Focus detecting optical system and camera provided with the same

1. A focus detecting optical system detecting a focus position of a photographing optical system from a positional relationship between at least one pair of secondary object images, wherein the focus detecting optical system comprises:

a condenser lens placed in the proximity of a preset imaging plane equivalent to an imaging plane of a photographic lens;

a pair of aperture stops that are placed on an exit side of the condenser lens and that divide a pupil of the photogrphic lens into two areas; and

a pair of re-imaging lenses for forming two secondary object images corresponding to the aperture stops, and satisfies the following conditions:

0.45 <|mg |<0.75

0.75 <|R 1 /R 2 |<1.25

| R 3 /R 4 |≦0.02

 where mg is an imaging magnification of the focus detecting optical system, R 1 is a radius of curvature of an entrance surface of the condenser lens, R 2 is a radius of curvature of an exit surface of the condenser lens, R 3 is a radius of curvature of an entrance surface of each of the re-imaging lenses, and R 4 is a radius of curvature of an exit surface of each of the re-imaging lenses.

2. A focus detecting optical system according to claim 1 , further comprising a light receiving element having a light-receiving surface, and satisfying the following condition:

|Δ|<1.55λ

where Δ is a difference of a position of a center of gravity between spots of C and F lines on the surface of the light-receiving element and λ is a wavelength of the E line.

3. A focus detecting optical system according to claim 1 , further satisfying the following condition:

3.50×10 −4 >|δd/D|

where δd is an image height error (mm) on a most peripheral side of a range measuring area and D is a distance (mm) between two images made by the pair of re-imaging lenses.

4. A focus detecting optical system according to claim 1 , wherein a plurality of sets, each of which is a combination of the pair of aperture stops with the pair of re-imaging lenses corresponding thereto, so that, in each set, centers of the aperture stops and the re-imaging lenses corresponding thereto are decentered from an optical axis of the photographic lens and an amount of decentration varies with each set.

5. A focus detecting optical system according to claim 1 , further comprising a light-receiving element having a light-receiving surface, and satisfying the following condition:

0.85 <LTL/fl <1.75

where LTL is an optical path length (a length from the preset imaging plane to the light-receiving surface) of the focus detecting optical system and fl is a focal length of an entire focus detecting optical system.

6. A focus detecting optical system according to claim 1 , wherein the re-imaging lenses are placed so that an optical axis of each of the re-imaging lenses is decentered farther away from a center of each of the aperture stops with respect to the optical axis of the photographic lens.

7. A camera comprising:

a focus detecting optical system;

a finder optical system;

a photographic lens;

path splitting means splitting an optical path from the photographic lens to direct the optical path toward an image sensor or a film and toward the finder optical system; and

a reflecting means conducting the optical path of the photographic lens to the focus detecting optical system,

the focus detecting optical system detecting a focus position of a photographing optical system from a positional relationship between at least one pair of secondary object images, wherein the focus detecting optical system comprises:

a condenser lens placed in the proximity of a preset imaging plane equivalent to an imaging plane of a photographic lens;

a pair of aperture stops that are placed on an exit side of the condenser lens and that divide a pupil of the photographic lens into two areas; and

a pair of re-imaging lenses for forming two secondary object images corresponding to the aperture stops,

and satisfies the following conditions:

0.45 <|mg |<0.75

0.75 <|R 1 /R 2 |<1.25

| R 3 /R 4 |≦0.02

where mg is an imaging magnification of the focus detecting optical system, R 1 is a radius of curvature of an entrance surface of the condenser lens, R 2 is a radius of curvature of an exit surface of the condenser lens, R 3 is a radius of curvature of an entrance surface of each of the re-imaging lenses, and R 4 is a radius of curvature of an exit surface of each of the re-imaging lenses.

8. A camera according to claim 7 , wherein a diameter of an image circle of the camera is substantially a half of the diameter of the image circle of a 135 format camera.

9. A camera according to claim 7 , further comprising the image sensor placed on the optical path of the photographic lens.

10. A camera comprising:

an image sensor provided with a light-receiving surface receiving light from a photographing optical system; and

a focus detecting optical system detecting a focus position of the photographing optical system from a positional relationship between at least one pair of secondary image objects with light from the photographing optical system,

wherein a diameter of an image circle of the camera is substantially a half of the diameter of the image circle of a 135 format camera.

11. A camera according to claim 10 , wherein the focus detecting optical system satisfies the following condition:

0.45 <|mg |<0.75

where mg is an imaging magnification of the focus detecting optical system.

12. A camera according to claim 10 , further comprising a reflecting mirror reflecting the light from the photographing optical system to conduct the light to the focus detecting optical system.

13. A camera according to claim 12 , wherein the reflecting mirror is moved to conduct the light from the photographing optical system to the image sensor.