Although 500mm (750mm equivalent on D70) seems to be very long focal length, for some wildlife shooting some extra milimeters could be beneficial. Therefore, I have tested 1.4x and 2x teleconverters (both Sigma and Kenko brands) in order to reach 700 and 1000 mm focal lengths. What cannot be seen here are focusing difficulties, darker viewfinder and very shallow dept of field. Picture quality is not so good with 2x TC, but pretty acceptable with 1.4x TC. Because of the mechanical construction, Sigma's teleconverters are impossible to use on many lenses, including Sigma's own. Even on Sigma 50-500, their TCs limit lower f.l. range to 100mm. There is a mechanical limiter on the lens, but if you forget to switc it on, you can damage both end lenses on TC and Bigma :-(