Roger- the reason not to use a 400 film, especially with smaller formats, in the studio is that you lose the ability to choose the aperture you want to use for depth-of-field control. The same is true for field use, sometimes to a greater degree than in the studio. If I have 400 speed film loaded, there's a good chance if I'm using strobes in the studio that I can't dial the power down enough to use f5.6 (for example) and keep the lighting quality that I want. When dealing with 35mm, it's even more likely to be a problem if the aperture you want to use is say f2.8 for depth-of-field purposes. Yes, there are ways to compensate by adjusting focus to have the main subject at the very rear edge of the depth-of-field, but that too has its own issues.