f/4

f/4 is where a frame stops being about one person. Two or three people at different distances hold together, and the middle of the room comes back.

It leans toward Deep Focus without arriving there. The far background still softens, so the shot has depth rather than flatness.

The sharp band runs roughly a metre at portrait distance on a normal lens, which covers a row of people standing at slightly different depths.

Sharpness across the frame is high. Corners have caught up with the centre on almost every lens, and diffraction has not started.

The set needs light. f/4 passes a quarter of what f/2 does, so an interior usually wants a proper key rather than the lamps already in the room.

Use it for group shots, for staged scenes where two distances matter, and for product work that has to hold the whole object.

Skip it when one face is the whole beat. At f/4 the background keeps enough detail to pull the eye.

f/2.8 keeps a single subject and softens the rest. f/4 brings the second person back into the shot.

f/8 goes further and hardens the background too. The test is the far wall: gently soft is f/4, legible is f/8.

Studio portrait and product work sits here because it holds a whole object sharp. A bottle, a face turned three quarters, a row of three people: all of them fit inside the band f/4 gives.

The model treats any f-number as a blur request and returns a shallow frame. Name the people or planes that have to be sharp, not only the stop.

The other failure is a frame sharp to the horizon. Say the far background stays gently soft, so the shot keeps its depth.