An optical imaging system includes a first lens having positive refractive power, a second lens, a third lens, and a fourth lens, arranged sequentially from an object side along an optical axis. The first lens through the fourth lens are spaced apart from each other along the optical axis in a paraxial region. A total focal length f of a lens unit including the first lens through the fourth lens and half (IMG HT) of a diagonal length of an imaging surface of an image sensor satisfy f/IMG HT >4.9. An effective aperture radius of an object-side surface of the first lens and an effective aperture radius of an object-side surface of the second lens are both greater than effective aperture radii of an object-side surface and an image-side surface of each of the lenses other than the first lens and the second lens.