Far-infrared camera lens, lens unit, and imaging apparatus

1. A far-infrared camera lens comprising:

a first lens that has a biconvex shape and is formed of ZnS;

a second lens that has a negative meniscus or biconcave shape and is formed of ZnS; and

a third lens that has a positive meniscus shape and is formed of ZnS, wherein

a diffraction surface is formed in either lens surface of at least one of the first lens, the second lens, or the third lens, and

a back focus is equal to or larger than 6 mm and equal to or smaller than 20 mm.

2. The far-infrared camera lens according to claim 1 , wherein

a total focal distance f is 10 mm to 30 mm, and

a focal distance f 12 of only the first and second lenses is 20 mm to 70 mm.

3. The far-infrared camera lens according to claim 1 , wherein

when the total focal distance of the three lenses is f and the focal distance of only the first and second lenses is f 12 , 1≦f 12 /f≦3.

4. The far-infrared camera lens according to claim 1 , wherein

the first, second, and third lenses formed of ZnS are manufactured by molding ZnS raw powder by heat compression using a lens-shaped mold.

5. The far-infrared camera lens according to claim 1 , wherein

in all of the first, second, and third lenses, sag amount<5 mm, 1.5 mm<central thickness<8 mm, and 1 mm<edge thickness<8 mm.

6. A lens unit using the lens according to claim 1 .

7. An imaging apparatus comprising:

the lens unit according to claim 6 ; and

an imaging unit that images an image formed on the lens unit.

8. The far-infrared camera lens according to claim 1 , wherein

the first lens is located at a first distance from an image plane,

the second lens is located at a second distance from the image plane, the second distance being smaller than the first distance, and

the third lens is located at a third distance from the image plane, the third distance being smaller than the second distance,

wherein the back focus is defined between the third lens and the image plane.