I didn't notice any problems at all with focus speed or sharpness. The two pictures of the RPD chopper came out pretty sharp and that's at the high end of 560mm. The auto focus worked great on the Canon 5D Mark III, but the auto focus won't work on my 70D, you have to manual focus. There are ways to work around this problem but I wouldn't try them. You do lose one stop of light with the Canon 1.4X III, and two stops with the Canon 2X III. I really don't know anything about the Nikon, Eric cold probably help you there.