Imaging optical system
An imaging optical system of a present disclosure includes, in a sequential order from an object side to an image side, a first lens, a second lens, a third lens, a fourth lens, and a fifth lens. The fourth lens is configured such that (i) the light output surface has an aspherical concave shape, (ii) the light output surface has a center portion through which an optical axis passes, and a periphery thereof, and (iii) the light output surface has a portion where the negative refractive power becomes gradually weaker from the center portion toward the periphery thereof. The fifth lens has positive refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side. Each of the light input and output surfaces has a convex shape toward a corresponding one of the object side and image side.
1 . An imaging optical system comprising, in a sequential order from an object side to an image side:
a first lens having negative refractive power;
a second lens having a meniscus-shape, positive refractive power, and a light input surface that faces toward the object side, the light input surface having a concave shape;
a third lens having positive refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side, each of the light input and output surfaces having a convex shape toward a corresponding one of the object side and image side;
a fourth lens having negative refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side,
the fourth lens being configured such that
the light output surface has an aspherical concave shape;
the light output surface has a center portion through which an optical axis passes, and a periphery thereof; and
the light output surface has a portion where the negative refractive power becomes gradually weaker from the center portion toward the periphery thereof; and
a fifth lens having positive refractive power, a light input surface that faces toward the object side, and a light output surface that faces toward the image side, each of the light input and output surfaces having a convex shape toward a corresponding one of the object side and image side, wherein:
the light input surface on the object side of the fourth lens has an aspherical shape;
the aspherical-shaped light input surface of the fourth lens has a center portion through which an optical axis passes, and a periphery thereof;
the aspherical-shaped light input surface of the fourth lens has a portion where the negative refractive power becomes gradually stronger from the center portion toward the periphery thereof;
the aspherical-shaped light input surface of the fourth lens has one or more inflection points, wherein a level of the negative refractive power at each of the one or more inflection points of the aspherical-shaped light input surface changes;
a change of a curvature of the light input surface on the object side of the fourth lens has an inflection point; and
a change of a curvature of the light output surface on the image side of the fourth lens decreases substantially monotonically form the center portion toward the periphery, and has no inflection points.
2 . The imaging optical system according to claim 1 , further comprising:
an aperture diaphragm located between the second lens and the third lens.
3 . The imaging optical system according to claim 1 , wherein:
the first lens has a light output surface that faces toward the image side, the light output surface of the first lens having an aspherical shape;
the light output surface of the first lens has a center portion through which an optical axis passes, and a periphery thereof; and
the light output surface of the first lens has a portion where the negative refractive power becomes gradually stronger from the center portion toward the periphery thereof.
4 . The imaging optical system according to claim 1 , wherein:
the first lens is configured to satisfy the following conditional expression (1):
1.3< |f 1 |/f< 3.1 (1)
where:
f represents a focal distance of a whole of the imaging optical system; and
f1 represents a focal distance of the first lens.
5 . The imaging optical system according to claim 4 , wherein:
the first lens is configured to satisfy the following conditional expression (2):
1.5< |f 1 /f< 2.0 (2)
6 . The imaging optical system according to claim 1 , wherein:
the second lens is configured to satisfy the following conditional expression (3):
2.9< |f 2 |/f (3)
where:
f represents a focal distance of a whole of the imaging optical system; and
f2 represents a focal distance of the second lens.
7 . The imaging optical system according to claim 6 , wherein:
the second lens is configured to satisfy the following conditional expression (4):
3.7<| f 2|/ f< 5.0 (4).
8 . The imaging optical system according to claim 1 , wherein:
the fourth lens is configured to satisfy the following conditional expression (5):
1.0<| f 4|/ f< 2.1 (5)
where:
f represents a focal distance of a whole of the imaging optical system; and
f4 represents a focal distance of the fourth lens.
9 . The imaging optical system according to claim 8 , wherein:
the fourth lens is configured to satisfy the following conditional expression (6):
1.2 <|f 4 |/f< 1.5 (6).
10 . The imaging optical system according to claim 1 , wherein:
the first lens has a light input surface that faces toward the object side; and
the light input surface of the first lens is configured to have a convex shape.
11 . The imaging optical system according to claim 1 , wherein:
the light input surface of the fourth lens has a concave shape.
12 . The imaging optical system according to claim 1 , wherein:
the second lens has a light output surface that faces toward the image side; and
at least one of the light input surface and light output surface of the second lens has an aspherical shape.
13 . The imaging optical system according to claim 1 , wherein:
at least one of the light input surface and light output surface of the third lens has an aspherical shape.
14 . The imaging optical system according to claim 1 , wherein:
the third lens has a relative refractive index, and has a temperature coefficient of the relative refractive index at a d-line of the third lens; and
the temperature coefficient of the relative refractive index at the d-line of the third lens under an ambient temperature of 20° C. is configured to satisfy the following conditional expression (7):
|( dn/dT ) 3 |<10×10 −6 /° C. (7)
where |(dn/dT) 3 | represents the temperature coefficient of the relative refractive index at the d-line of the third lens.
15 . The imaging optical system according to claim 14 , wherein:
the temperature coefficient of the relative refractive index at the d-line of the third lens under the ambient temperature of 20° C. is configured to satisfy the following conditional expression (8):
|( dn/dT ) 3 |<6×10 −6 /° C. (8).
16 . The imaging optical system according to claim 1 , wherein:
the fifth lens has a relative refractive index, and has a temperature coefficient of the relative refractive index at a d-line of the fifth lens; and
the temperature coefficient of the relative refractive index at the d-line of the fifth lens under an ambient temperature of 20° C. is configured to satisfy the following conditional expression (9):
|( dn/dT ) 5 |<10×10 −6 /° C. (9)
where |(dn/dT) 5 | represents the temperature coefficient of the relative refractive index at the d-line of the fifth lens.
17 . The imaging optical system according to claim 16 , wherein:
the temperature coefficient of the relative refractive index at the d-line of the fifth lens under the ambient temperature of 20° C. is configured to satisfy the following conditional expression (10):
|( dn/dT ) 5 |<6×10 −6 /° C. (10).