Sigma EF-610 DG ST Electronic Flash for Pentax Digital SLR Cameras is the most powerful one yet, now offering a Guide number of 61 which translates into 61/m (200/ft) designed to work with the latest TTL auto exposure systems of all the most popular digital and film SLR cameras The auto zoom function automatically sets the optimum illumination angle in accordance with the lens' focal length in a range from 24mm to 105mm Sigma EF-610 DG ST Electronic Flash