Optical low pass filter and, image capturing apparatus and image capturing unit having the same

1. An optical low pass filter arranged on a side of a light-receiving surface of an image-capturing element, the optical low pass filter comprising:

a first birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface is parallel to a long side of the image-capturing element;

a second birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface is parallel to a short side of the image-capturing element;

a third birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface forms an angle of 45° to the short side of the image-capturing element; and

a fourth birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface forms an angle of 90° to the orthogonal projection of the optic axis of the third birefringent plate onto the light-receiving surface,

wherein the following conditional expression is satisfied:

1.43≤ b/a< 3,

where a represents a separation width of an incident light ray in each of the first birefringent plate and the second birefringent plate, and b represents a separation width of an incident light ray in each of the third birefringent plate and the fourth birefringent plate.

2. The optical low pass filter according to claim 1 , wherein

either the third birefringent plate or the fourth birefringent plate is arranged between the first birefringent plate and the second birefringent plate, and

either the first birefringent plate or the second birefringent plate is arranged between the third birefringent plate and the fourth birefringent plate.

3. The optical low pass filter according to claim 1 , wherein

both of a value of MTF in a direction parallel to the orthogonal projection of the optic axis of the third birefringent plate onto the light-receiving surface and a value of MTF in a direction parallel to the orthogonal projection of the optic axis of the fourth birefringent plate onto the light-receiving surface are equal to or less than 25% in a range of a frequency not less than a cut-off frequency and not more than 300 lp/mm.

4. The optical low pass filter according to claim 1 , wherein the optical low pass filter separates an incident light ray into 16 or more light rays.

5. An optical low pass filter arranged on a side of a light-receiving surface of an image-capturing element, the optical low pass filter comprising:

a first birefringent plate that separates an incident light ray in a first direction parallel to a long side of the image-capturing element;

a second birefringent plate that separates an incident light ray in a second direction parallel to a short side of the image-capturing element;

a third birefringent plate that separates an incident light ray in a third direction that forms an angle of 45° to the short side of the image-capturing element on the light-receiving surface; and

a fourth birefringent plate that separates an incident light ray in a fourth direction that forms an angle of 90° to the third direction on the light-receiving surface,

wherein the following conditional expression is satisfied:

1.43≤ b/a< 3,

where a represents a separation width of an incident light ray in each of the first birefringent plate and the second birefringent plate, and b represents a separation width of an incident light ray in each of the third birefringent plate and the fourth birefringent plate.

6. The optical low pass filter according to claim 5 , wherein

either the third birefringent plate or the fourth birefringent plate is arranged between the first birefringent plate and the second birefringent plate, and

either the first birefringent plate or the second birefringent plate is arranged between the third birefringent plate and the fourth birefringent plate.

7. The optical low pass filter according to claim 5 , wherein

both of a value of MTF in a direction parallel to orthogonal projection of an optic axis of the third birefringent plate onto the light-receiving surface and a value of MTF in a direction parallel to orthogonal projection of an optic axis of the fourth birefringent plate onto the light-receiving surface are equal to or less than 25% in a range of a frequency not less than a cut-off frequency and not more than 300 lp/mm.

8. The optical low pass filter according to claim 5 , wherein the optical low pass filter separates an incident light ray into 16 or more light rays.

9. An image capturing apparatus comprising:

an optical low pass filter and an image-capturing element that receives a light ray output from the optical low pass filter and performs photoelectric conversion thereto, wherein the optical low pass filter includes:

a first birefringent plate of which an orthogonal projection of an optic axis onto a light-receiving surface of the image-capturing element is parallel to a long side of the image-capturing element,

a second birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface is parallel to a short side of the image-capturing element,

a third birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface forms an angle of 45° to the short side of the image-capturing element, and

a fourth birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface forms an angle of 90° to the orthogonal projection of the optic axis of the third birefringent plate onto the light-receiving surface, and

the following conditional expression is satisfied:

1.43≤ b/a< 3,

where a represents a separation width of an incident light ray in each of the first birefringent plate and the second birefringent plate, and b represents a separation width of an incident light ray in each of the third birefringent plate and the fourth birefringent plate.

10. The image capturing apparatus according to claim 9 , wherein

the image-capturing element includes a plurality of pixels, and

the following conditional expression is satisfied:

0.309<( a 2+ b 2)/ p 2<0.797,

where p represents a pitch between the plurality of pixels.

11. The image capturing apparatus according to claim 9 , wherein

the image-capturing element includes a color filter of a Bayer array.

12. The image capturing apparatus according to claim 9 , wherein

the image-capturing element includes a plurality of pixels, and

a pitch between the plurality of pixels is not less than 3 μm and not more than 7 μm.

13. The image capturing apparatus according to claim 9 , wherein the optical low pass filter separates an incident light ray into 16 or more light rays.

14. An image capturing unit comprising:

an optical low pass filter and an image-capturing element that receives a light ray output from the optical low pass filter and performs photoelectric conversion thereto, wherein the optical low pass filter includes:

a first birefringent plate of which an orthogonal projection of an optic axis onto a light-receiving surface of the image-capturing element is parallel to a long side of the image-capturing element,

a second birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface is parallel to a short side of the image-capturing element,

a third birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface forms an angle of 45° to the short side of the image-capturing element, and

a fourth birefringent plate of which an orthogonal projection of an optic axis onto the light-receiving surface forms an angle of 90° to the orthogonal projection of the optic axis of the third birefringent plate onto the light-receiving surface, and

the following conditional expression is satisfied:

1.43≤ b/a< 3,

where a represents a separation width of an incident light ray in each of the first birefringent plate and the second birefringent plate, and b represents a separation width of an incident light ray in each of the third birefringent plate and the fourth birefringent plate.

15. The image capturing unit according to claim 14 , wherein

the image-capturing element includes a plurality of pixels, and

the following conditional expression is satisfied:

0.309<( a 2+ b 2)/ p 2<0.797,

where p represents a pitch between the plurality of pixels.

16. The image capturing unit according to claim 14 , wherein

the image-capturing element includes a color filter of a Bayer array.

17. The image capturing unit according to claim 14 , wherein

the image-capturing element includes a plurality of pixels, and

a pitch between the plurality of pixels is not less than 3 μm and not more than 7 μm.

18. The image capturing unit according to claim 14 , wherein the optical low pass filter separates an incident light ray into 16 or more light rays.