Depth of field

Depth of field is the slice of distance, in front of and behind your focus point, that still reads as sharp. Shallow means a thin slice: the eyes are crisp and the ears are already going soft. Deep means everything from the front grass to the far ridge holds detail. Three things move it directly: a wider aperture, a longer lens, and a closer subject all make it shallower. Sensor size gets in through the side door. Bigger sensor, same optics, same distance, and depth of field does not budge; the difference shows up once you match the framing, because filling the frame on a big sensor means a longer lens or a closer stance, and that is what thins the slice.

Reach for shallow when the subject has to win: portraits, a single product on a busy table, food. Reach for deep when the whole frame is the point: architecture, a flat lay, a group shot where nobody is allowed to be soft, a scene you want to feel documentary and honest.

Gotcha: phone 'portrait mode' is part optics, part guesswork. A fast phone lens close to a subject does produce real background blur, but most of what you see is computed from a depth map and painted on, which is why it still eats hair, glasses arms, and coffee steam. The AI-tool version of the same trap is the opposite problem: image models default to creamy blur because their training data is full of it, so deep focus is the thing you have to ask for by name.