Optical system and image pickup apparatus having the same
An optical system includes a plurality of lenses and a diaphragm. The plurality of lenses consist of, in order from an object side to an image side, a first lens having a positive refractive power, a second lens having a positive refractive power, a third lens having a negative refractive power, a fourth lens, a fifth lens, and a sixth lens. The diaphragm is located between the first lens and the second lens. A predetermined condition is satisfied.
1. An optical system comprising a plurality of lenses and a diaphragm,
wherein the plurality of lenses consist of, in order from an object side to an image side, a first lens having a positive refractive power, a second lens having a positive refractive power, a third lens having a negative refractive power, a fourth lens, a fifth lens, and a sixth lens,
wherein the diaphragm is located between the first lens and the second lens, and
wherein the following inequalities are satisfied:
0.5 <SPIP/TTL <1.0
1.65 <PNdave <2.00
−0.70<3/ f<− 0.25
0.20< BF/TTL< 0.40
where SPIP is a distance on an optical axis from the diaphragm to an image plane when a backfocus is expressed by an air-equivalent length, TTL is a distance on the optical axis from a lens surface on the object side of the first lens to the image plane when the backfocus is expressed by the air-equivalent length, PNdave is an average value of refractive indexes of all materials of positive lenses included in the optical system for d-line, f3 is a focal length of the third lens, f is a focal length of the optical system, and BF is an air-equivalent length of a distance on the optical axis from a lens surface on the image side of the sixth lens to the image plane.
2. The optical system according to claim 1 , wherein the following inequality is satisfied:
0.80< f 1/ f< 2.00
where f1 is a focal length of the first lens, and f is a focal length of the optical system.
3. The optical system according to claim 1 , wherein the following inequality is satisfied:
0.30< f 2/ f< 0.80
where f2 is a focal length of the second lens.
4. The optical system according to claim 1 , wherein the following inequality is satisfied:
1.5<( L 1 R 2+ L 1 R 1)/( L 1 R 2− L 1 R 1)<4.0
where L1R1 is a radius of curvature of a lens surface on the object side of the first lens, and L1R2 is a radius of curvature of a lens surface on the image side of the first lens.
5. The optical system according to claim 1 , wherein the optical system includes two or more negative lenses, and the following inequality is satisfied:
15 <Nvdave< 30
where Nvdave is an average value of Abbe numbers for the d-line of all materials of the negative lenses included in the optical system.
6. The optical system according to claim 1 , wherein the following inequality is satisfied:
0.90< TTL/f< 1.40.
7. The optical system according to claim 1 , wherein the fourth lens includes an area having a negative refractive power near the optical axis,
wherein aspherical surfaces are formed on both sides of the fourth lens,
wherein a lens surface on the object side of the fourth lens in the area is concave, and
wherein a lens surface on the image side of the fourth lens in the area is convex.
8. The optical system according to claim 1 , wherein the fifth lens includes an area having a positive refractive power near the optical axis,
wherein aspherical surfaces are formed on both sides of the fifth lens,
wherein a lens surface on the object side of the fifth lens in the area is concave, and
wherein a lens surface on the image side of the fifth lens in the area is convex.
9. The optical system according to claim 1 , wherein the sixth lens includes an area having a positive refractive power near the optical axis,
wherein aspherical surfaces are formed on both sides of the sixth lens,
wherein a lens surface on the object side of the sixth lens in the area is convex, and
wherein the lens surface on the image side of the sixth lens in the area is concave.
10. An image pickup apparatus comprising:
an optical system; and
an image sensor configured to receives an image formed by the optical system,
wherein the optical system includes a plurality of lenses and a diaphragm,
wherein the plurality of lenses consist of, in order from an object side to an image side, a first lens having a positive refractive power, a second lens having a positive refractive power, a third lens having a negative refractive power, a fourth lens, a fifth lens, and a sixth lens,
wherein the diaphragm is located between the first lens and the second lens, and
wherein the following inequalities are satisfied:
0.5 <SPIP/TTL <1.0
1.65 <PNdave <2.00
−0.70<3/ f<− 0.25
0.20< BF/TTL< 0.40
where SPIP is a distance on an optical axis from the diaphragm to an image plane when a backfocus is expressed by an air-equivalent length, TTL is a distance on the optical axis from a lens surface on the object side of the first lens to the image plane when the backfocus is expressed by the air-equivalent length, and PNdave is an average value of refractive indexes of all materials of positive lenses included in the optical system for d-line, f3 is a focal length of the third lens, f is a focal length of the optical system, and BF is an air-equivalent length of a distance on the optical axis from a lens surface on the image side of the sixth lens to the image plane.