Imaging optical system and imaging apparatus

1. An imaging optical system, comprising: a front group, a stop, and a rear group, in order, from an object side,

wherein a lens arranged closest to the stop in the front group is a positive lens comprising a concave surface disposed on the object side, and a convex surface disposed on an image side,

wherein the front group further comprises at least one additional positive lens comprising a concave surface disposed on the object side, and a convex surface disposed on the image side,

wherein a lens arranged closest to the stop in the rear group is a positive lens, and

wherein a conditional expression (1) and a conditional expression (2) below are satisfied:

1.40<|dn3/dn4<9.60   (1)

0.52<y/f·sin(θ1)<1.35   (2)

wherein

dn3: a temperature coefficient of a relative refractive index at a temperature 20° C-40° C. to a d line of the positive lens arranged closest to the stop in the front group

dn4: a temperature coefficient of a relative refractive index at the temperature 20° C.-40° C. to a d line of the positive lens arranged closest to the stop in the rear group

f: a focal length of the imaging optical system

y: a maximum image height of the imaging optical system, and

θ1: a maximum half image viewing angle of the imaging optical system.

2. The imaging optical system according to claim 1 ,

wherein the front group comprises a first lens on the object side of the positive lens arranged closest to the stop and the at least one additional positive lens,

wherein the first lens is a negative lens, a surface of which on the object side is a convex surface and is also an aspherical surface that weakens convex power with distance from an optical axis, and is arranged more on the object side than the second lens.

3. The imaging optical system according to claim 1 ,

wherein a conditional expression (3) and a condition expression (4) below are satisfied:

v2<47   (3)

v3>65   (4)

wherein

v2: an Abbe constant of the positive lens of a smallest Abbe constant to the d line, among the positive lenses included in the front group, and

v3: an Abbe constant of the positive lens of a largest Abbe constant to the d line, among the positive lenses included in the front group.

4. The imaging optical system according to claim 1 ,

wherein a conditional expression (5) below is satisfied:

0.15<N2-N3   (5)

wherein

N2: a refractive index of the positive lens of a largest refractive index to the d line, among the positive lenses included in the front group, and

N3: a refractive index of the positive lens of a smallest refractive index to the d line, among the positive lenses included in the front group.

5. The imaging optical system according to claim 1 ,

wherein the front group comprises a first lens, which is a negative lens, disposed on the object side of the positive lens arranged closest to the stop and the at least one additional positive lens

wherein the rear group comprises a fourth lens, which is the positive lens arranged closest to the stop, a fifth lens, which is a negative lens, and a sixth lens, which is a positive lens in order from the object side,

the imaging optical system being practically composed of the six lenses.

6. The imaging optical system according to claim 5 , satisfying a conditional expression (6) below:

1.3<|f12/f|<9.5   (6)

wherein

f: the focal length of the imaging optical system, and

f12: the focal length of the second lens.

7. The imaging optical system according to claim 5 , satisfying a conditional expression (7) below:

2.2<|f13/f|<8.9   (7)

wherein

f: the focal length of the imaging optical system, and

f13: the focal length of the third lens.

8. An imaging apparatus including an imaging optical system described in claim 1 , and an image sensor that converts an optical image formed by the imaging optical system to an electric signal, on an image side of the imaging optical system.