As the artist conceptualizes their work, they designate a specific subject or focus for that work. There is a particular message that they wish to convey with their subject and as a result, a given selection of details that must be emphasized in their work.
In a particular work, the artist will need to select the appropriate 'focus' for the piece. This focus will reveal a subject in greater detail while de-emphasizing the surrounding subjects to provide a greater contrast that allows the subject to stand out. This technique of emphasizing a particular subject with a selected focus is commonly described as a depth of field in photography and cinematography. In a visual sense it is the distance between the nearest and farthest objects in frame that appear 'acceptably sharp.'
A deep focus or large depth of field is common in cinematography and is also quite common in fantasy novels as well. In a deep focus shot the foreground, middle-ground and background are all in focus. This emphasizes moderate detail across all planes of the frame and in narrative terms this is revealed as detailed descriptions of the world, characters, atmosphere even political machinations. While there will be given subjects within the frame they will only exhibit an equal level of detail as that of the rest of the frame regardless of positional emphasis(if they're the main characters, subjects, etc.).
Ex: The rusted mechanical horse slowed its piston-timed pace until it brought the carriage to a stop before us. It was clear that the Algenie were being selective about transit updates in this section of the city. "Select your destination sir." The robotic voice echoed.
The counterpoint, a shallow focus or small depth of field is often more effective. In a shallow focus the subject is emphasized while de-emphasizing the foreground and background. This emphasizes great detail on a single plane and significantly less detail on surrounding subjects and in a narrative sense, is common in most contemporary novels. The subjects in frame will exhibit varying levels of detail depending on their positional emphasis.
Ex: The cab swerved across traffic in response to my hail drawing to a stop in front of us. I jumped in and tried not to recoil at the stench. The cabbie looked over his shoulder, "Where to boss?"
A balance between the shallow focus and deep focus is the selective focus, a large depth of field with selected emphasis. This focal method allows for a larger detailed frame than a shallow focus but also provides more detail than a deep focus on selected subjects in the frame. In a narrative sense this is more uncommon but also generates an air of inconsistency.
Ex: The battered yellow cab swerved around the sanitation truck as the workers hopped off. The car dodged a pristine Mercedes and slid to a stop in front of me almost taking my toes off the curb. I shook my head as horns sounded in the distance and I pulled the door handle. The scent of sweat and decay greeted me as I slid in on the cheap vinyl seat. "Where to boss?" The large cabbie asked with a thick Brooklyn accent.
Every creative piece has a selected depth of field, a focal length that establishes what in frame is emphasized and de-emphasized. Once a focal length is selected, it is abundantly apparent to the audience when that focal length changes. If there is a shift within the focus it often weakens the piece and often distinguishes a professional piece from the amateur. Do you know what focal length you're using?