Endoscope objective optical system

An endoscope objective optical system, comprising in order from an object side:

• a front group having a negative refractive power;
• an aperture stop;
• and
• a rear group having a positive refractive power, wherein
• the front group includes a first negative lens and a second negative lens, and
• the rear group includes a first positive lens, a second positive lens, and a cemented lens, and
• the cemented lens includes a lens having a positive refractive power and a lens having a negative refractive power, and
• the following conditional expressions (1), (2), (3′), and (4) are satisfied:
• 1<Iw/ft<1.8 (1),
• 4<Lt/Iw<9.5 (2),
• 1.49≦Lsf/ft<2.30 (3′), and
• 0.38<ΣLa/Lsf<0.6 (4)
• where,
• Iw denotes a maximum image height,
• ft denotes a focal length of the overall endoscope objective optical system,
• Lt denotes an overall length of the endoscope objective optical system,
• Lsf denotes a distance from an object-side first surface up to the aperture stop, and
• ΣLa denotes a sum of air spaces between the object-side first surface and the aperture stop, and in this case,
• the overall length is a distance from the object-side first surface up to an image position, and
• the object-side first surface is a lens surface positioned nearest to object in the endoscope objective optical system.

An endoscope objective optical system, comprising in order from an object side:

• a front group having a negative refractive power;
• an aperture stop;
• and
• a rear group having a positive refractive power, wherein
• the front group includes a first negative lens and a second negative lens,
• the rear group includes a first positive lens, a second positive lens, and a cemented lens,
• the cemented lens includes a lens having a positive refractive power and a lens having a negative refractive power, and
• the following conditional expressions (1), (2), (3), (4), (5), (6), and (7) are satisfied:
• 1<Iw/ft<1.8 (1),
• 4<Lt/Iw<9.5 (2),
• Lsf/ft≦2.8 (3),
• 0.38<ΣLa/Lsf<0.6 (4)
• 0.25<PSp12×ft (5),
• 0.5<|PSp12/PSn12|<1.1 (6), and
• 0.02<PS3×ft (7)
• where,
• Iw denotes a maximum image height,
• ft denotes a focal length of the overall endoscope objective optical system,
• Lt denotes an overall length of the endoscope objective optical system,
• Lsf denotes a distance from an object-side first surface up to the aperture stop, and
• ΣLa denotes a sum of air spaces between the object-side first surface and the aperture stop, and in this case,
• the overall length is a distance from the object-side first surface up to an image position, and
• the object-side first surface is a lens surface positioned nearest to object in the endoscope objective optical system,
• PSp12 denotes Petzval sum for the first positive lens and the second positive lens,
• PSn12 denotes Petzval sum for the first negative lens and the second negative lens, and
• PS3 denotes Petzval sum for the cemented lens.

An endoscope objective optical system, comprising in order from an object side:

• a front group having a negative refractive power;
• an aperture stop;
• and
• a rear group having a positive refractive power, wherein
• the front group includes a first negative lens and a second negative lens,
• the rear group includes a first positive lens, a second positive lens, and a cemented lens,
• the cemented lens includes a lens having a positive refractive power and a lens having a negative refractive power,
• the following conditional expressions (1), (2), (3), and (4) are satisfied, and
• a refractive index of the second negative lens is not less than 1.75:
• 1<Iw/ft<1.8 (1),
• 4<Lt/Iw<9.5 (2),
• Lsf/ft≦2.8 (3), and
• 0.38<ΣLa/Lsf<0.6 (4)
• where,
• Iw denotes a maximum image height,
• ft denotes a focal length of the overall endoscope objective optical system,
• Lt denotes an overall length of the endoscope objective optical system,
• Lsf denotes a distance from an object-side first surface up to the aperture stop, and
• ΣLa denotes a sum of air spaces between the object-side first surface and the aperture stop, and in this case,
• the overall length is a distance from the object-side first surface up to an image position, and
• the object-side first surface is a lens surface positioned nearest to object in the endoscope objective optical system.

An endoscope objective optical system, comprising in order from an object side:

• a front group having a negative refractive power;
• an aperture stop;
• and
• a rear group having a positive refractive power, wherein
• the front group includes a first negative lens and a second negative lens,
• the rear group includes a first positive lens, a second positive lens, and a cemented lens,
• the cemented lens includes a lens having a positive refractive power and a lens having a negative refractive power,
• the following conditional expressions (1), (2), (3), and (4) are satisfied, and
• a refractive index of the second positive lens is higher than the refractive index of the first positive lens:
• 1<Iw/ft<1.8 (1),
• 4<Lt/Iw<9.5 (2),
• Lsf/ft<2.8 (3), and
• 0.38<ΣLa/Lsf<0.6 (4)
• where,
• Iw denotes a maximum image height,
• ft denotes a focal length of the overall endoscope objective optical system,
• Lt denotes an overall length of the endoscope objective optical system,
• Lsf denotes a distance from an object-side first surface up to the aperture stop, and
• ΣLa denotes a sum of air spaces between the object-side first surface and the aperture stop, and in this case,
• the overall length is a distance from the object-side first surface up to an image position, and
• the object-side first surface is a lens surface positioned nearest to object in the endoscope objective optical system.