Patent ID: 7440198

Claim:
A focus adjustable optical system arranged on an optical axis between an object side and an image side, comprising: a first lens unit, said first lens unit comprising at least one lens set on said optical axis, and defining a fixed distance from said lens to the image plane on the optical axis; a second lens unit spaced between said first lens unit and the image plane, said second lens unit comprising at least one lens movable along said optical axis to change the distance between the at least one lens of said second lens and the image plane; a third lens unit spaced between said second lens unit and the image plane, said third lens unit comprising at least one lens that defines with the image plane a fixed distance; wherein said first lens unit is comprised of a first lens having a positive diopter and a second lens having a negative diopter, said first lens and said second lens being arranged along said optical axis in direction from said object side to said second lens unit; said first lens unit wholly has a positive diopter; said second lens unit is comprised of a third lens having a positive diopter; said third lens unit is comprised of a fourth lens having a negative diopter; and wherein said focus adjustable optical system further comprising a diaphragm arranged on said optical axis before the object-side lens surface of said first lens.