DIFFRACTION OPTICAL ELEMENT
An imaging optical system according to the present invention includes a lens that has first and second surfaces and that has a diffraction grating on only one of the first and second surfaces. If the diameter of an effective area, which is defined by a light ray that has entered the lens with a maximum angle of view, is D when measured on the surface with the diffraction grating, an F number of the imaging optical system at the maximum angle of view is Fno, a d-line Abbe number of the lens is νd, and an F number of an axial bundle of rays is F, then the average diffracting ring zone pitch of the effective area satisfies 0.008 ≤ Λ D × Fno ≤ 0.00031 · vd · F
1 . An imaging optical system comprising a plurality of lenses which includes a lens that has first and second surfaces and that has a diffraction grating on only one of the first and second surfaces, the plurality of lenses being arranged in an optical axis direction,
wherein if the diameter of an effective area, which is defined by a light ray that has entered the lens with a maximum angle of view, is D when measured on the surface with the diffraction grating, an F number of the imaging optical system at the maximum angle of view is Fno, a d-line Abbe number of the lens is vd, and an F number of an axial bundle of rays is F, then the average diffracting ring zone pitch of the effective area satisfies
0.008
≤
Λ
D
×
Fno
≤
0.00031
·
vd
·
F
2 . The imaging optical system of claim 1 , wherein the average diffracting ring zone pitch Λ satisfies
0.01
≤
Λ
D
×
Fno
≤
0.00021
·
vd
·
F
3 . The imaging optical system of claim 2 , wherein the order of diffraction of the diffraction grating is second-order or a higher order.
4 . The imaging optical system of claim 3 , further comprising an optical adjustment layer, which has been formed on the surface with the diffraction grating and which satisfies
0.9
m
λ
n
1
(
λ
)
-
n
2
(
λ
)
≤
d
≤
1.1
m
λ
n
1
(
λ
)
-
n
2
(
λ
)
where d is the depth of the diffraction grating, m is the order of diffraction, λ is the wavelength, n 1 (λ) is the refractive index of the lens at the wavelength λ, and n 2 (λ) is the refractive index of the optical adjustment layer at the wavelength λ.
5 . The imaging optical system of claim 4 , wherein if a light ray passes with a full angle of view through an area on the surface of the lens with the diffraction grating, the diffraction grating covers only a part of that area and does not cover the other part of that area.
6 . The imaging optical system of claim 5 , wherein if a light ray passes with the full angle of view through the area on the surface of the lens with the diffraction grating, the diffraction grating covers only a part of that area that is located closer to the optical axis of the lens than a predetermined radial location is, and does not cover the other part of that area that is located more distant from the optical axis than the predetermined radial location is.