I currently own a Canon 70-200 f4 IS USM, but have an interest in trading it in for the longer reach of the Canon 100-400 "L' lens. But before I would do so, I'd like to know just how a cropped image taken with the shorter zoom would compare with an uncropped image taken with the longer lens. I could even utilize a top-notch 1.4X TC with the shorter zoom to help reach the 400mm with less cropping. So, for the sake of a simplified comparison, how would the IQ compare of an image taken at 400mm with the 100-400mm lens versus that same image taken with the 70-200 f4 lens without (and even with) a 1.4X Canon TC attached to the 70-200 lens.....at the very same finished size. Need some opinions before ditching a lens that has served me well for a longer reach which I just might be able to get with some careful cropping...and not have to part with my 70-200mm. Thank you.