Optical system

An optical system comprising:

• an object lens group;
• an erector lens group;
• and
• an eyepiece group;
• wherein the object lens group, the erector lens group, and the eyepiece group are arranged in order from an object side to an image side along an optical axis;
• wherein the object lens group comprises a first lens, a second lens, a third lens, a fourth lens, and an optical element;
• wherein the optical element is disposed between the second lens and the third lens or the optical element is disposed between the third lens and the fourth lens;
• wherein the first lens is with refractive power and comprises a convex surface facing the object side;
• wherein the second lens is with refractive power;
• wherein the third lens is with refractive power;
• wherein the fourth lens is with refractive power;
• wherein the refractive power of the fourth lens is opposite to the refractive power of the third lens;
• wherein the first lens, the second lens, the third lens, and the fourth lens are arranged in order from the object side to the image side along the optical axis;
• wherein the eyepiece group comprises a fifth lens, a sixth lens, a seventh lens, and an eighth lens;
• wherein the fifth lens is with refractive power and comprises a concave surface facing the object side;
• wherein the sixth lens is with refractive power and comprises a convex surface facing the image side;
• wherein the seventh lens is a biconvex lens with positive refractive power and comprises a convex surface facing the object side and another convex surface facing the image side;
• wherein the eighth lens is a biconvex lens with positive refractive power and comprises a convex surface facing the object side and another convex surface facing the image side;
• wherein the fifth lens, the sixth lens, the seventh lens, and the eighth lens are arranged in order from the object side to the image side along the optical axis.