Hi everyone. I am currently upgrading from a nikon d40 with 55-200 to a Canon 40D. (giving the d40 to my brother in law, since he has taken a great interest in photography.) I want to get a prime lens with my 40d. I do a lot of wildlife, mainly in the morning to afternoon, then again at night as the sun is going down. I mainly take a walk through a few fields, then its straight into the woods, under some briers and vines and other crawling around, then my route stops at a small swamp area, which is always full of birds. So I need a good all around lens. I am wondering how much a difference in reach is the 300 to 400? Will the f/4 to f/5.6 make that big of difference? I shoot a lot of birds, squirrels, rabbits, and deer. Kind of a wide variety. Would the 400 f/5.6 do fine in the woods and cloudy days? Even if it was just a monopod? I would really prefer to handhold as much as possible since I do a lot of crawling around. I am just worried the 300 wont bring me the reach I am hoping for, but I could be wrong.