Lens and depth of field
Focal length sets the angle of view (wide-angle, 35 mm, telephoto); depth of field says what is sharp and what is blurred.
Why it matters in production
Writing a focal length in the prompt (“35 mm”, “85 mm telephoto”) gives an image consistent with a real camera, and controlled background blur (“shallow depth of field”). A phone or action-cam wide-angle makes a realistic film more believable; a telephoto isolates a face. Too much blur is a tell of a generated image.
Other names
focal length · bokeh · shallow focus