Abstract: The present invention provides both continuously variable spot size illuminators and discretely variable spot size illuminators that can be utilized in many applications, e.g., photo-dynamic therapy, to provide a light spot on a treatment plane, e.g., a patient's retina, whose size can be continuously or discretely adjusted while ensuring that the light fluence and the working focal distance remain substantially constant at all spot sizes, thereby preserving parfocality with other optical systems. In a continuous variable spot size illuminator of the invention, a focusing lens system is employed to form an image of a light source on an intermediate plane, and a variable aperture is utilized to select a portion of the intermediate image. An objective lens system then images the selected portion of the intermediate image onto a treatment plane.