In Citizen Kane Welles made significant use of deep focus cameras in several scenes. Fast forward 70 years to today and I hardly ever see a deep focus camera being used; more often I’ll be watching a scene where there’s a character in the foreground (closeup), and several people in the middle, and they are all out of focus. It’s just jarring when it’s that badly done, and tends to take me out of the movie a bit. So why in this postmodern day and age do they still use the more conventional narrow focus cameras? Are the deep focus ones still too expensive for general use? Are the associated technical requirements (brighter lighting) too daunting?
Deep focus requires faster film and a smaller aperture. These have their tradeoffs.
Artistically, deep focus is used when the composition or the action serves to let the viewer know what the focal point of the frame is–or when it’s not important. Watch a recent animated film for examples of the usefulness of shallower focus: when there’s only thing on the screen that’s in focus, there’s no question about where the director wants you to look.
In short, it’s not that simple, and there are many complex reasons for the choices cinematographers make.