Depth of Field Prompts: Controlling Blur and Separation

Depth of field is the single fastest way to change how an image reads. A subject cut out of a soft wash of background looks deliberate and expensive; the same subject sitting inside a background rendered as sharply as they are looks like a snapshot. Both are correct in the right place, and the prompt decides which one you get.

Say Where the Focus Is, Not Just That There Is Blur

The common failure is asking for “blurry background” and getting blur smeared across the whole frame, including the subject’s face. The model has been told that blur belongs in the picture but not what should stay crisp, so it distributes it.

Fix that by naming the focus plane before you name the blur. “Sharp focus on the woman’s eyes, background falling away into soft blur” gives the model an anchor. The word order matters less than the fact that something is explicitly declared sharp. A prompt that only describes softness will produce a soft image.

The same applies in reverse. If you want everything sharp — a landscape, an architectural shot, a flat-lay of tools — say so directly. “Everything from the foreground rocks to the distant ridge in sharp focus” is a real instruction. Simply not mentioning depth of field is not; you will get whatever the model’s default is for that subject, which for portraits tends toward soft and for wide scenes tends toward sharp, but is never reliable.

Aperture Numbers Work as a Signal, Not as Physics

Writing f/1.4 or f/16 does something. It is not doing optics — no lens is being simulated — but those strings appear constantly alongside images with the corresponding look, so they behave as a shorthand for it. f/1.4 pushes toward a very shallow band of focus and a background dissolved into colour. f/8 tends toward a normal, mostly-sharp look. f/16 reads as everything-in-focus.

The catch is that the effect is coarse. f/1.4 and f/1.8 are not meaningfully different to the model, and asking for f/2.8 will not consistently give you a wider focus band than f/1.4. Treat the numbers as three or four buckets — wide open, middling, stopped down — rather than a dial. If you need a specific degree of softness, describe it in words: “background gently softened, individual leaves still recognisable” gets you closer than any f-number.

Describe What the Blur Looks Like

Shallow focus is not one effect. Background lights can render as smooth round discs, as hard-edged polygons, as long streaks. Foliage can turn into a creamy wash or break into busy, nervous shapes. If the character of the blur matters to your image, describe it: “distant streetlights rendered as large soft circles of light” or “background dissolved into flat bands of colour with no detail.”

Foreground blur is worth knowing about too, and almost nobody asks for it. A branch or a shoulder passing out of focus across the near edge of the frame adds a strong sense of depth and of someone standing in a real place. “Out-of-focus leaves in the immediate foreground, partly across the frame” is enough to get it, and it costs one clause.