(35mm lens format used for questions). Can someone let me know if this logic is correct? I am looking at the Cannon 400mm f5.6. I have a 300mm f4 and a teleconverter. The questions below are for the lens without the TC. The question is this: as the lens gets longer, does the aperature necessary to create a "fuzzy" background (i.e. shallow DOF) get smaller such that a 400mm f5.6 lens will have about as muted a background as a 300mm lens at f4? My logic is that the hyperfocal (depth of full focus) distance becomes larger as aperture becomes smaller so by inference doesn't hyperfocal distance (and thus depth of field) become smaller as aperture increases do sort of equalize the decrease in maximum aperture size for the longer lens with its increase in mm's)? I don't want to buy the 400mm lens if it doesn't have the same relative ability to drop the background into out-of-focus to make sure the subject is not distracted with background objects. Thanks in advance and I hope this question makes sense.