JPS63243906A - Focus detection device for camera - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus detection device for a camera that receives light passing through a photographic lens and detects the focus of the photographic lens.

``Prior Art'' Various types of focus detection devices are known to be installed in photographic cameras, and one of them is a focus detection device having the configuration shown in FIG.

FIG. 1 shows the basic optical system of the focus detection device, in which 11 is a photographing lens, 12 is a condenser lens, 13 and 14 are a pair of imaging lenses, and 15 and 16 are light receiving element arrays.

The condenser lens 12 is the optical axis 17 of the photographic lens 11.
The imaging lenses 13.14 are arranged directly after the predetermined focal plane 18 of the taking lens 11 above, and the imaging lenses 13.14 are symmetrical with respect to the optical axis, with the optical axis of each of the reimaging lenses 13.14 being parallel to the optical axis 17. The light receiving element arrays 15 and 16 are arranged so that
are arranged at positions that serve as the imaging planes of the respective imaging lenses 13 and 14.

In this focus detection device, when the photographing lens 11 is in focus, an image 19a thereof is formed on the predetermined focal plane 18, and furthermore, the imaging lens 13 forms a first image 20a, and the imaging lens 14 forms a second image 21a. is formed.

When the photographing lens 11 is in the rear focus state, the image 1
9b is formed behind the focused image 19a, and the first image 20b1 second image 21b formed by the imaging lens 13.14 is closer to the optical axis 17 than the focused first image 20a3 and second image 21a. Formed at vertically separated positions.

Contrary to the above, when the front focus is achieved, the image 19C of the photographic lens 11 is formed further forward than the image 19a at the time of focus, and both the first image 20c and the second image 21C are further along the optical axis 17 than at the time of focus. It is formed at a position close to .

In this way, in the above focus detection device, the light receiving element array 1
The focusing state can be detected by detecting the distance between the first image and the second image from the illuminance distribution on 5.16.

"Problem to be Solved by the Invention" In order to obtain accurate focus detection in the focus detection device described above, the first image and the second image formed by the imaging lenses 13 and 14 must always correspond to each other. It must have a uniform illuminance distribution.

However, due to the effects of distortion that occurs when the condenser lens 12 is a spherical lens, chromatic aberration that occurs in the optical system, etc., the first and second images do not have equal illuminance distribution and are not accurate. Focus detection may not be possible.

"Means for Solving the Problems" The present invention was developed in view of the above problems.
The purpose is to improve focus detection accuracy by suppressing the influence of chromatic aberration generated by an optical system as much as possible.

Therefore, in the present invention, a condenser lens is arranged behind the planned focal plane of the photographing lens, and a pair of imaging lenses are arranged behind the condenser lens symmetrically with respect to the optical axis. In a camera focus detection device configured to detect the focal point of the photographic lens by comparing images of planned focal planes with each other, the pair of imaging lenses described above are formed on the rear side to form a convex surface toward the rear, and on the front side. A focus detection device for a camera is proposed, which is characterized by being provided with an optical member having a concave surface rotationally symmetrical with respect to an optical axis.

"Function" As mentioned above, since the incident light side of the pair of imaging lenses is formed into a concave surface, the influence of chromatic aberration of the optical system can be suppressed as much as possible, and the imaging lens has a flat or convex surface on the incident light side. The detection error is extremely small compared to .

"Example" Next, embodiments of the present invention will be described with reference to the drawings.

2 and 3 are imaging lenses 13 and 14 shown in FIG. 1.
2 is a cross-sectional view taken along the line A--A in FIG. 3, and FIG. 3 is a side view of the optical member 22 viewed from the direction of the light emitted from the imaging lens.

This optical member 22 has a pair of imaging lenses 23 on its back side.
24 are integrally formed. These imaging lenses 23 and 24 are symmetrical with respect to the optical axis and have convex surfaces in the direction of the emitted light.

Further, a concave surface 25 that is rotationally symmetrical with respect to the optical axis 17 is formed on the front side of this optical member 22, and the structure is such that light enters each imaging lens 23, 24 from this concave surface 25.

A focus detection device equipped with such an optical member 22 as an imaging lens is less affected by chromatic aberration, and detection errors are improved.

FIG. 4 is a graph showing the relationship between detection errors when the radius of curvature R of the concave surface 25 is changed as a parameter. As can be seen from this figure, the detection error decreases by changing from R1 = 25 mm toward R1-■, but R
It can be reduced as much as possible by setting it to be negative with respect to the condenser lens 12, such as 1=-25 mm.

In addition, R1 is 20 mm to 30 mm, and each image formation is 1/7 z 23
．． If the radius of curvature R2 of 24 is set to about 1.6 mm, R
Since the curvature of R1 is gentler than that of 2, a high degree of alignment accuracy is not required.

Next, FIG. 5 shows a basic optical system showing an embodiment of (i).

In this embodiment, a mask 26 is provided at or near the focal point of the condenser lens 12, and the above-mentioned imaging lenses 23 and 26 pass through the pupils 26a and 26b of this mask 26.
This is a configuration in which light is incident on the area.

With this configuration, the first image 20 and second image 21 of the imaging lens 23 and 24 move in response to the amount of movement X of the image 19 of the photographing lens 11! 1y becomes a linear equation of y=kx, which facilitates focus detection. Note that k is a constant.

In other words, the amount of blur between the first image 20 and the second image 21 is proportional to the amount of movement of the center of each image 20.21.

If the above-mentioned linear equation does not hold true, an error will occur in the predicted value of defocus, resulting in problems such as an increase in the number of distance measurements. Although it is possible to store errors in predicted values in advance as data and correct them, this is not preferable because the calculation process becomes complicated.

According to the above embodiment, the amount of movement between the first image 20 and the second image 21 is linear on the light receiving element array 15, 16, and the detection accuracy is improved.

The specific configuration of the optical system of this embodiment is shown in Table 1 below based on FIG.

Table 1 Radius of curvature On-axis spacing Refractive index (unit mm) 1st order mm) Image 19 r1=ψ dl=4 L d a =11.7 1 Mask 26 r4=” d4-0.045 1
da=4.3 1 light receiving element array 15.16 r 7 = o.

Note that the focal length M f −12,4 of the condenser lens 12
Back focus b of 5mm 1 condenser lens 12
r -11,76mm? be.

FIG. 7 is a graph showing the detection error determined by the above optical system configuration.

As shown in this graph, the detection accuracy is highest when d s =11.7 mm. This graph shows d3=
11.7mm ±Q, 5mm shifted from m d 3
Although the other case of =12.2''mXd3=11.2mm was also shown, it can be seen that when shifted in this way, the detection error increases rapidly.

On the other hand, in the above optical system configuration, when the image 19 is formed on the planned focal plane 18 of the photographic lens 11, the influence of distortion caused by the condenser lens 12 is small, but the front focus or rear focus state is large. As the situation increases, its influence increases.

Therefore, as shown in FIG. 6, for the condenser lens 12 of the optical system shown in FIG. 5, it is effective to reduce the curvature of the rear surface toward the lens periphery to form an aspheric surface 27. It is true.

Note that the dotted line in the figure indicates the spherical surface 28.

It is preferable that the aspherical surface 27 is determined according to the following formula.

In addition, C-1/rs, K-0,, D=3.5 x 10
, E=0.

=4 However, it is only necessary to satisfy the condition O<D<5xlO.

A graph showing the degree of influence of distortion when the rear surface of the condenser lens 12 is formed into an aspherical surface 27 by determining the value of Z as a function of H as described above is shown in FIG. 8, and similar to FIG. Graphs regarding detection errors are shown in FIG. 9.

Note that in FIG. 8, DF indicates defocus.

"Effects of the Invention" As described above, in the focus detection device according to the present invention, a pair of imaging lenses provided symmetrically with respect to the optical axis of the photographic lens are attached to an optical member integrally formed with a convex surface. Since a rotationally symmetrical concave surface is formed and light enters the re-imaging lens from this concave surface, the influence of chromatic aberration caused by the optical system that constitutes the focus detection device is minimized, making detection of this type of device Accuracy can be sufficiently improved.

Further, since the formation of the concave surface of the optical member does not require particularly high alignment accuracy, implementation is easy.

[Brief explanation of drawings]

FIG. 1 shows a basic optical system of a focus detection device shown as a conventional example, and FIGS. 2 and 3 show an optical member equipped with an imaging lens, which is an embodiment of the present invention. Figure 2 is the third
3 is a side view of the optical member seen from the imaging lens side; FIG. 4 is a graph showing the detection error of the focus detection device when equipped with the optical member; FIG. 5 is a diagram showing a partial optical system of a focus detection device showing another embodiment of the present invention equipped with the above-mentioned optical member and mask, FIG. 6 is a diagram showing a condenser lens provided in the focus detection device, and FIG. Fig. 7 is a graph showing the detection error of the embodiment shown in Fig. 5, and Figs. 8 and 9 are graphs showing the degree of influence of distortion and the detection error when the condenser lens shown in Fig. 6 is installed. be. 11... Photographing lens 12... Condenser lens 15. 16... Light receiving element array 19... Photographing lens image 20... First image 21... Second image 22... Optical member 23 .24...Imaging lens 25...Concave surface 26...Mask 27...Aspherical surface Fig. 6 Fig. 7

Claims (1)

[Claims] A condenser lens is placed behind the planned focal plane of the photographic lens.
A pair of imaging lenses are arranged behind the condenser lens symmetrically with respect to the optical axis, and the focal point of the photographing lens is detected by comparing images of a predetermined focal plane formed on each of the pair of imaging lenses. In the camera focus detection device having the above configuration, the pair of imaging lenses described above are formed on the rear side with convex surfaces facing toward the rear, and an optical member is provided on the front side with a concave surface rotationally symmetrical with respect to the optical axis. Features: Focus detection device for cameras.