I use the 400 5.6, and love it. I went through an evolution however. I calibrated the lens with Focal, and then I upped the shutter speed to over 1/focal length and put 5DIII with lens on sturdy tripod, and now when I use it, get great shots (when I don't, it's my fault). Learning curve in NOT steep, and with only a bit of practice, you can will get excellent images from edge to edge by all standards, and for a great price. Also, it is easy to carry as it is light, and relatively small, so it is a great lens.