okay great thanks everybody. I realize that for example F6.3 vs F6.7 is not usually too crucial but good to know how it works. It is much easier to alter the aperture in thirds on the big RB67 lenses than on the small Nikon ones, so I will probably be rounding to the nearest half stop on the Nikon most of the time anyway. I just got into the habit of thinking in 1/3 increments because that is what is marked on my sekonic studio meter.