I am ready to buy the 150mm lens for Mamiya 645. Question is simply is there enough of a difference between f/4 and f/3.5 to be noticeable? I fully anticipate I will use this lens on a tripod with some artificial lighting. I fully anticipate I would be taking some portraits and might actually need f/4 or greater for DOF. Is the f/3.5 or the f/2.8 that much brighter in terms of focusing? TIA!