DESC: Electronic Flash EF-500 DG ST can be used with Sigma’s and other manufacturers’ AF 35mm SLR cameras and Digital SLR cameras. It has a maximum guide number of 165 in feet or 50 in meter at 105mm focal length (ISO 100). Projection angle of the flashlight is set automatically to match the lens’ focal length ranging from 28mm wide-angle to 105mm medium-telephoto. It incorporates a wide panel that expands the angle of illumination that of a 17mm focal length lens.