Converter lens, interchangeable lens, and image pickup apparatus
A converter lens includes three or more negative lenses and increases the focal length of an entire system, in which an average refractive index Ndave at the d-line (wavelength of 587.56 nm) of a material of all the negative lenses included in the converter lens is defined.
1. A converter lens which has a negative refractive power and increases a focal length of an entire system when arranged on an image side of a master lens, the converter lens consisting of:
a first lens unit having positive refractive power; and
a second lens unit having negative refractive power and arranged on an image side of the first lens unit,
wherein the first lens unit consists of a negative lens and a first positive lens,
wherein the second lens unit including a second positive lens arranged closest to an image side of the converter lens,
wherein three or more negative lenses are included in the converter lens, and
wherein the following inequalities are satisfied:
1.92 <Nd ave<2.10, and
1.30<( R 1+ R 2)/( R 1− R 2)<2.50,
where Ndave represents an average refractive index at a d-line of a material of all negative lenses included in the converter lens, R1 represents a curvature radius of a surface of the second positive lens on an object side, and R2 represents a curvature radius of a surface of the second positive lens on the image side.
2. The converter lens according to claim 1 ,
wherein the following inequality is satisfied:
0.10< sk/TD< 0.50
where sk represents an air-converted length from a surface of the converter lens closest to the image side to an image plane while the converter lens is arranged on the image side of the master lens, and TD represents a length on an optical axis from a surface of the converter lens closest to the object side to a surface closest to the image side.
3. The converter lens according to claim 1 ,
wherein the negative lens and the first positive lens are cemented to each other.
4. The converter lens according to claim 1 ,
wherein the following inequality is satisfied:
−2.30< f 1/ f 2<−0.95,
where f1 represents a focal length of the first lens unit and f2 represents a focal length of the second lens unit.
5. The converter lens according to claim 1 ,
wherein the following inequality is satisfied:
−1.40< f 1/ f<− 0.30,
where f1 represents a focal length of the first lens unit and f represents a focal length of the converter lens.
6. The converter lens according to claim 1 ,
wherein the second lens unit includes three or more negative lenses.
7. The converter lens according to claim 6 ,
wherein the second lens unit comprises four or more negative lenses.
8. An interchangeable lens comprising:
a master lens; and
a converter lens which has a negative refractive power and increases a focal length of an entire system when arranged on an image side of the master lens,
wherein the converter lens consists of
a first lens unit having positive refractive power and
second lens unit having negative refractive power and arranged on an image side of the first lens unit,
wherein the first lens unit consists of a negative lens and a first positive lens,
wherein the second lens unit including a second positive lens arranged closest to an image side of the converter lens,
wherein three or more negative lenses are included in the converter lens, and
wherein the following inequalities are satisfied:
1.92 <Nd ave<2.10, and
1.30<( R 1+ R 2)/( R 1− R 2)<2.50,
where Ndave represents an average refractive index at the d-line of a material of all negative lenses included in the converter lens, R1 represents a curvature radius of a surface of the second positive lens on an object side, and R2 represents a curvature radius of a surface of the second positive lens on the image side.
9. The interchangeable lens according to claim 8 ,
wherein the following inequality is satisfied:
0.10< sk/TD< 0.50
where sk represents an air-converted length from a surface of the converter lens closest to the image side to an image plane while the converter lens is arranged on the image side of the master lens, and TD represents a length on an optical axis from a surface of the converter lens closest to the object side to a surface closest to the image side.
10. The interchangeable lens according to claim 8 ,
wherein the negative lens and the first positive lens are cemented to each other.
11. The interchangeable lens according to claim 8 ,
wherein the following inequality is satisfied:
−2.30< f 1/ f 2<−0.95,
where f1 represents a focal length of the first lens unit and f2 represents a focal length of the second lens unit.
12. The interchangeable lens according to claim 8 ,
wherein the following inequality is satisfied:
−1.40< f 1/ f<− 0.30,
where f1 represents a focal length of the first lens unit and f represents a focal length of the converter lens.
13. An apparatus comprising:
a master lens;
a converter lens which has a negative refractive power and increases a focal length of an entire system when arranged on an image side of the master lens; and
an imaging device configured to receive an image formed by the master lens and the converter lens,
wherein the converter lens consists of
a first lens unit having positive refractive power and
second lens unit having negative refractive power and arranged on an image side of the first lens unit,
wherein the first lens unit consists of a negative lens and a first positive lens,
wherein the second lens unit uncluding a second positive lens arranged closest to an image side of the converter lens,
wherein three or more negative lenses are included in the converter lens, and
wherein the following inequalities are satisfied:
1.92 <Nd ave<2.10, and
1.30<( R 1+ R 2)/( R 1− R 2)<2.50,
where Ndave represents an average refractive index at the d-line of a material of all negative lenses included in the converter lens, R1 represents a curvature radius of a surface of the second positive lens on an object side, and R2 represents a curvature radius of a surface of the second positive lens on the image side.
14. The apparatus according to claim 13 ,
wherein the following inequality is satisfied:
0.10< sk/TD< 0.50
where sk represents an air-converted length from a surface of the converter lens closest to the image side to an image plane while the converter lens is arranged on the image side of the master lens, and TD represents a length on an optical axis from a surface of the converter lens closest to the object side to a surface closest to the image side.
15. The apparatus according to claim 13 ,
wherein the negative lens and the first positive lens are cemented to each other.
16. The apparatus according to claim 13 ,
wherein the following inequality is satisfied:
−2.30< f 1/ f 2<−0.95,
where f1 represents a focal length of the first lens unit and f2 represents a focal length of the second lens unit.
17. The apparatus according to claim 13 ,
wherein the following inequality is satisfied:
−1.40< f 1/ f<− 0.30,
where f1 represents a focal length of the first lens unit and f represents a focal length of the converter lens.