Objective for an endoscope

Claim 2

An objective for an endoscope comprising a GRIN lens whose surface on the object side is arranged as a planar surface or convex surface and whose surface on the image side is arranged as a convex surface, and an aperture stop located on the object side of said GRIN lens, said GRIN lens being arranged that the refractive index n thereof is expressed by the formula shown below when the refractive index of the central portion thereof is represented by reference symbol n 0 and the radial distance from the optical axis is represented by reference symbol r, said objective for an endoscope fulfilling the conditions (1), (2) and (3) shown below: n.sup.2 (r)=n.sub.0.sup.2 {1-(gr).sup.2 +h.sub.4 (gr).sup.4 +h.sub.6 (gr).sup.6 + . . . } ##EQU2## where, reference symbol g represents a parameter showing the degree of the gradient of the refractive index, reference symbols h.sub.4, h.sub.6, . . . respectively represent the coefficients of distribution of refractive indices in the terms of the fourth order, sixth order and so on, reference symbol I represents the image height, reference symbol φ represents the outer diameter of the GRIN lens, reference symbol Z represents the thickness of the central portion of the GRIN lens, and reference symbol R.sub.1 and R.sub.2 respectively represent radii of curvature of the surface on the object side and surface on the image side of the GRIN lens.

Claim 3

An objective for an endoscope according to claim 1 further fulfilling the conditions (4), (5) and (6) shown below: |R.sub.2 /I|≦3 (4) g≧0.58 (5) h.sub.4 ≧0 (6).

Claim 4

An objective for an endoscope according to claim 2 comprising said GRIN lens and a cover glass wherein said GRIN lens is arranged to have a plano-convex shape and arranged that said aperture stop is provided on the front surface of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.491, F = 2.99, 2ω = 69.2° I = 0.3, φ = 0.6 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = ∞ (= R.sub.1) d.sub.2 = 0.6632 n.sub.1 = 1.6000 (*) r.sub.3 = -0.4500 ( = R.sub.2) d.sub.3 = 0.1900 r.sub.4 = ∞ d.sub.4 = 0.3000 n.sub.2 = 1.51633 ν = 64.15 r.sub.5 = ∞ ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 0, h.sub.6 = 0 ##STR73## ##STR74## ##STR75## ##STR76## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop and cover glass, reference symbols d 1 , d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 , n 2 , . . . respectively represent refractive indices of the lens, and reference symbol ν 2 represents Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 5

An objective for an endoscope according to claim 2 comprising said GRIN lens wherein said GRIN is arranged to have a plano-convex shape and arranged that said aperture stop is provided on the front surface of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.551, F = 2.99, 2ω = 60.8° I = 0.3, φ = 0.6 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = ∞ (= R.sub.1) d.sub.2 = 0.7363 n.sub.1 = 1.6000 (*) r.sub.3 = -0.6000 ( = R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 0, h.sub.6 = 0 ##STR77## ##STR78## ##STR79## ##STR80## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order form the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 6

An objective for an endoscope according to claim 2 comprising said GRIN lens wherein said GRIN lens is arranged to have a plano-convex shape and arranged that said aperture stop is provided on the front surface of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.589, F = 2.99, 2ω = 56.6° I = 0.3, φ 0.6 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = ∞ (= R.sub.1) d.sub.2 = 0.7868 n.sub.1 = 1.6000 (*) r.sub.3 = -0.7500 ( = R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 0, h.sub.6 = 0 ##STR81## ##STR82## ##STR83## ##STR84## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances beteen respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 7

An objective for an endoscope according to claim 2 comprising said GRIN lens wherein said GRIN lens is arranged to have a plano-convex shape and arranged that said aperture stop is provided on the front surface of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.490, F = 3.01, 2ω = 67.6° I = 0.3, φ = 0.65 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = ∞ (= R.sub.1) d.sub.2 = 0.6774 n.sub.1 = 1.6000 (*) r.sub.3 = -0.4500 ( = R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 3, h.sub.6 = 0 ##STR85## ##STR86## ##STR87## ##STR88## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbols n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 8

An objective for an endoscope according to claim 2 comprising said GRIN lens wherein said GRIN lens is arranged to have a plano-convex shape and arranged that said aperture stop is provided on the front surface of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.310, F = 2.99, 2ω = 111.9° I = 0.3, φ = 0.65 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = ∞ (= R.sub.1) d.sub.2 = 0.6238 n.sub.1 = 1.8000 (*) r.sub.3 = -0.4500 ( = R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.5, h.sub.4 = 2.5, h.sub.6 = -0.5 ##STR89## ##STR90## ##STR91## ##STR92## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 9

An objective for an endoscope according to claim 2 comprising a plane-parallel plate made of a homogeneous medium and said GRIN lens wherein said GRIN lens has a plano-convex shape, said plane-parallel plate and said GRIN lens are cemented together, and said aperture stop is provided at the cemented surface between said plane-parallel plate and said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.203, F = 2.98, 2ω = 83.9° I = 0.15, φ= 0.35 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.1500 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ (stop) d.sub.2 = 0 r.sub.3 = ∞ (= R.sub.1 ) d.sub.3 = 0.3930 n.sub.2 = 1.65000 (*) r.sub.2 = -0.2400 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 2.5, h.sub.4 = 1.5, h.sub.6 = 1 ##STR93## ##STR94## ##STR95## ##STR96## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 , d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 and n 2 respectively represent refractive indices of the lens, and reference symbol ν 1 represents Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 12

An objective for an endoscope according to claim 2 comprising said GRIN lens wherein said GRIN lens is arranged to have a plano-convex shape and arranged that said aperture stop is provided on the front surface of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.219, F = 2.99, 2ω = 77.8° I = 0.15, φ = 0.35 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = ∞ (= R.sub.1) d.sub.1 = 0.3192 n.sub.1 = 1.65000 (*) r.sub.3 = -0.2000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 2, h.sub.4 = 4, h.sub.6 = -1.5 ##STR97## ##STR98## ##STR99## ##STR100## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 13

An objective for an endoscope according to claim 2 comprising a plane-parallel plate made of a homogeneous medium and said GRIN lens wherein said GRIN lens has a plano-convex shape, said plane-parallel plate and said GRIN lens are cemented together, and said aperture stop is provided at the cemented surface between said plane-parallel plate and said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.734, F = 3.00, 2ω = 61.4° I = 0.4, φ = 1.0 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.4000 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ (stop) d.sub.2 = 0 r.sub.3 = ∞ (= R.sub.1) d.sub.3 = 0.9025 n.sub.2 = 1.60000 (*) r.sub.4 = -0.6000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 0.61, h.sub.4 = 2.2, h.sub.6 = -30 ##STR101## ##STR102## ##STR103## ##STR104## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 , d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 and n 2 respectively represent refractive indices of the lens, and reference symbol ν 1 represents Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 14

An objective for an endoscope according to claim 2 comprising a plane-parallel plate made of a homogeneous medium, said GRIN lens and a cover glass wherein said GRIN lens has a plano-convex shape, said plane-parallel plate and said GRIN lens are cemented together, and said aperture stop is provided at the cemented surface between said plane-parallel plate and said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.707, F = 3.01, 2ω = 95.7° I = 0.6, φ = 0.9 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.4000 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ (stop) d.sub.2 = 0 r.sub.3 = ∞ (= R.sub.1) d.sub.3 = 0.7888 n.sub.2 = 1.7000 (*) r.sub.4 = -0.7000 (= R.sub.2) d.sub.4 = 0.4700 r.sub.5 = ∞ d.sub.5 = 0.3000 n.sub.3 = 1.51633 ν.sub.3 = 64.15 r.sub.6 = ∞ ______________________________________ (Coefficients of power distribution of GRIN lens) g = 0.65, h.sub.4 = 2, h.sub.6 = 10, ##STR105## ##STR106## ##STR107## ##STR108## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop and cover glass, reference symbols d 1 , d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbol n 1 , n 2 , . . . respectively represent refractive indices of the lens, and reference symbols ν 1 and ν 3 respectively represent Abbe's numbers of lenses made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 15

An objective for an endoscope according to claim 2 comprising said GRIN lens, which is arranged to have a biconvex shape, and said aperture stop arranged in front of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.495, F = 3.00, 2ω = 68.8° I = 0.3, φ = 0.6 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = 0.9000 (= R.sub.1 ) d.sub.2 = 0.7761 n.sub.1 = 1.6000 (*) r.sub.3 = -0.6000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 0, h.sub.6 = 0 ##STR109## ##STR110## ##STR111## ##STR112## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 16

An objective for an endoscope according to claim 2 comprising said GRIN lens, which is arranged to have a biconvex shape, and said aperture stop arranged in front of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.535, F = 2.99, 2ω = 63.2° I = 0.3, φ = 0.6 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = 0.9000 (= R.sub.1 ) d.sub.2 = 0.8794 n.sub.2 = 1.6000 (*) r.sub.3 = -0.9000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 0, h.sub.6 = 0 ##STR113## ##STR114## ##STR115## ##STR116## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 17

An objective for an endoscope according to claim 2 comprising said GRIN lens, which is arranged to have a biconvex shape, and said aperture stop arranged in front of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.475, F = 2.99, 2ω = 70.1° I = 0.3, φ = 0.65 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = 1.0000 (= R.sub.1) d.sub.2 = 0.7435 n.sub.1 = 1.60000 (*) r.sub.3 = -0.5000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 3, h.sub.6 = 0 ##STR117## ##STR118## ##STR119## ##STR120## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 18

An objective for an endoscope according to claim 2 comprising said GRIN lens, which is arranged to have a biconvex shape, and said aperture stop arranged in front of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.507, F = 3.01, 2ω = 65.0° I = 0.3, φ = 0.65 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = 0.7000 (= R.sub.1) d.sub.2 = 0.8774 n.sub.1 = 1.60000 (*) r.sub.3 = -0.7000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.0, h.sub.4 = 3, h.sub.6 = 0 ##STR121## ##STR122## ##STR123## ##STR124## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 19

An objective for an endoscope according to claim 2 comprising said GRIN lens, which is arranged to have a biconvex shape, and said aperture stop arranged in front of said GRIN lens, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.317, F = 3.00, 2ω = 108.9° I = 0.3, φ = 0.65 ______________________________________ r.sub.1 = ∞ (stop) d.sub.1 = 0 r.sub.2 = 1.0000 (= R.sub.1) d.sub.2 = 0.6798 n.sub.1 = 1.80000 (*) r.sub.3 = -0.5000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 1.5, h.sub.4 = 2.5, h.sub.6 = -0.5 ##STR125## ##STR126## ##STR127## ##STR128## ______________________________________ where, reference symbols r 1 , r 2 and r 3 respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 respectively represent distances between respective surfaces in the order from the object side, and reference symbol n 1 represents refractive index of the lens, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 20

An objective for an endoscope according to claim 2 comprising a plane-parallel plate, which is made of a homogeneous medium and arranged that said aperture stop is provided on the rear surface thereof, and said GRIN lens which is arranged to have a biconvex shape, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.201, F = 2.99, 2ω = 87.8° I = 0.15, φ = 0.35 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.1500 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ d.sub.2 = 0 r.sub.3 = ∞ (stop) d.sub.3 = 0.0500 r.sub.4 = 0.4000 (= R.sub.1) d.sub.4 = 0.2429 n.sub.2 = 1.65000 (*) r.sub.5 = -0.3000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 2.5, h.sub.4 = 1.5, h.sub.6 = 1 ##STR129## ##STR130## ##STR131## ##STR132## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, referenc symbols d 1 and d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 and n 2 respectively represent refractive indices of the lens, and reference symbol ν 1 represents Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 21

An objective for an endoscope according to claim 2 comprising a plane-parallel plate, which is made of a homogeneous medium and arranged that said aperture stop is provided on the rear surface thereof, and said GRIN lens which is arranged to have a biconvex shape, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.236, F = 3.04, 2ω = 71.5° I = 0.15, φ = 0.35 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.1500 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ d.sub.2 = 0 r.sub.3 = ∞ (stop) d.sub.3 = 0.0500 r.sub.4 = 0.4500 (= R.sub.1) d.sub.4 = 0.3785 n.sub.2 = 1.65000 (*) r.sub.5 = -0.3000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 2, h.sub.4 = 4, h.sub.6 = -1.5 ##STR133## ##STR134## ##STR135## ##STR136## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 , d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 and n 2 respectively represent refractive indices of the lens, and reference symbol ν 1 represents Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 22

An objective for an endoscope according to claim 2 comprising a plane-parallel plate, which is made of a homoheneous medium and arranged that said aperture stop is provided on the rear surface thereof, said GRIN lens which is arranged to have a biconvex shape, and a cover glass, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.830, F = 3.00, 2ω = 80.6° I = 0.6, φ = 1.0 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.4000 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ d.sub.2 = 0 r.sub.3 = ∞ (stop) d.sub.3 = 0.1000 r.sub.4 = 2.5000 (= R.sub.1) d.sub.4 = 0.3785 n.sub.2 = 1.60000 (*) r.sub.5 = -0.8000 (= R.sub.2) d.sub.5 = 0.3900 r.sub.6 = ∞ d.sub.6 = 0.6000 n.sub.3 = 1.51633 ν.sub.3 = 64.15 r.sub.7 = ∞ ______________________________________ (Coefficients of power distribution of GRIN lens) g = 0.61, h.sub.4 = 2.2 h.sub.6 = -30 ##STR137## ##STR138## ##STR139## ##STR140## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop and cover glass, reference symbols d 1 , d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 , n 1 , . . . respectively represent refractive indices of the lens, and reference symbols ν 1 and ν 3 respectively represent Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).

Claim 23

An objective for an endoscope according to claim 2 comprising a plane-parallel plate, which is made of a homogeneous medium and arranged that said aperture stop is provided on the rear surface thereof, and said GRIN lens which is arranged to have a biconvex shape, said objective for an endoscope having the following numerical data: ______________________________________ f = 0.625, F = 3.00, 2ω = 76.3° I = 0.4, φ = 0.9 ______________________________________ r.sub.1 = ∞ d.sub.1 = 0.4000 n.sub.1 = 1.51633 ν.sub.1 = 64.15 r.sub.2 = ∞ d.sub.2 = 0 r.sub.3 = ∞ (stop) d.sub.3 = 0.1000 r.sub.4 = 1.4000 (= R.sub.1) d.sub.4 = 0.9581 n.sub.2 = 1.70000 (*) r.sub.5 = -0.7000 (= R.sub.2) ______________________________________ (Coefficients of power distribution of GRIN lens) g = 0.65, h.sub.4 = 2, h.sub.6 = 10 ##STR141## ##STR142## ##STR143## ##STR144## ______________________________________ where, reference symbols r 1 , r 2 , . . . respectively represent radii of curvature of respective surfaces, in the order from the object side, including the stop, reference symbols d 1 and d 2 , . . . respectively represent distances between respective surfaces in the order from the object side, reference symbols n 1 and n 2 respectively represent refractive indices of the lens, and reference symbol ν 1 represents Abbe's number of the lens made of a homogeneous medium, and where, for the GRIN lens, the refractive index of the central portion thereof is shown and is marked with an asterisk (*).