This afternoon I took a drive down south Arizona to look for pronghorn. A main purpose was to test the 100-400 to see how it performs with my 1.4x extender attached. It performs quite well. This photo I posted in the gallery http://www.zoochat.com/729/chihuahuan-pronghorn-461210/ is with that combination in very low light (and a tripod). The lens is zoomed out to the maximum 400mm, with the extender making it a total of 560mm. I am pleased. A shot I took earlier in better light (of cattle) looks even sharper. Edit - I forgot to mention the posted image is about a 100 percent crop.