Optical system and image pickup apparatus having the same

Abstract

An optical system images an object with light with a wavelength of 8 μm or higher, and includes a diaphragm and an optical element having an aspherical surface and disposed at a position different from that of the diaphragm. In a section including an optical axis, a thickness of an optical element monotonously increases from an on-axis to an outermost off-axis or the optical element is the thinnest at a position other than an on-axis and an outermost off-axis. A predetermined condition is satisfied.

What is claimed is: 
     
         1 . An optical system configured to image an object with light with a wavelength of 8 μm or longer, the optical system comprising:
 a diaphragm; and 
 an optical element having an aspherical surface and disposed at a position different from that of the diaphragm, 
 wherein in a section including an optical axis, a thickness of an optical element monotonously increases from an on-axis to an outermost off-axis, and 
 wherein the following conditional expression is satisfied:
   0.0<| f/Pf 1|<0.3 
 
 
       where f is a focal length of the optical system, and Pf1 is a focal length of the optical element. 
     
     
         2 . The optical system according to  claim 1 , further comprising a first lens closest to the object,
 wherein the following conditional expression is satisfied:
   0.1<| f 1/ f|< 6.0 
   
       where f1 is a focal length of the first lens. 
     
     
         3 . The optical system according to  claim 1 , further comprising a first lens closest to the object, and a second lens adjacent to the first lens,
 wherein the following conditional expression is satisfied:
   0.1<| f 2/ f|< 20.0 
   
       where f2 is a focal length of the second lens. 
     
     
         4 . The optical system according to  claim 1 , further comprising a first lens closest to the object, a second lens adjacent to the first lens, and a third lens disposed on an image side of the second lens,
 wherein the following conditional expression is satisfied:
   0.1<| f 3/ f|< 5.0 
   
       where f3 is a focal length of the third lens. 
     
     
         5 . An optical system configured to image an object with light with a wavelength of 8 μm or longer, the optical system comprising:
 a diaphragm; and 
 an optical element having an aspherical surface and disposed at a position different from that of the diaphragm, 
 wherein in a section including an optical axis, the optical element is the thinnest at a position other than an on-axis and an outermost off-axis, and 
 wherein the following conditional expression is satisfied:
   0.0<| f/Pf 1|<0.3 
 
 
       where f is a focal length of the optical system, and Pf1 is a focal length of the optical element. 
     
     
         6 . The optical system according to  claim 5 , wherein the aspherical surface has a shape represented by a function of a sixth order or longer. 
     
     
         7 . The optical system according to  claim 5 , further comprising a first lens closest to the object,
 wherein the optical element is disposed on an object side of the diaphragm, and   wherein the following conditional expression is satisfied:
     PZ/LP< 0.8 
   
       where PZ is a distance from the diaphragm to the optical element, and LP is a distance from the diaphragm to the first lens. 
     
     
         8 . The optical system according to  claim 5 , wherein the optical element is disposed on an image side of the diaphragm, and
 wherein the following conditional expression is satisfied:
     PZ/LS< 0.8 
   
       where PZ is a distance from the diaphragm to the optical element, and LS is a distance from the diaphragm to an image plane. 
     
     
         9 . The optical system according to  claim 5 , further comprising a first lens closest to the object,
 wherein the following conditional expression is satisfied:
   0.1<| f 1/ f|< 5.0 
   
       where f1 is a focal length of the first lens. 
     
     
         10 . The optical system according to  claim 5 , further comprising a first lens closest to the object, and a second lens adjacent to the first lens,
 wherein the following conditional expression is satisfied:
   0.1< f 2/ f< 10.0 
   
       where f2 is a focal length of the second lens. 
     
     
         11 . The optical system according to  claim 5 , further comprising a first lens closest to the object, a second lens adjacent to the first lens, and a third lens disposed on an image side of the second lens,
 wherein the following conditional expression is satisfied:
   0.1< f 3/ f< 10.0 
   
       where f3 is a focal length of the third lens. 
     
     
         12 . The optical system according to  claim 1 , wherein the following conditional expression is satisfied:
   0.05≤ T≤ 1.00
   
       where T [mm] is a thickness of the optical element on an optical axis. 
     
     
         13 . An optical system configured to image an object with light with a wavelength of 8 μm or longer, the optical system comprising:
 a diaphragm; and 
 an optical element having an aspherical surface and disposed at a position different from that of the diaphragm, 
 wherein the following conditional expressions are satisfied:
   20≤( N 10−1)/( N 8−− N 12)≤800
 
   0.0<| f/Pf 2|<0.3 
 
 
       where N8 is a refractive index of a material of an optical element at a wavelength of 8 μm, N10 is a refractive index of the material at a wavelength of 10 μm, N12 is a refractive index of the material at a wavelength of 12 μm, f is a focal length of the optical system, and Pf2 is a focal length of the optical element. 
     
     
         14 . The optical system according to  claim 13 , wherein the optical element is disposed on an image side of the diaphragm, and
 wherein the following conditional expression is satisfied:
   0.3< PZ/LS< 1.0 
   
       where PZ is a distance from the diaphragm to the optical element, and LS is a distance from the diaphragm to an image plane. 
     
     
         15 . The optical system according to  claim 13 , further comprising a first lens closest to the object,
 wherein the following conditional expression is satisfied:
   0.01<| f 1/ Pf 2|<0.80 
   
       where f1 is a focal length of the first lens. 
     
     
         16 . The optical system according to  claim 13 , further comprising a first lens closest to the object, and a second lens adjacent to the first lens,
 wherein the following conditional expression is satisfied:
   0.01< f 2/ Pf 2<0.80 
   
       where f2 is a focal length of the second lens. 
     
     
         17 . The optical system according to  claim 13 , further comprising a first lens closest to the object, a second lens adjacent to the first lens, and a third lens disposed on an image side of the second lens,
 wherein the following conditional expression is satisfied:
   0.1< f 3/ Pf 2<0.80 
   
       where f3 is a focal length of the third lens. 
     
     
         18 . The optical system according to  claim 1 , wherein the optical element is made of a silicon material. 
     
     
         19 . The optical system according to  claim 1 , further comprising another optical element made of a silicon material or a germanium material. 
     
     
         20 . An image pickup apparatus comprising:
 the optical system according to  claim 1 ; and   an image sensor configured to receive light from the optical system.