How is deep focus achieved?
Internet Filmmakers' FAQ
Deep focus is achieved by stopping the lens down. This requires a lot of light. Even though colour negative films are now faster than the black and white stocks of the 1940s, deep focus has never been used much in colour (there are a few exceptions). My theory is that deep focus is less attractive in colour. In black and white, everything is monochromatic and reduced to tone and texture - so a red curtain in the background is not going to distract from a face in the foreground even if both are in focus. In colour, a lot of details in the background are going to jump out at you due to their colour, thus competing too strongly for the viewer's attention. Therefore, deep focus would only work well in colour if there were complete control over the art direction (such as in a period film.)
The old-school "deep focus" king was VistaVision which featured a 28mm focal length lens and had a field of view which slightly exceeded the field of view of the 35mm focal length B&L CinemaScope lens (which was seldom used because of its relatively severe barrel distortion).
Skipping the films that use tricks like split-dioptres and slant-focus to achieve deep focus, there are few colour films to study that have been lit to a high f/stop for deep focus. You might look at the Raiders of the Lost Ark films or much of DP Douglas Slocombe's other work like Lady Jane; he often shoots for deep focus. Polanski's Frantic averaged an f/8 for interior scenes where the DP could use a lot of light. Some of Bogdanovich's work like in Texasville also tried to work with deep focus (although that film also points out how colour can get uglier when everything is in focus.)
Another problem with modern films is that it's impossible to shoot an entire film in deep focus because some locations require you to use a certain amount of available light. For example, maybe you could light a hotel room to f/11 but in the next scene you could be in a parking garage with low ceilings and bare fluorescent tubes working at an f/2.8 - and then be filming a car driving out into a city street at night and shooting at f/1.3.
Another problem is that many directors don't know how to stage for deep focus - so what's the point of lighting to an f/11 if every actor stands ten feet away from the camera or the whole scene is shot in close-ups against a blank wall?