Patent ID: 8422140

Claim:
A fixed focal length lens, adapted to be disposed between a magnified side and a reduced side, the fixed focal length lens comprising: a first lens group, disposed between the magnified side and the reduced side and having a positive refractive power, wherein the first lens group comprises a first lens and a second lens arranged in sequence from the magnified side towards the reduced side, and refractive powers of the first lens and the second lens are respectively negative and positive; a second lens group, movably disposed between the first lens group and the reduced side for focusing and having a positive refractive power, wherein the second lens group comprises a third lens, a fourth lens, and a fifth lens arranged in sequence from the magnified side towards the reduced side, and refractive powers of the third lens, the fourth lens, and the fifth lens are respectively positive, negative, and positive; and a third lens group, disposed between the second lens group and the reduced side and having a positive refractive power, wherein the third lens group comprises a sixth lens, and a refractive power of the sixth lens is positive, wherein positions of the lens group and the third lens group are maintained fixed relative to the fixed focal length lens, and the second lens group is moved relative to the first lens group and the third lens group to focus, and the fixed focal length lens projects an image from the reduced side to the magnified side.