JPS61284714A - Focus detecting device - Google Patents

Abstract

PURPOSE:To decrease an error in focus detection due to the difference in the wavelength of an illumination light source for a subject by making the Abbe number of a condenser lens larger than the Abbe number of an image splitting optical element. CONSTITUTION:The material of the condenser lens L0 is so selected that the Abbe number nu of the condenser lens L0 is larger than the Abbe number nu2 or image forming lenses L1 and L2. When the materials of the image forming lenses L1 and L2 have a smaller Abbe number nu (large difference in refractive index due to wavelength), an error in focus detection due to the wavelength difference of the light source is corrected effectively and further the larger the Abbe number nu1 of the condenser lens L0 is, the better. Crown glass such as BK7 or a material with a large Abbe number nu such as acryl PMMA is applied for the condenser lens L0 and flint glass, etc., are applied as the material of the image forming lens.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a focus detection device for a camera that detects the focus of a photographic lens by receiving light that has passed through a photographic lens.

Conventionally, various focus detection devices as described above are known. One of them is a condenser lens arranged behind the expected focal plane of the photographing lens, and a pair of imaging lenses arranged symmetrically on the optical axis behind the condenser lens. Some lenses detect the focus of the photographic lens by comparing the images of the intended focal plane formed by both imaging lenses. Figure 7 shows the basic optical system of such a focus detection device.
TL) is the photographic lens, (L,) is the condenser lens, (
Ll) and (L2) are respectively imaging lenses, and both imaging lenses (L,) (Lm) are symmetrical with respect to the optical axis (X) of the photographing lens (TL), and both imaging lenses ( Ll
) (L2) are parallel to the optical axis (X), respectively.
To ni 5 Ell baby Tsume E + Y /
deceased)? ↓礪-shi 1. "+ - (TL) is the planned focal plane, (FR) is the imaging plane of both imaging lenses, and the imaging plane (
A light receiving element array is arranged on the FR).

With this configuration, when the photographing lens (TL) is in focus on a single subject, its image (A) is formed on the expected focal plane (F), and the image forming lens (L,) (L
2), a first image (A1) and a second image (A2) are respectively formed. When the photographing lens (TL) is rear focused, the image (B) is formed behind the focused image (A) and is further focused by the imaging lens (L, ) (L2). The first image (B, ) is located at a position farther away from the first image (AI) in focus in the direction perpendicular to the optical axis (X) than the 12th image (A2).
A second image (B2) is formed respectively. Conversely, when the front is in focus, the photographic lens (TL) forms an image (C) in front of the in-focus image (A), and the first image (C1
) and the second image (C2) are both formed at a position closer to the optical axis (X) than when in focus. Therefore, the distance between two images separated by such a focus detection device is called the imaging plane (FR).
By measuring using the light-receiving element array placed above, focus detection information for front focus, focus, and back focus can be obtained.

Here, the role of the condenser lens (Lo) is to connect the pupils of the pair of imaging lenses (L, ) (L2) to the photographing lens (TL).
is to project the image onto the exit pupil position. If there is no condenser lens (Lo), the peripheral light flux will be easily vignetted by the photographing lens (TL), and the secondary image (AI) (A2
)(B, )(B2)(CI)(C2) The illuminance changes depending on the image height, which causes a focus detection error.

However, even in such a focus detection device, focus detection errors may occur due to differences in the light sources illuminating the subject. In particular, focus detection errors tend to occur when photographing using a telephoto lens under sunlight or under artificial light sources such as fluorescent lamps or tungsten lamps.

SUMMARY OF THE INVENTION An object of the present invention is to provide a focus detection device that can suppress the occurrence of focus detection errors due to differences in illumination light sources for objects. The present invention will be explained in detail below.

FIG. 8 is a graph showing the spectral sensitivity distribution of the light receiving element array, the spectral distribution of typical daylight, and the spectral distribution obtained by combining them. In Fig. 8, (SS) is a silicon 7 photodiode (SPD) which is usually used as a light receiving element.
) and has sufficient sensitivity in a very wide band. (SD) shows the typical daylight spectral distribution, and (KD) shows (S S ) and (S
D) shows the combined spectral distribution. Therefore, the spectral sensitivity distribution under daylight of the focus detection device using the above-mentioned silicon 7-otodiode as a light receiving element is as shown in (KD). Then, this spectral sensitivity distribution (KD
), this focus detection device has a sensitivity peak at about 590n 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. In FIG. 9, (SA) shows the spectral distribution of the tungsten lamp, and (SS) shows the spectral sensitivity distribution of the light receiving element similar to that in FIG. Therefore, the spectral sensitivity distribution of the focus detection device under tungsten lamp illumination is expressed by (KA), which is a combination of (SA) and (SS). As is clear from FIG. 9, the spectral sensitivity distribution (KA) of the focus detection device under tungsten lamp illumination has a sensitivity peak at about 68 onI11. Therefore, the sensitivity peak of the focus detection device differs by about 90 nm between daylight and tungsten lamp illumination. In order to prevent focus detection errors due to this difference, (R) is shown in Figures 8 and 9.
Although it is conceivable to use an infrared cut filter having the spectral characteristics shown in the following, it is not preferable because the amount of light incident on the light receiving element will be drastically reduced and the low brightness limit of focus detection will be worsened.

On the other hand, although photographic lenses (TL) are designed to minimize chromatic aberration, chromatic aberration is not completely eliminated, and the relationship between the incident light wavelength and image point position of a typical lens is As shown by ■■ in the graph of FIG. 10, the image point positions are designed to be almost the same for the G line (436 nm) and the C line (656 na+). In Figure 10, (KDP) is the wavelength of the sensitivity peak of the spectral sensitivity in daylight (590 nm), and (K A P ) is the wavelength of the sensitivity peak in the case of tungsten lamp illumination (680 nm).
n+) respectively. As is clear from FIG. 10, the focal position detected by the focus detection device may change by as much as 0.2 mm from 0.1 fflI11 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 photographic lens (TL) to the planned focal plane (as shown by the solid line A). F), and two secondary images are projected onto the light receiving element (S) through the condenser lens (L, ) and the imaging lens (L, ) (L2).

On the other hand, when a tungsten lamp (A) is used as the illumination light source, the reflected light from the subject proceeds as shown by the broken line, and an image is formed after the intended focal plane (F), and the condenser lens (Lo ) and imaging lens (L, )
Two secondary images are projected onto the light receiving element (S) through (L2). And, compared to the image interval in the case of sunlight illumination, the image interval in the case of tungsten lamp illumination is L゛,
The focus detection error is due to the wavelength difference of the light source by the distance in the optical axis direction.

Next, we will discuss the phenomenon in which focus detection errors occur due to differences in the refractive index of the condenser lens due to differences in the wavelength of the light source.
This will be explained using Figure 2. In the basic optical system shown in Figure 17,
The condenser lens (Lo) is the imaging lens (Ll) (L2
) has the role of photographing the exit pupil of the photographic lens (TL). In Fig. 12, (M) is the imaging lens (
It is a mask that determines the pupil diameter of (L, HL2), and a pair of apertures (M, ) (M2) are formed, and each aperture (M
+) (M2) are arranged so that their centers coincide with the optical axes of the imaging lenses (Ll) and (L2), respectively.

In the f512 diagram, (MD) and (MA) respectively indicate the images of the mask (M) projected onto the photographing lens (TL) by the condenser lens (Lo). M D ), and (MA) in the case of a tungsten lamp. In other words, in the case of a tungsten lamp illumination that contains many long wavelength components (reddish), the refractive index of the condenser lens (LO) is the same as that in the case of sunlight. It is smaller than the refractive index of the condenser lens (
Since the focal length of Lo) is longer in the case of tungsten light illumination than in the case of sunlight illumination, the image (MA) is focused farther than the image (MD). The smaller the difference in refractive index due to the wavelength difference of the illumination light source, the smaller the difference between the image (MA) and the image (
M D ) are projected at positions close to each other. Therefore, it is better for the Abbe number ν1 of the condenser lens X'(L,) to be large and for the change in refractive index with wavelength to be small.

Here, as is clear from FIG. 11, normally, because of the photographing lens (TL) itself, light with strong long wavelength components tends to be focused at the rear, so the image spacing on the photodetector array tends to widen. In addition, in the configuration of the focusing optical system as shown in Fig. 12, the distance between images due to the longer wavelength light tends to become wider, so when the taking lens and the focusing optical system are combined, the rear focus is further increased. On the other hand, if we look at the lens (L,) (L2), Figure 13 shows that the imaging lens (L,) and the mask (M) This is an enlarged view of the part.If the light with a strong short wavelength component is A and the light with a strong long wavelength component is B, then the refractive index of the material for the long wavelength component is smaller than the refractive index for the short wavelength component. , the light B with a strong long wavelength component has a weaker bending of the light ray, and the distance between the images becomes shorter (this is in the direction of correcting the error caused by the wavelength difference of the light source at the focus position, including the photographic lens. Therefore, , 7-beam number v of the imaging lens
2 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, in the present invention, a condenser lens is arranged near or behind the planned focal plane of the photographic lens, and an image separation optical element for separating an image into two is arranged behind the condenser lens symmetrically with the optical axis. The present invention is a focus detection device that performs focus detection on a predetermined focal plane of a photographic lens by comparing two images obtained by the present invention, and is characterized by satisfying the following conditions.

(1) ν1>ν2 where, ν is the Abbe number of the condenser lens, ν
2 is the Abbe number of the image separation optical element.

Examples of the present invention will be described in detail below.

FIG. 1 is a schematic diagram showing a focus detection device according to a first embodiment of the present invention. In the fpJ1 diagram, the condenser lens (
The Atsube number ν1 of Lo) is the imaging lens (Ll) (L2)
The material is selected so that it is larger than the Abbe number ν2. From the consideration in Fig. 13, it is clear that the material for the imaging lens (L, > (L2)) with a small Abbe number ν (the difference in refractive index depending on the wavelength is 1) will be the focal point due to the wavelength difference of the light source. This is because it is effective in correcting detection errors,
Furthermore, from the explanation in Fig. 12, it is better to have a larger Atsube number ν of the condenser lens (Lo>.As for the material used for the condenser lens (Lo), BK7, BK
Crown glass or acrylic such as S, 5K16 (
Materials with a large Atsube number ν such as PMMA) are used, while materials for the imaging lens include printed glass,
Materials with a small Atsube number ν such as polystyrene and polycarbonate are used.

In FIG. 1, the light ray (dl) indicates the light ray under sunlight illumination, and the light M(al) indicates the light ray under tungsten lamp illumination. The light beam (dl) is largely refracted by the condenser lens (Lo), but is not significantly refracted by the imaging lenses (Ll) (L2). On the other hand, the light ray (al) is not greatly refracted by the condenser lens (Lo), but the imaging lens (Ll) (
L2), both rays (dl)
Both (al) reach the same position on the light receiving element (S). Therefore, it is possible to satisfactorily correct focus detection errors due to wavelength differences between light sources.

In this example, as shown enlarged in FIG. 2, the imaging lens (L=) (L2) has its optical axis (x,
) (x2) is the opening (Ml) (M2) of the mask (M)
It is arranged a distance r11 outside the center of . By configuring in this way, the imaging lens (L, ) (
L2) has a prism-like effect, so that the light ray on the longer wavelength side (AI) shown in Fig. 2 is closer to the optical axis (TL) of the photographing lens (TL) than the light ray on the short wavelength side (DI). By passing through a position close to X), the distance between the two images of the long-wavelength light ray can be made as small as possible, thereby reducing focus detection errors due to the wavelength difference between the light sources. This will be explained in more detail by replacing this with the prism (P) in FIG.

In Figure 3, if the light rays directed at the prism (P)l and the two people are (ml), then the light M emitted from the prism (P)
The angle difference α from (o+2) is expressed as α=n·θ (2). Here, the mouth is the refractive index of the prism (P), θ
is the wedge angle of the prism (P). From equation (2), the larger the wedge angle θ is, the more the prism (P)
The larger the refractive index of , the more easily the light ray is deflected, so
The smaller the Abbe number of the imaging lenses (Ll) (L2), the larger the deflection angle, and the better the focus detection error due to the difference in wavelength can be corrected.

In the first embodiment described above, since the sensitivity of the light receiving element normally extends to the long wavelength side, the pair of imaging lenses (L, ) (LX
) of the optical axis (x, ) (x2) of the mask (M)
, > (M2) from the center of the photographing lens (TL) optical axis (X
) to correct the phenomenon of rear focus on the long wavelength side, but the first
As is clear from Figure 0, there is a very short wavelength component (400n
Even if there are many cases (near m), it will be a back pin. Therefore, in the case of a special purpose focus detection device that uses a special light source that produces a lot of light on the short wavelength side, conversely, the optical axes (x, ) (x2) of the imaging lenses (Ll) (L2) are connected to the mask (M ) opening (Ml) (
By moving the lens closer to the optical axis (X) of the imaging lens than the center of M2), it is possible to correct the phenomenon of back focus on the short wavelength side, and in this case, the heat of the imaging lens (Ll) (L2) can be corrected. The smaller the number ν2, the better the correction.The Atsube number of the condenser lens (Lo) is ν1, and the imaging lens (Ll) (
Letting the Abbe number of L2) be ν2, the focus detection error due to wavelength can be well corrected when the relationship of 2 and 2 holds.

FIG. 4 is a sectional view showing 12 embodiments that are effective in the above cases. In Figure 4, a pair of imaging lenses (
L, ) (L2) is such that its optical axis (xl) (x2) is closer to the optical axis (X
) is placed close to. Therefore, if (a) is one daylight beam with many long wavelength components, and (b) is one tungsten beam with many short wavelength components, in the conventional configuration, (b) is detected as a rear focus. However,
As in this embodiment, the optical axis (
×1) (×2) are arranged in the optical axis (X) direction of the photographing lens (TL) with respect to the center of the opening (M,) (M2) of the mask, so the prism effect is the second
The situation is the opposite of the case shown in the figure, and the light of the short wavelength component is largely bent by the imaging lenses (Ll) (L2), so that the focus detection error due to the difference in wavelength is corrected.

FIG. 5 is a sectional view showing a third embodiment of the present invention,
In this embodiment, a pair of integrated prisms (P, ) (P2) are used in place of the pair of imaging lenses (Ll) (L2) used in the above embodiments. In front of the prisms (P 1 ) (P 2 ), there is a lens (L 3 ).
) is arranged, this lens ('Lj) has an imaging function, and the function of separating images is a prism (Pl) (P2).
has. Here, prism (P,) (P2)
The position may be in front of the lens (L,).

FIG. 6 is a sectional view showing a fourth embodiment of the present invention,
In this embodiment, the lens (L,) used in the third embodiment shown in the fjSS diagram and a pair of prisms (PI) (P2) are integrated into one image separation element (DE). be.

Incidentally, in all of the above embodiments, an infrared light cut filter that blocks infrared light is not used, but the present invention is not limited to this, and is applicable to those using an infrared light cut filter. is also valid. When using an infrared light cut filter, by applying the present invention,
The amount of light received by the light receiving element can be increased by widening the wavelength range detected by the light receiving element instead of making it very narrow as in the conventional case.

As described in detail above, the present invention provides a condenser lens that is arranged near or behind the planned focal plane of a photographing lens, and an image separation optical element that separates an image into two parts symmetrically on the optical axis behind the condenser lens. In a focus detection device that detects the focus on the predetermined focal plane of the photographing lens by comparing the two separated images, ν1 is the Abbe number of the condenser lens, and ν2 is the Abbe number of the image separation optical element. This feature satisfies the conditions ν, >ν2 when .

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the first embodiment of the present invention, tjrJ
Fig. 2 is an enlarged view of the main part, Fig. 3 is a sectional view of the prism for explaining the prism action of the imaging lens, Fig. 4 is a sectional view showing the second embodiment of the present invention, and Fig. 5. is a sectional view showing the third embodiment, Fig. f56 is a sectional view showing the fourth embodiment, Fig. fjS7 is a sectional view showing the optical system of the focus detection device used in the present invention, and Fig. 8 is a sectional view showing the optical system of the focus detection device used in the present invention. Silicon 7
A graph showing the spectral sensitivity distribution of a light-receiving element consisting of an optodiode, Figure 9 is a graph showing the spectral sensitivity distribution of the light-receiving element under a tungsten lamp, and fjS10 is a graph showing the difference in image point position depending on the wavelength of the photographic lens. , Fig. 11 is a cross-sectional view showing the cause of focus detection error due to chromatic aberration of a conventional photographic lens, Fig. 12 is a cross-sectional view showing the cause of focus detection error due to the difference in refractive index depending on the wavelength of a condenser lens, and Fig. 13 is a cross-sectional view showing the cause of focus detection error due to the difference in refractive index depending on the wavelength of a condenser lens. FIG. 3 is a cross-sectional view showing the cause of focus detection error due to a difference in refractive index depending on the wavelength of an imaging lens. (T L ＞”, m shadow, (L, ): condenser lens, (L ,) (L 2) (P I) (P 2) (D
E): Image separation optical system. Applicant Minolta Camera Co., Ltd. Figure 1O Figure 5 Round Table (JL#L)

Claims (1)

[Claims] 1. A condenser lens is disposed near or behind the planned focal plane of the photographic lens, and an image separation optical element for separating an image into two parts symmetrically on the optical axis is disposed behind the condenser lens, A focus detection device that performs focus detection on a predetermined focal plane of a photographing lens by comparing two separated images with each other, and is characterized in that it satisfies the following conditions: ν_1>ν_2 However, here where ν_1 is the Abbe number of the condenser lens, ν_2 is the Abbe number of the image separation optical element. 2. The focus detection device according to claim 1, wherein the image separation optical element is a pair of lenses arranged symmetrically with respect to the optical axis of the photographing lens. 3. The focus detection device according to claim 1, wherein the image separation optical element is a pair of prisms arranged symmetrically with respect to the optical axis of the photographing lens.