Provided are a light scanning unit and an image forming apparatus including the light scanning unit. The light scanning unit can include a source, a deflector, and an optical imaging system. The deflector can deflect the light generated by the source. The optical imaging system can form the deflected light into an image on a photosensitive medium and can have a first and second optical imaging lenses collectively configured: to have the functionality of an f-theta lens. The optical imaging system can be such that a first ratio (k/fm) is between about 0.81 and about 0.88, and a second ratio (fm/fm1) is between about 0.6 and about 0.91, where k is an f-theta scanning coefficient of the optical imaging system, fm is a main scanning focal distance of the optical imaging system, and fm1 is a main scanning focal distance of the first optical imaging lens.