Optical system

An optical system comprising, in order from an object side to an image side:

• a first lens group comprising:
• a first lens element with positive refractive power having a convex object-side surface;
• a second lens group comprising, in order from the object side to the image side:
• a second lens element with negative refractive power having a concave image-side surface;
• a third lens element with positive refractive power having a convex image-side surface;
• and
• a fourth lens element with negative refractive power having a concave object-side surface and a concave image-side surface at a paraxial region, wherein the image-side surface of the fourth lens element changes from concave at the paraxial region to convex at a peripheral region, and the object-side surface and the image-side surface of the fourth lens element are aspheric;
• wherein an object moves closer towards or further away the optical system, the focusing adjustment is performed by moving the first lens element along an optical axis;
• the optical system has a total of four lens elements with refractive power, a difference of an axial distance between the first lens element and the second lens element of the optical system focused on the object at a close distance and infinity is ΔT12, a central thickness of the second lens element is CT2, and the following relationship is satisfied:
• 0.05<|ΔT12|/CT2<0.80.