imaging lenses (L ₁ ) (L ₂ ) to the exit pupil position of the taking lens (TL). If there is no condenser lens (L ₀ ), the peripheral light flux will be easily eclipsed by the taking lens (TL), and the secondary images (A ₁ ) (A ₂ ) (B ₁ ) (B ₂ ).
This is because the illuminance changes depending on the image heights of (C ₁ ) and (C ₂ ) and causes focus detection error.

However, even in such a focus detection device, a focus detection error may occur due to a difference in the light source illuminated by the subject. In particular, a focus detection error tends to occur easily when shooting under sunlight using a telephoto lens and when shooting under an artificial light source such as a fluorescent lamp or a tungsten lamp.

It is an object of the present invention to provide a focus detection device capable of suppressing the occurrence of focus detection error due to such a difference in illumination light source of a subject. Hereinafter, the present invention will be described in detail.

FIG. 8 is a graph showing a spectral sensitivity distribution of the light receiving element array, a typical daylight spectral distribution, and a spectral distribution obtained by combining them. In FIG. 8, (SS) shows the spectral sensitivity distribution of a silicon photodiode (SPD) normally used as a light receiving element, which has sufficient sensitivity in a very wide band. (SD) shows a typical daylight spectral distribution, and (KD) shows a spectral distribution obtained by combining (SS) and (SD). Therefore, the spectral sensitivity distribution under daylight of the focus detection device using the above-mentioned silicon photodiode as a light receiving element is as shown in (KD). Then, as is clear from this spectral sensitivity distribution (KD), this focus detection device has a sensitivity peak at about 590 nm with respect to daylight.

On the other hand, FIG. 9 is a graph showing the spectral distribution of the tungsten lamp and the spectral sensitivity distribution of the focus detection device under the illumination thereof. In FIG. 9, (SA) shows the spectral distribution of the tungsten lamp, and (SS) shows the spectral sensitivity distribution of the same light receiving element as in FIG. Therefore, the spectral sensitivity distribution of the focus detection device under tungsten lamp illumination is (SA) and (SS)
It becomes as shown by (KA) which is a combination of and. As is clear from FIG. 9, the spectral sensitivity distribution (KA) of the focus detection device under the illumination of the tungsten lamp has a sensitivity peak at about 680 nm. Therefore, the sensitivity peak of the focus detection device is
There is a difference of about 90 nm under daylight and under tungsten lamp illumination. In order to prevent focus detection error due to this difference,
Although it is conceivable to use an infrared cut filter having a spectral characteristic shown by (R) in FIGS. 8 and 9,
This is not preferable because the amount of light incident on the light receiving element is significantly reduced and the low brightness limit of focus detection is deteriorated.

On the other hand, the taking lens (TL) is designed to minimize chromatic aberration, but it is not completely chromatic, and the relationship between the incident light wavelength and the image point position of a typical lens is
As shown in the graph in Fig. 10, the G line (43
It is designed so that the image point positions of the 6 nm) and the C line (656 nm) are substantially the same. In Fig. 10, (KDP) is the wavelength (590 nm) of the sensitivity peak of the spectral sensitivity in the case of daylight.
, (KAP) shows the wavelength (680 nm) of the sensitivity peak in the case of tungsten lamp illumination. As is clear from FIG. 10, it is considered that the focus position detected by the focus detection device changes from 0.1 mm to 0.2 mm due to the difference in the illumination light source.

For example, as shown in FIG. 11, when the subject is illuminated by the sun (D), the reflected light from the subject passes through the taking lens (TL) and the projected focal plane ( The image is formed on F), and two 2's are formed on the light receiving element (S) through the condenser lens (L ₀ ) and the imaging lens (L ₁ ) (L ₂ ).
The next image is projected. On the other hand, when the tungsten lamp (A) is used as the illumination light source, the reflected light from the subject advances as shown by the broken line B, and the planned focal plane (F)
An image is formed after that, and two secondary images are projected on the light receiving element (S) through the condenser lens (L ₀ ) and the imaging lenses (L ₁ ) (L ₂ ). Then, compared with the image distance L in the case of the sunlight illumination, the image distance L'in the case of the tungsten lamp illumination becomes, and there is a focus detection error due to the wavelength difference of the light source by the distance D in the optical axis direction.

Next, regarding the phenomenon in which focus detection error occurs due to the difference in the refractive index of the condenser lens due to the difference in the wavelength of the light source,
It will be described with reference to the drawings. In the basic optical system shown in FIG. 7, the condenser lens (L ₀ ) has a function of photographing the pupils of the imaging lenses (L ₁ ) (L ₂ ) as the exit pupils of the photographing lens (TL). In FIG. 12, (M) is an imaging lens (L ₁ ) (L ₂ )
Is a mask that determines the pupil diameter of a pair of apertures (M ₁ )
(M ₂ ) is formed, and the centers of the openings (M ₁ ) (M ₂ ) are arranged so as to coincide with the optical axes of the imaging lenses (L ₁ ) (L ₂ ), respectively.

In FIG. 12, (MD) and (MA) show images of the mask (M) projected on the taking lens (TL) by the condenser lens (L ₀ ) respectively, and in the case of sunlight due to the wavelength difference of the illumination light source. Is (MD) and for tungsten lamps is (MA). That is, in the case of illumination of a tungsten lamp containing a lot of long-wavelength components (reddish), the refractive index of the condenser lens (L ₀ ) becomes smaller than that of sunlight, and the condenser lens (L ₀ Since the focal length of) is longer in the case of tungsten light illumination than in the case of sunlight illumination, the image (MA) is formed farther than the image (MD). The image (MA) and the image (MD) are projected closer to each other as the difference in refractive index due to the wavelength difference of the illumination light source is smaller. Therefore, it is preferable that the Abbe number ν ₁ of the condenser lens (L ₀ ) is large and the change in the refractive index with wavelength is small.

Here, as is generally clear from FIG. 11, the light having a long wavelength component tends to be back-focused due to the taking lens (TL) itself, so that the image interval tends to widen on the light receiving element array. In addition to that, in the structure of the focusing optical system as shown in FIG. 12, the image interval due to the light on the long wavelength side tends to be wider, so if the taking lens and the focusing optical system are combined, Information tends to be released.

On the other hand, considering the imaging lenses (L ₁ ) and (L ₂ ), first, FIG. 13 is an enlarged view of the portions of the imaging lens (L ₁ ) and the mask (M). Assuming that the light having a short wavelength component is A and the light having a long wavelength component is B, the refractive index of the material for the long wavelength component is smaller than the refractive index for the short wavelength component. The light beam is bent more weakly and the image interval is shortened, and the error due to the wavelength difference of the light source at the focus position including the photographing lens is corrected. Therefore, it is better that the Abbe number ν ₂ of the imaging lens is small and the difference in refractive index between the long wavelength component and the short wavelength component is large in order to correct the focus detection error due to the difference in wavelength of the light source.

Therefore, according to the present invention, the condenser lens is arranged in the vicinity of or behind the planned issue surface of the taking lens, and the image separating optical element for separating the image into two parts is arranged behind the condenser lens symmetrically with respect to the optical axis. A mask having an aperture that determines the pupil diameter of the image separation optical element is arranged in front of the separation optical element, and the two separated images are compared with each other to perform focus detection with respect to the planned focal plane of the taking lens. In the focus detection device, the optical axis of the image separation optical element and the center of the opening of the mask are displaced to give the image separation optical element a prism effect, and further, the following conditions are satisfied. .

(1) ν ₁ > ν ₂ where ν ₁ is the Abbe number of the condenser lens, ν
₂ is the Abbe number of the image separation optical element.

Hereinafter, examples of the present invention will be described in detail. FIG. 1 is a schematic diagram showing a focus detection device according to a first embodiment of the present invention. In FIG. 1, the material is selected so that the Abbe number ν ₁ of the condenser lens (L ₀ ) is larger than the Abbe number ν ₂ of the imaging lenses (L ₁ ) and (L ₂ ). It can be seen from Fig. 13 that the material of the imaging lenses (L ₁ ) and (L ₂ ) has a small Abbe number ν (a large difference in the refractive index depending on the wavelength) to detect the focus due to the wavelength difference of the light source. This is because it is effective in correcting the error, and it is better that the Abbe number ν of the condenser lens (L ₀ ) is larger than that described in FIG. As a material used for the condenser lens (L ₀ ), a crown glass such as BK7, BKS, SK16 or a material having a large Abbe number ν such as acrylic (PMMA) is applied, while a material for the imaging lens is flint glass,
A material having a small Abbe number ν such as polystyrene or polycarbonate is applied.

In FIG. 1, a light ray (dl) shows a light ray under sunlight illumination, and a light ray (al) shows a light ray under tungsten lamp illumination. Ray (dl) is condenser lens (L ₀ ).
Is greatly refracted by the imaging lens (L ₁ )
It cannot be greatly refracted by (L ₂ ). On the other hand, the light ray (al) is not greatly refracted by the condenser lens (L ₀ ), but is largely refracted by the imaging lens (L ₁ ) (L ₂ ), and both light rays (dl) (al) are light receiving elements. In (S), the positions reach each other. Therefore,
The focus detection error due to the wavelength difference of the light source can be corrected well.

In this embodiment, as shown in the enlarged view of FIG. 2, the imaging lens (L ₁ ) (L ₂ ) has its optical axis (x ₁ )
(X ₂ ) is arranged outside the center of the openings (M ₁ ) (M ₂ ) of the mask (M) by a distance H. With this configuration, the imaging lenses (L ₁ ) and (L ₂ ) have prismatic functions, and the long-wavelength ray (Al) shown in FIG. So that it passes through a position closer to the shooting lens (TL) optical axis (X) than the light beam (Dl) of
It is possible to reduce the focus detection error due to the wavelength difference of the light source by making the distance between the two images due to the light beam on the long wavelength side as small as possible. This will be described in more detail by replacing it with the prism (P) in FIG. In FIG. 3, assuming that the light ray incident on the prism (P) is (m1), the angle difference α with the light ray (m2) emitted from the prism (P) is α = n · θ. -Represented by (2). Here, n is the refractive index of the prism (P), θ
Is the wedge angle of the prism (P). From the equation (2), the larger the wedge angle θ, and the larger the prism (P)
The higher the refractive index of
The smaller the Abbe number of the imaging lenses (L ₁ ) and (L ₂ ) is, the larger the deflection angle is, and the focus detection error due to the wavelength difference can be corrected well.

In the first embodiment, since usually extends the sensitivity long wavelength side of the light receiving element, a pair of imaging lenses (L ₁₎ the optical axis of the _{(L 2) (x 1)} (x 2) a mask ( M) opening (M ₁ ) (M ₂ )
We tried to correct the phenomenon of rear focusing on the long-wavelength side by arranging the lens so that it would move away from the center of the lens, away from the optical axis (X) of the taking lens (TL). Even if there are many very short wavelength components (around 400 nm), it will become a back focus. Therefore, in the case of a special-purpose focus detection device that uses a special light source with a large amount of light on the short wavelength side, the optical axes (x ₁ ) (x ₂ ) of the imaging lenses (L ₁ ) (L ₂ ) should be reversed. By bringing the opening (M ₁ ) (M ₂ ) of the mask (M) closer to the direction of the image pickup lens optical axis (X) than the center of the opening (M ₁ ) (M ₂ ), it is possible to correct the phenomenon of back focusing on the short wavelength side. In this case, the smaller the imaging lens (L ₁ ) (L ₂ ) and the Abbe number ν ₂ are, the better the correction is. The Abbe number of the condenser lens (L ₀ ) is ν ₁ , the imaging lens (L ₁ ) (L ₂ ), The Abbe number is ν ₂ , ν ₁ > ν
_When the relationship of _{2 is established,} the focus detection error due to the wavelength is corrected well.

FIG. 4 is a sectional view showing a second embodiment which is effective in the above case. In FIG. 4, the optical axes (x ₁ ) (x ₂ ) of the pair of imaging lenses (L ₁ ) (L ₂ ) are larger than the centers of the openings (M ₁ ) (M ₂ ) of the pair of masks. It is arranged so as to be close to the optical axis (X). Therefore, assuming that (a) is one daylight ray with many long-wavelength components and (b) is one tungsten light ray with many short-wavelength components, in the conventional configuration, (b)
In this case, the optical axis (x ₁ ) (x ₂ ) of the imaging lens (L ₁ ) (L ₂ ) is detected as a rear pin, but the aperture (M ₁ ) ( Since it is arranged in the optical axis (X) direction of the taking lens (TL) with respect to the center of M ₂ ),
The prism effect is the opposite of that shown in FIG. 2, and the light of the short wavelength component is largely deflected by the imaging lenses (L ₁ ) and (L ₂ ) to correct the focus detection error due to the wavelength difference. In the direction.

FIG. 5 is a cross-sectional view showing a first reference example related to the present invention. In this reference example, the pair of imaging lenses (L ₁ ) and (L ₂ ) used in the above-mentioned examples are used. Therefore, a pair of integrated prisms (P ₁ ) (P ₂ ) is used. The prism (P1) in front of (P2) is arranged the lens (L _3), the lens (L ₃₎ have an imaging action, act to separate the image prism (P ₁₎ ( P ₂ ) has. here,
The position of the prism (P ₁ ) (P ₂ ) may be in front of the lens (L ₃ ).

FIG. 6 is a cross-sectional view showing a second reference example relating to the present invention. This reference example shows the lens (L ₃ ) used in the first reference example shown in FIG. 5 and a pair of prisms (P ₁ ) (P ₂ ) are integrated into one image separation element (DE).

Incidentally, in the above embodiment, the infrared light cut filter that blocks all infrared light was not used, but the present invention is not limited to this, as compared with one using an infrared light cut filter. However, it is effective. When an infrared light cut filter is used, by applying the present invention, the wavelength range detected by the light receiving element is not made extremely narrow as in the past, but is set wide to increase the amount of light received by the light receiving element. Can be made.

As described above in detail, according to the present invention, the condenser lens is arranged in the vicinity of or behind the planned focal plane of the taking lens, and the image separating optical element for separating the image into two in the optical axis symmetry is arranged behind the condenser lens. In addition, a mask having an aperture that determines the pupil diameter of the image separation optical element is arranged in front of the image separation optical element, and the two separated images are compared with each other, so that the planned focal plane of the taking lens is In a focus detection device that performs focus detection, the optical axis of the image separation optical element and the center of the opening of the mask are shifted to give the image separation optical element a prism effect, and ν ₁ is the Abbe number of the condenser lens, It is characterized by satisfying the condition of ν ₁ > ν ₂ when ₂ is the Abbe number of the image separation optical element. The focus detection error due to can be reduced.

[Brief description of drawings]

FIG. 1 is a sectional view showing a first embodiment of the present invention, FIG. 2 is an enlarged view of a main part thereof, FIG. 3 is a sectional view of a prism for explaining the prism action of the imaging lens, and FIG. FIG. 7 is a sectional view showing a second embodiment of the present invention, FIG. 5 is a sectional view showing a first reference example, and FIG. 6 is a sectional view showing a second reference example.
FIG. 7 is a sectional view showing a fourth embodiment of the present invention, and FIG. 7 is a sectional view showing an optical system of a focus detection device used in the present invention.
FIG. 8 is a graph showing the spectral sensitivity distribution of a light receiving element composed of a silicon photodiode under sunlight, FIG. 9 is a graph showing the spectral sensitivity distribution of the light receiving element under a tungsten lamp, and FIG. 10 is a photograph. A graph showing the difference in image point position depending on the wavelength of the lens, Fig. 11 is a sectional view showing the cause of focus detection error due to the chromatic aberration of the conventional taking lens, and Fig. 12 is focus detection due to the difference in refractive index depending on the wavelength of the condenser lens. FIG. 13 is a sectional view showing the cause of the error, and FIG. 13 is a sectional view showing the cause of the focus detection error due to the difference in the refractive index depending on the wavelength of the imaging lens. (TL): Shooting lens, (L ₀ ): Condenser lens,
(L ₁ ) (L ₂ ) (P ₁ ) (P ₂ ) (DE): Image separation optical system.

Claims (1)

[Claims] 1. A condenser lens is arranged in the vicinity of or behind a planned focal plane of a taking lens, and an image separation optical element for separating an image into two symmetrically with respect to the optical axis is arranged behind the condenser lens, and at the same time, an image separation optical system. A focus detection is performed in which a mask having an aperture that determines the pupil diameter of the image separation optical element is arranged in front of the element, and the two separated images are compared with each other, thereby performing focus detection with respect to the planned focal plane of the taking lens. In the device, the optical axis of the image separation optical element and the center of the opening of the mask are displaced to give the image separation optical element a prism effect,
Further, a focus detection device characterized by satisfying the following conditions: ν ₁ > ν ₂ where ν ₁ ; Abbe number of condenser lens ν ₂ ; Abbe number of image separation optical element.