a zoom lens system has four or five lens units with the optical powers being +/-/+/- or +/-/+/+/-, respectively , from the object side . the image side lens unit has a negative optical power and is movable , along with at least one other lens unit , in a zooming operation . the zoom lens system satisfies the following conditions : 0 . 6 & lt ; lbw / fw & lt ; 1 . 7 and 0 . 6 & lt ;| fn / fw |& lt ; 1 . 7 , where lbw represents a back focus length at the shortest focal length condition ; fw represents a focal length of the zoom lens system at the shortest focal length condition ; and fn represents a focal length of the image side lens unit . the lens system can be employed with a solid state imaging device , and an optical low - pass filter and an infrared blocking filter can be positioned between the lens system and the imaging unit .