Objective lens system

1. An objective lens system, comprising, in sequence from an object side to an image side:

a first lens group having a focal length F 1 , having a positive first lens assembly with a focal length F 1 a and a negative second lens assembly with a focal length F 1 b , and the first lens assembly being used for focusing;

a second lens group being a system of magnification variation with a focal length F 2 ;

a third lens group for compensation having a focal length F 3 , the third lens group providing an image point of the second lens group in a location within the focal length F 3 ; and

a fourth lens group having a focal length F 4 ;

wherein the first, second, third and fourth lens groups form a finite conjugative system of magnification variation, and the system as a whole has a negative focal length, wherein the focal length F 1 a of the first lens assembly and the focal length F 1 of the first lens group satisfy the following requirement: 0.8F 1 ≦F 1 a≦ 1.2F 1 , and wherein the focal lengths F 1 , F 2 of the first and second lens groups meet the requirement: 1.8F 1 ≦F 2 ≦2.2F 1 .

2. The objective lens system as claimed claim 1 , wherein the focal length F 3 of the third lens group satisfies the following formula: 6(D 1 +D 2 +D 3 )≦F 3 ≦8(D 1 +D 2 +D 3 ), wherein D 1 , D 2 and D 3 are respectively distances between the first and second lens groups, between the second and third lens groups, and between the third and fourth lens groups.

3. The objective lens system as claimed in claim 1 , wherein, from the object side to the image side, the first lens assembly of the first lens group comprises a double-plane lens, a concave-convex lens and a double-convex lens, the second lens assembly comprises a double-convex lens and a double-concave lens, and the first lens group totally contains ten surfaces r 1 to r 10 .

4. The objective lens system as claimed in claim 3 , wherein the last surface r 10 of the first lens group is an aspherical surface for correcting possible aperture aberration in case of high magnification thereby improving the axial spherical aberration.

5. The objective lens system as claimed in claim 4 , wherein the last surface r 10 of the first lens group has parameters including: conic coefficient K=0.003035479, curvature radius R=1.738237, and aspherical coefficients of the fourth, sixth, eighth and tenth orders thereof being respectively 0.0021409099, −0.022174842, 0.010690177 and −0.0099233468.

6. The objective lens system as claimed in claim 5 , wherein, from the object side to the image side, the second lens groups comprises a double-convex lens and a double-concave lens, and contains four surfaces r 11 to r 14 .

7. The objective lens system as claimed in claim 6 , wherein, from the object side to the image side, the third lens group comprises a double-concave lens, a convex-concave lens and a double-convex lens, and totally contains six surfaces r 15 to r 20 .

8. The objective lens system as claimed in claim 7 , wherein, from the object side to the image side, the fourth lens group comprises a double-convex lens, a concave-convex lens and two double-plane lenses, and totally contains eight surfaces r 21 to r 28 .

9. The objective lens system as claimed in claim 8 , wherein the first surface of the fourth lens group is an aspherical surface for correcting possible aberration within the field of view.

10. The objective lens system as claimed in claim 9 , wherein the first surface r 21 of the fourth lens group has parameters including: conic coefficient K=−0.6602577, curvature radius R−2.504539, thickness being 0.007282585, refractive index being 1.520200, the Abbe-number being 52.020000, and the aspherical coefficients of the fourth, sixth, eighth and tenth orders being respectively 0.039342084, 0.01913038, −0.0085947166 and 0.018000121.

11. The objective lens system as claimed in claim 10 , wherein the last surface r 28 of the fourth lens group is a plane surface.