Unlike the screens on manual cameras, screens on AF cameras the screen will not show DOF at less than f/2.8, so if you focus at f/1.4 or f/2.8 the VF looks the same. The rangefinder will focus over a wider range than you actually want with fast lenses, I had an F100 in the past and at close range and f/1.4 you could turn the ring and still be in focus over a rather wide area, shows you how accurate AF is...
The only exception to that is the Ec-S screens Canon makes that will show DOF to f/1.8. I have one and it is a vast improvement over the stock screens, I can accurately focus anywhere on the screen without resorting to focus points and the rangefinder. I hacked one once to try and install it on a F100. It kind of worked but the screen was too thick and sat slightly at angle, meaning the extremes of the screen would front/back focus...shame Nikon doesn't make a plain screen that helps in this regard...