I have had my mark III for about two weeks now and have been shooting a lot of hummingbird and macro shots. My previous camera was a 40d which I thought took excellent pictures.... that is until now! While I still think it takes great pictures, the 5d is just amazing. I was really worried about losing the crop factor for some pics, especially the birds. However, the Mark III more than makes up for this with it's low light capability and the substantial bump in resolution. I have been amazed at how much I can crop some of my full frame pics and they remain excellent. I am getting shots now that I never dreamed of with the 40d, and the 61 point autofocus, don't even get me started on how much I already love that! Anyone with a crop body that has concerns about upgrading and losing the crop factor, don't worry about it if you upgrade to the Mark III. I can't pick up my 40d anymore, it just can't compare. I was going to keep it so I would have two bodies, but I'm starting to think that it may go and I may buy another full frame, possibly a Mark II.