Objective optical system, image pickup apparatus, and endoscope
An objective optical system includes a predetermined lens disposed near the pupil of the objective optical system. The predetermined lens has a first refractive region including an optical axis and a second refractive region located outside the first refractive region. The refractive power of the second refractive region is greater than the refractive power of the first refractive region. A light beam reaching an image plane includes a first light beam and a second light beam. The first light beam is a light beam transmitted through the first refractive region and includes at least one of a beam of white light and a beam of near-infrared light. The second light beam is a light beam transmitted through the second refractive region and includes only the beam of near-infrared light. A predetermined conditional expression (1) is satisfied.
1 . An endoscope comprising a predetermined lens disposed near a pupil of an objective optical system,
wherein:
the predetermined lens has a first refractive region including an optical axis and a second refractive region located outside the first refractive region,
a refractive power of the second refractive region is greater than a refractive power of the first refractive region,
a light beam reaching an image plane includes a first light beam and a second light beam,
the first light beam is a light beam transmitted through the first refractive region and includes at least one of a beam of white light and a beam of near-infrared light,
the second light beam is a light beam transmitted through the second refractive region and includes only the beam of near-infrared light, and
the following Conditional Expression (1) is satisfied:
0<{( Dir/Dw ) 2 ×( Tir/Dir )}/{( fir−fe )/(Pich× Fno )}<0.1 (1)
where;
Dir is an aperture diameter that allows the near-infrared light to pass through,
Dw is an aperture diameter that allows the white light to pass through,
Tir is a spherical segment amount in the second refractive region,
fir is a focal length of the objective optical system at a predetermined wavelength,
fe is a focal length of the objective optical system at e-line,
Fno is an F-number of the objective optical system at e-line,
Pich is a pixel pitch of an imager,
the spherical segment amount is a maximum distance among distances between a virtual plane and the second refractive region,
the virtual plane is a plane passing through a boundary between the first refractive region and the second refractive region on a lens surface of the lens, and is orthogonal to the optical axis,
the maximum distance is a distance in a direction parallel to the optical axis, and
the predetermined wavelength is 835 nm.
2 . The endoscope according to claim 1 , wherein a paraxial radius of curvature at the lens surface in the second refractive region is smaller than a paraxial radius of curvature at the lens surface in the first refractive region.
3 . The endoscope according to claim 1 , wherein;
the first refractive region has an optical film provided on the lens surface, and
the optical film has a characteristic of transmitting the white light and the near-infrared light.
4 . The endoscope according to claim 1 , wherein:
the second refractive region has an optical film provided on the lens surface, and
the optical film has a characteristic of transmitting only the near-infrared light.
5 . The endoscope according to claim 1 , wherein:
a flat plate is disposed near the predetermined lens,
the flat plate has a first transmission region and a second transmission region,
the second transmission region has a characteristic of transmitting only the near-infrared light,
the first light beam includes light transmitted through the first transmission region, and
the second light beam includes light transmitted through the second transmission region.