An optical imaging system includes a first lens group having a positive or negative optical power, an aperture diaphragm, and a second lens group having a positive optical power arranged in order from an object side. The first lens group includes a first front lens group having at least two negative lenses and a first rear lens group having at least one positive lens with an airspace in-between the first front and rear lens groups. The second lens group includes a second front lens group having first positive and negative lenses, second negative and positive lenses arranged in order from the object side, and a second rear lens group including at least one aspherical lens. When focusing on an object in a close range from infinity, the second lens group moves to the object side while an interval between the second front and rear lens groups are shortened.