that helps a lot. basicaly the the 400 5.6 would become a f8 and not autofocus . f8 probably wont work for me anyways as i do a lot of wildlife shots at the end of the day and cant push the iso real high without loosing image quality. im probably gonna go with the 400 5.6 but the is of the 300f4 has me thinking hard about it. i had a cheaper 75-300 for a while and it was a bit short . i wish i was rich , i would buy them both and a 5dmrkIII lol. thanks for the explination.